On the main page of this website, I suggested that modern humans may have interbred with two groups in different regions, Neanderthals in Europe, and Denisovans in Asia. This suggests that Neanderthals, Denisovans, and modern humans should not be classified as three different species, Homo neanderthalensis, Homo denisova, and Homo sapiens, but rather three related sub-species, Homo sapiens neanderthalensis, Homo sapiens denisova, and Homo sapiens sapiens.

This is not merely a question of taxonomy. To begin with, it relates to our evolutionary history, the story of how humans evolved, and is thus fascinating and important. Second, it relates to our principal subject, race and genetics.  In particular, it is susceptible to dangerous misinterpretation. The idea that humans may have interbred with Neanderthals and Denisovans provides a convenient way to separate humans into “Europeans,” “Asians,” and “Africans,” so providing a basis for racial classification. I have never encountered a racist argument framed in precisely these terms, but the potential exists, and so I think it’s important to address these issues clearly.

To understand the story of Neanderthals and Denisovans, we have to define three confusing terms: hominin, archaic human, and modern human. Consider the figure below, which shows the evolution of humans and our close ancestors, from time when the human and chimpanzee lines split, approximately 6 millions years ago, to the present. Hominins are upright-walking, large-brained primates. Therefore, all of the species in this picture could be considered hominins, except perhaps the oldest species, Orrorin tugensis and Sahelanthropus tchadensis, which show intermediate features between primates and hominins, and may or may not have walked upright. Humans, in the broadest sense, are members of the family Homo, therefore, species such as Ardipithecus ramidus and Australopithecus africanus are not considered humans, while species such as Homo habilis and Homo erectus are. The human family is further divided into archaic humans, that lived before approximately 40,000 years ago, such as Homo erectus and Homo neanderthalensis, and modern humans, Homo sapiens.

Figure 1. Hominin diversity. Adapted from Klein, 2009. The original included images of stone artifacts associated with different periods.

This figure clearly shows that, at different times in the past, diverse hominins existed in Africa. How were these hominins related to each other? How did they spread around the world? Why did only humans survive?

Here we enter the perilous world of archeology and physical anthropology, where consensus is rare, and controversy is common. As I have said before, the discovery of a single toe or tooth can completely rewrite human history, or, at least, push it in new directions, and introduce new questions. Therefore, what I can offer is only one possible story of human evolution. There are various parts of the story that are subject to ongoing discussion, and I would not be surprised if many details, or even major chapters, changed in the coming decades. That said, I do think this interpretation represents something close to our best current understanding of human evolution.

Before I go any further, it’s important to describe where the information to construct these stories comes from, the important facts that need to be explained, and a brief history of past theories, to provide context.

Most of our information about human evolution comes from fossils of varying kinds, such as bones, tools, traces of fire, and so on, which can be considered traditional forms of evidence. More recently, genetic analysis has proved a powerful tool to elucidate the relationships between different species, when sufficient DNA can been extracted from preserved remains. These analyses typically involve comparison of DNA from different individuals, including modern and archaic humans, as well as techniques that allow researches to determine paternal and maternal lineages, and so trace wandering lines back in time across generations. Finally, there are alternative fields of study, such as evolutionary linguistics, that can provide support for traditional and genetic evidence. Using these approaches, researchers try to determine the most parsimonious theories that explain their observations. The best theories are supported by multiple lines of evidence, are leave the fewest questions unanswered. This is the kind of story that I will try to tell below.

Any theory of human evolution has to explain some basic facts: 1) Archaic humans left Africa and spread through Eurasia about 1.8 million years ago, 2) All modern humans are genetically very similar to each other, 3) African populations are more genetically diverse than populations in other parts of the world, and 4) Modern humans in Europe share some features with archaic humans who lived there, and modern humans in Asia share some features with archaic humans who lived there, while these features are not shared by modern humans in Africa.

The first theories of human evolution relevant to this discussion are generally called polygenism or polygenetic theories. These theories emerged in the Eighteenth and early Nineteenth centuries, during the age of exploration and colonialism, when Europeans traveled to other continents, encountered people living there who appeared very different from themselves, and tried to account for these differences. Predictably, these early attempts were, by today’s scientific and cultural standards, inadequate and offensive. Essentially, polygenism holds that modern humans evolved independently on each continent—Africa, Europe, Asia, and the Americas—and that people from each continent are completely independent of and different from each other. Polygenism implies that only people from Europe are truly human, while all other people are sub-human. Naturally, this provided an intellectual framework to rationalize the conquest, colonialism, slavery, and racism that followed. The popularity of polygenism began to decline in the late Nineteenth century, as it was replaced by more accurate and powerful theories. Presently, no educated person, and certainly not any practicing scientist, believes in polygenism, however, as we shall see, there are elements of polygenism that can still inform our understanding of human evolution.

The second relevant theories of human evolution are monogenism or monogenetic theories. Monogenism is based on the idea that humans evolved in one place, now understood to be Africa, and from there spread to other parts of the world. It requires that all humans are related to each other, and that our distant ancestors were African, however difficult to accept that may be for those who would believe otherwise. Monogenism began to take hold in the late Nineteenth century, and is now commonly accepted as true. All of the remaining theories described below are monogenetic theories.

Monogenetic theories can be broadly separated into two groups; multiregional theories, and recent African origin theories. Multiregional theories state that archaic humans originated in Africa, dispersed throughout Eurasia, and then developed into modern humans independently, in multiple regions of the world. Multiregional theories assume that, following our initial expansion from Africa, humans followed much the same evolutionary path in Africa, Europe, Asia, India, and so on. Some people believe there was little or no mixing between archaic humans from different regions, while others believe there was considerable mixing, producing a single, globally distributed species.

Recent African origin theories, like multiregional theories, state that archaic humans originated in Africa, and dispersed throughout Eurasia. Then, relatively recently, modern humans left Africa, and spread around the world. The modern humans we observe today are the ancestors of this recent migration. As above, there are various interpretations of the recent African origin theories. Some people believe that modern humans completely replaced archaic humans, with no interbreeding, or genetic exchange, between them. Others believe that modern humans assimilated, or absorbed, archaic humans, by interbreeding with them. These two interpretations are known respectively as the replacement theory and the assimilation theory.

Multiregional or recent African origin, mixing or no mixing, replacement or assimilation—which theory is right? As you might imagine, our best understanding of human evolution involves a combination of different theories, with various overlapping components, and unexpected twists.

I will begin the tale at the time when last common ancestor of the human (Homo) and chimpanzee (Pan) lines is estimated to have lived, somewhere between 13 and 4 million years ago. That might seem like a broad time period—the uncertainty is high—but the split between the human and chimpanzee lines was probably a drawn out process, with many hybrids, species, and sub-species, which lived together for thousands of years before they finally separated. It’s difficult to be sure, because fossils are scant, and were have little other evidence to elucidate this division. However, because the first fossils tentatively classified as hominins, such as Orrorin tugensis and Sahelanthropus tchadensis, appear about six million years ago, it’s reasonable to assume that the division was underway by this time. We believe that it occurred in Africa—the little evidence we have is found there, and the great majority of later hominin fossils are also found in Africa. Nonetheless, it is possible that, in other parts of the world, ape-like creatures came down from the trees, and began walking around on two legs, either occasionally or most of the time. There is some evidence that hominins may have evolved outside of Africa, however, if true, they did not survive, nor interbreed with hominins from Africa, to the best of our knowledge. Therefore, although upright walking apes may have evolved in several parts of the world—a kind of proto-polygenesis—these tentative forays on two legs appear to have been evolutionary dead ends, while, in contrast, in Africa, they lead eventually to ourselves.

I will now skip over a long period in human evolution, from split between the human and chimpanzee lines, until the first large migration from Africa, approximately 2 million years. This is not for lack of representative fossils. On the contrary, this period offers a wealth of evidence of diverse hominins. To highlight several briefly, there is Australopithecus afarensis, the southern ape, who lived approximately 3.9-2.9 million years ago, and who clearly walked upright. We know this because they left footprints behind, the tracks of two individuals, walking across a flat plain, preserved in volcanic ash, in present-day Tanzania. The footprints were made by one larger and one smaller individual, perhaps a mother and child, or brother and sister, or mating pair, walking across the prehistoric landscape, an evocative and magical scene, all the more powerful because it is true. There is Homo habilis, the handy man, who lived approximately 2.1 to 1.5 million years ago, so named because he constructed simple stone tools. There is Homo erectus, who lived approximately 1.9 million to 143,000 thousand years ago, who learned how to control fire. And there are many others, such as the Ardipithecus family, the Kenyanthropus family, the Paranthropus family. Some groups are well defined, while others are known only by a few fossils. There is considerable debate about how each should be classified, the relationships between them, and to what extent they did or did not contribute to the Homo family. Only more evidence will clarify these issues, but we can only assume that, as our knowledge grows, new questions will arise, and our understanding of this chaotic period, the bountiful, adolescent flowering of mankind, may never be complete.

What we do know is that by 1.8 million years ago Homo erectus left Africa and spread throughout Eurasia. This was the first significant human migration from Africa, and subsequently Homo erectus was established across the continent, including Southern and Western Europe, the Middle East, India, Southern Central Asia, and Southern Eastern Asia. The evidence of Homo erectus in these regions is widespread, and includes many well-known fossils, such as Java man, Peking man, and remains from Turkey, Georgia, and Hungary. It’s important to note that some members of Homo erectus remained in Africa, including Ethiopia, Eritrea, and Kenya. The figure below shows the approximate geographical range of Homo erectus.

Figure 3. The geographical range of Homo erectus. From http://www.abroadintheyard.com.

The African branch of Homo erectus continued to evolve, and was perhaps the direct ancestor of the next important human in our story, Homo heidelbergensis. Homo heidelbergensis, named after a fossil discovered near Heidelberg, Germany, appeared in Africa approximately 700,000 years ago. Like Homo erectus, Homo heidelbergensis left Africa and spread throughout Eurasia. This is the second significant human migration from Africa, although there were probably many smaller migrations that left no traces, or, at least, none that we have yet found. As I discussed, there are competing theories about whether successive waves of migrants replaced or assimilated established groups, however, I think we can assume that there was at least some interbreeding between Homo heidelbergensis and Homo erectus, if only because there is evidence of similar interbreeding later, as I will discuss. Homo heidelbergensis moved into habitat previously occupied by Homo erectus, and established populations large enough to leave traces in North Africa, the Western Middle East, and Southern and Western Europe, roughly around the border of the Mediterranean Sea. Perhaps Homo heidelbergensis should be called the Mediterranean man. The figure below shows the approximate geographical range of Homo heidelbergensis.

Figure 4. The geographical range of Homo heidelbergensis. From http://www.abroadintheyard.com.

At this point, an interesting event occurred, central to the story of Neanderthals and Denisovans. The Homo heidelbergensis population split into three branches; one branch remained in Africa, one branch established itself Western Europe, and became Neanderthals, and the other branch moved to Asia, and became Denisovans.

Neanderthals are named after the Neander valley, in Germany, where they were first discovered. They lived approximately 250,000 to 40,000 years ago, in Western Europe and Central Asia, but not in Africa. They left behind numerous fossils and other artifacts, and their anatomy and way of life have been studied in detail. For reasons that will become clear, it’s probably worthwhile to pause for a moment and consider who and what Neanderthals were. Neanderthals are often portrayed as primitive, hulking brutes, but the truth is that they were probably far closer to modern humans than we care to admit. They were of comparable height, although their body and limb proportions were different, with more robust builds, larger barrel chests, and more powerful arms and hands. Their skulls were also of comparable size, but they had different facial features, including receding foreheads and sloping chins, larger noses and more pronounced brow ridges, as well as, surprisingly, larger brains. Some people think they were covered with hair, to protect them from the cold environment of Northern latitudes, while others claim that they had had red or blond hair, and light skin. They are sometimes depicted with blue eyes, but this is probably supposition.

Neanderthals made stone tools, used fire, and appear to have lived in small and sparsely distributed groups of hunters and gatherers. They prepared rough shelters, and simple hides, perhaps for use as blankets and ponchos, however, there is no evidence of more sophisticated shelters, clothes, or footwear. Some evidence suggests that they decorated themselves with flowers, feathers, pigments, bones, and shells, but this is controversial. Equally controversial is the claim that Neanderthals had the capacity for symbolic thought, produced art, and buried their dead. In any case, it’s clear that, while Neanderthals were relatively advanced compared to any other previous group, there were nonetheless very different in terms of their anatomy and behavior from modern humans.

Far less in know about Denisovans. They were discovered in the Denisova cave, in the Altai mountains of Central Asia, near the border with China and Mongolia. From this cave, researchers recovered a finger bone and several teeth, which were determined to be approximately 41,000 years old. Genetic analysis revealed that the inhabitants of the cave were sufficiently different from Neanderthals to warrant their own classification as an independent species or sub-species. No further fossils have been discovered, although several skull caps, 125,000 to 105,00 years old, unearthed in China, may belong to Denisovans. Considering the lack of evidence, it goes without saying that little is known about their way of life, but it’s probably safe to assume that they very similar to Neanderthals. The figure below shows the division between Neanderthals and Denisovans, and the approximate geographic range of each group. I will discuss the geographic distribution of Denisovans in more detail later.

Figure 5. The division between Neanderthals and Denisovans, and the approximate geographic range of each group. Note that the gray shaded area indicates Homo erectus. The extent of Denisovan habitat is not yet clear. Arrows indicate evidence of gene flow. From Veeramah and Hammer, 2014.

While Neanderthals and Denisovans developed in Eurasia, the African branch of Homo heidelbergensis, as above, continued to evolve. We believe that Homo heidelbergensis or their close ancestors gave rise to Homo sapiens, the wise man, who appeared in Africa approximately 200,000 years ago. This marks the arrival of anatomically modern humans, or modern humans indistinguishable, in terms of their anatomy, but not their behavior, from people today. Modern humans left Africa approximately 70,000 years ago. This was the third and last great migration from Africa. Modern humans spread into areas occupied Neanderthals, Denisovans, small groups of surviving Homo heidelbergensis and Homo erectus, and perhaps other groups of archaic humans. As they did so, they gradually pushed other groups aside, through some combination of replacement and assimilation, until, finally, somewhere between 40,000 to 30,000 years ago, only Homo sapiens remained. The figure below shows a possible evolutionary history of Homo sapiens, including the division of Homo heidelbergensis into Neanderthals and Denisovans, and the continued evolution of Homo heidelbergensis in Africa. Our direct ancestors remain unknown.

Figure 6. Possible evolutionary path of Homo sapiens. From Stringer, 2012.

Once modern humans were established throughout Eurasia, they began their gradual dispersal worldwide, from island to island in the Pacific Ocean, and across the Bearing Strait to the Americas.  Evidence for a west-to-east Pacific crossing, or for the arrival of modern humans in North America by way of Greenland, remains controversial. It’s possible that modern humans migrated along these routes, but they apparently did not do so in sufficient numbers, and over a long enough time period, to leave significant physical or genetic evidence. In any case, by approximately 10,000 years ago, modern humans reached Patagonia, having successfully traversed the full length of the Americas. The astonishing habitat expansion of Homo sapiens was complete.

Figure 7. Global migration of Homo sapiens. Blue arrows indicate genetic markers from paternally-inherited Y chromosomes, and  orange arrows indicate genetic markers from maternally-inherited mitochondrial DNA.  From National Geographic Maps.

That’s the story of human evolution, or, at least, one possible story, as I emphasized at the start. The Homo line developed in Africa and spread through Eurasia by way of three migrations, first Homo erectus, then Homo heidelbergensis, and finally Homo sapiens. In each region—Africa, Europe, the Middle East, India, Asia—evolution followed slightly different paths, resulting in slightly different groups of archaic humans. There was considerable mixing and gene flow between these groups, but not so much as to render them indistinguishable. Likewise, with each successive wave of migration, there was some degree of assimilation, at least enough to leave traces in our genes.

This story explains the basic facts that I outlined at the start. It accounts for the first great migration from Africa 1.8 million years ago. It provides an explanation for why all modern humans are genetically very similar (modern humans are derived from a small group of migrants), and why African populations are more genetically diverse (Africans represent larger ancestral populations). And it explains why modern humans in Europe share some features with archaic humans who lived there, and modern humans in Asia share some features with archaic humans who lived there, while these features are not shared by modern humans in Africa. As above, modern humans interbred with Neanderthals in Europe, and Denisovans in Asia, as well as other groups, and the evidence of these encounters is present in our genes.

Many questions remain. Did Homo sapiens emerge in Africa much earlier than previously thought, perhaps 300,000 years ago? Fossils found in Morocco suggest this might be the case. What of Homo floresiensis, the petite human, only one meter high, discovered on the island of Flores, in Indonesia? These people appear to have survived until at least 50,000 years ago. Where did Homo floresiensis come from, and how are they related to other humans? And, perhaps most important, who are the direct ancestors of modern humans, Homo heidelbergensis, or other groups, such as Homo rhodesiensis, or Homo sapiens idaltu? Hopefully, in the future, we will find answers to these questions.

Now that we have some understanding of human evolution, we can return to Neanderthals and Denisovans. In 2008, an international research team succeeded in sequencing the full Neanderthal genome. Later, the same team compared the Neanderthal genome to the genomes of five modern humans, one each from France, China, and Papua New Guinea, and two from sub-Saharan Africa. Results showed that one to four percent of the genomes of non-African modern humans are derived from Neanderthals, indicating modern humans did interbreed with Neanderthals. Because the proportions of Neanderthal DNA in non-African modern humans from different regions is roughly equal, the researchers concluded that modern humans interbred with Neanderthals relatively soon after they left Africa, perhaps in the Middle East, before they dispersed widely through Eurasia.

Subsequent work has largely upheld these findings, although the story, predictably, has grown more complex. First, it seems that modern humans probably interbred with Neanderthals more than once, at different times, and in different regions, creating a complex admixture of genes. This explains why Eurasians from different regions appear to have different amounts and kinds of Neanderthal DNA, though generally falling within the range cited above.

Second, researchers studying the Y chromosome (paternally-inherited) and mitochondrial DNA (maternally-inherited) believe that reproduction between modern humans and Neanderthals was not always successful or symmetrical. Evidence suggests that mating between male modern humans and female Neanderthals did not produce offspring, or produced offspring that did not survive in large numbers, or were infertile. Likewise, mating between male Neanderthals and modern human females produced healthy female offspring, while male offspring were absent, scarce, or infertile.

In the case of Denisovans, the genetic admixture is even more complex. Despite the fact that Denisovans left behind few fossils, from these remains researchers extracted sufficient DNA to sequence the Denisovan genome in 2010. Denisovans apparently share a significant portion of their genome with Neanderthals, which is not surprising, considering both branched from same ancestor, Homo heidelbergensis. In addition, the Denisovan genome includes contributions from other unknown archaic humans, which is also not surprising, considering the complexity of our past.

When researchers compared the Denisovan genome to the genomes of six modern humans, including a ǃKung South African, a Nigerian, a Frenchman, a Papua New Guinean, a Melanesian, and a Han Chinese, they found that a small fraction of the genomes of non-African modern humans are derived from Denisovans. Modern humans interbred with Denisovans after they left Africa.

The geographical distribution of Denisovan DNA is interesting. Up to six percent of the genomes of people from Melanesia, including Papua New Guinea, are derived from Denisovans. The same is true, to a lesser extent, of Australian Aboriginals, and Philippines islanders, but not people from the rest of Asia. This suggests that Denisovans, or groups that mixed with Denisovans, traveled over vast distances, from the Denisova cave in Siberia, to Melanesia, Australia, and the Philippines, but did not necessarily spread uniformly through Asia. In many ways, this supports our understanding of human evolution, multiple migrations and frequent mixing, constrained by geography, climate, culture, and so on.

The evidence is strong that modern humans interbred with Neanderthals and Denisovans. Nonetheless, we cannot rule out the possibility that the genetic similarity between modern humans, Neanderthals, and Denisovans, can be explained by the fact that all three share a common ancestor in Africa. Also, the studies cited above involve a very limited number of subjects. With more data our understanding of human evolution might change considerably.

We can now turn our attention to what all of this means for race and genetics. The first and most important lesson is that everything I have related above is history. Exactly who our direct ancestors were, how we migrated from Africa, and whether or not we interbred with Neanderthals and Denisovans does not in any way change the facts that we observe today. Humans are overwhelmingly more genetically similar that they are different, the small genetic variation that does exist is not enough to classify human as sub-species or races, there is no evidence of fundamental differences between humans from different parts of the world, and so on. In short, our history does not call into question the scientific basis for racial equality, precisely because it belongs to our past, and only describes how we became what we are today, a single human race.

The second lesson is related to the definition of species and sub-species. Recall that the definition of a species is, “A group of living things that breeds successfully under normal conditions in the wild,” and that a sub-species is, “A population that has become sufficiently different to deserve independent classification.” Because we apparently did interbreed with Neanderthals and Denisovans, even to a limited degree, we should probably change the classification of these humans to Homo sapiens neanderthalensis, Homo sapiens denisova, and Homo sapiens sapiens.

I argued that human genetic variation falls well below commonly accepted thresholds to classify large mammals as sub-species, therefore, there are no human races. Faced now with true sub-species, we can more fully appreciate the great similarity between modern humans, and the argument that there are no human races.

Neanderthals, as I explained, were visibly different from modern humans. Their body and limb proportions, their facial features, the size of their brains, and, perhaps, the hair on their skin, all fell outside the normal range of modern humans. Neanderthals were more different from modern humans than, for example, a San Bushman and Kenyan tribesman, an Ainu islander and native Quechan, a Papua New Guinean and Berber nomad. Furthermore, their behavior was very different that of modern humans. They did not approach our level of sophistication in tool use, clothing, footwear, shelters, symbolic thought, art, religion, and, presumably, language and intelligence. Finally, as I explained above, reproduction between Neanderthals and modern humans was not always successful, and involved significant reproductive barriers. The same is likely true of Denisovans.

This is obviously not true for modern humans. The seemingly great variation between people from different parts of the world is limited to superficial physical appearance. We have found no significant differences in fundamental characteristics between people from different regions. And it goes without saying that all modern humans can and do reproduce freely, without any sort of barriers. This strengthens the case for racial equality. There is no basis to separate modern humans into sub-species. The differences between us are simply too small.

Finally, we can return to the question that prompted this discussion, “Does the fact that we interbred with Neanderthals and Denisovans provide any basis to separate modern humans into groups, such as Africans, Europeans, or Asians? The answer, as I hope I have shown, is, “No.” However, it’s likely that encounters between modern and archaic humans contributed to the genetic variation, however small, that we observe today. In this sense, interbreeding with Neanderthals and Denisovans is just one small part of our story. It can help us understand how we came to be human, but it does not change our essential nature.

How will our understanding of race and genetics evolve? Do we anticipate great discoveries that will significantly change our views about racial equality? What does the future hold?

To begin with, we can only assume that all of the fields that contribute to our current understanding of race and genetics, such as biology, geography, archeology, anthropology, sociology, and history, will continue to advance. As I mentioned, the identification of a single toe bone or tooth fragment could rewrite human history. However, considering that accepted theories are based on multiple overlapping lines of evidence, I do not think that our understanding will change radically. Instead, it will  become more refined.

For example, we will learn more about how we lived before we migrated from Africa, the territory we occupied, the plants and animals found there, the climate at that time, and details of events such as the Toba Catastrophe, that steered the course of human history. Likewise, the dates and paths of our various migrations from Africa and to other parts of the world will become more clear, as well as the gene flow that occurred between these areas, and the varying degrees of isolation, imposed by geographical barriers, that effected our genetic composition.

We will learn more about how humans intermingled with and eventually replaced Neanderthals and Denisovans, and how much and what parts of our genome derive from this union. Perhaps we will also discover that humans met and interbred with other hominins, such as Homo floresiensis, the small Pacific islanders. If so, we may add more extinct sub species to the human lineage, and our picture of the genes we have carried forward through time will become more complex.

Additionally, we will learn more recent human history–for example, the last 10,000 years. More details about how the environment shaped history will emerge, we will clarify the historical events and processes that are not determined by the environment, and our understanding of the importance of culture, and individual, will improve.

There are two areas of research that I believe have the potential to change our understanding of race and genetics in more profound ways. The first is the study of genetic variation. Recall that Lewontin’s 1972 study employed seventeen genetic markers, while Rosenberg’s 2003 study employed 993 markers. In the future, studies will routinely employ thousands of markers. In addition, the number of people sampled will increase. This is particularly important for areas previously considered homogeneous, including entire continents, such as Africa. Our understanding of human genetic variation will improve, and we will be able to make sense of the great admixture of genes in any one person, and trace our ancestry with greater precision.

The second area of research is the ongoing quest to relate particular genes to particular characteristics, or, to use the parlance of biology, to relate genotype to phenotype. Considering the remarkable achievements of genetic research, and the incredible amount of money and resources directed into this field, it is astonishing that we know so little about genes.

Consider a characteristic such as height. We know that height exhibits a high degree of heritability, that is, our height can be accurately predicted from the heights of our parents, so it must be encoded somewhere in our genes. We also know that height was shaped by natural selection. Therefore, you might expect that we have a solid understanding of the genetic basis for height. In fact, attempts to find the genes that determine height have been largely unsuccessful. Studies of genetic variation show that about half of the variation in height within a population can be explained by as many as 300,000 genetic markers. We also know there are mutations in certain genes that produce developmental disorders related to height, and so we assume that these genes play some role in determining height, at least when they do not function properly. However, we have no real understanding of which genes, or set of genes, control height in healthy humans. If this is true of height, a relatively simple physical characteristic, with high heritability, shaped by natural selection, how can we hope to understand more complex traits, influenced less by genes, and more by the environment, which have not changed so clearly over time?

When we have a better understanding of how genes and characteristic are related, we will be able to study how they are distributed through populations and over time. For example, we may be able to link specific characteristics, such as height or skin color, to specific genes, define how these genes vary between people from different parts of the world, and trace the evolution of these genes. Our understanding of human variation will consequently improve.

I should say that this kind of research could easily be misinterpreted. For the sake of argument, imagine that we identified a gene that determined a fundamental characteristic. Imagine, furthermore, that we determined the distribution of this genes in various groups of people. The potential to support racist beliefs or policies would be enormous. I hope it’s clear why this scenario will most likely never occur. To begin with, fundamental characteristics are almost certainly too complex to be precisely determined by one or even many genes. Furthermore, we have no reason to believe that fundamental characteristics vary around the world in any systematic way, for all of the reasons described elsewhere on this website.

Therefore, while our understanding of our history and origin, and our knowledge of genetics, will certainly improve in the future, there is no reason to suspect that the scientific basis for racial equality will be overturned.

I would now like to turn my attention to an extremely important question. In many ways, it constitutes the second chapter of any discussion of race and genetics, and I usually present it to my students only after we have discussed the fundamentals in some detail.

If we accept that there are no fundamental differences between people around the world, how can we explain the last five hundred years of history? Why have some people succeeded while others have not? Why is there such an enormous disparity in the global distribution of power and wealth? Why didn’t Inca conquistadors, wearing helmets made of gold, sail across the Pacific, and lay siege to the Catholic citadels of Portugal and Spain? Why didn’t Zulu warriors, riding zebras or elephants, conquer Europe, and establish a white slave trade? Why didn’t Chinese or Indian vessels sail into British ports, and develop colonial trading outposts?

The answers to these questions have traditionally been racist answers, namely, that white people have succeeded because they are superior, genetically, biologically, culturally, and so on. We now have more informed answers, based on geography.

A full treatment of this subject can be found in Jared Diamond’s remarkable book, Guns, Germs, and Steel: The Fates of Human Societies. Diamond’s book is triumph of reasoning and research, backed up extensive evidence. I will not do justice to his work in this short space, but I will try to sketch the rough outlines.

The figure below shows several factors that contributed to the success of particular societies around the world, from approximately the time of the agricultural revolution, to the present. Although the figure is not strictly a timeline, you can think of top as the past, and the bottom as the present. More distant factors (ultimate factors) are shown above, and more recent factors (proximate factors) are shown below, with arrows defining the causal relationships between them.

In summary, the east-west orientation of the long axis of Eurasia created a vast region with a stable and hospitable climate, and numerous plant and animal species available for domestication. This contributed to the agricultural revolution, food surpluses, and the ability to sustain large dense societies. Once these societies developed, opportunities for trade, communication, and the rapid spread of technology followed. Diseases were also able to move freely, and populations resistant to those diseases emerged and thrived. European societies were therefore able to develop technology that allowed them to conquer other societies, as well as the economic, political and social structures that impelled them to do so, and the diseases they carried with them, toward which people from other continents had no defense. As the title of the book suggests, guns, germs and steel largely determined the fates of societies, and these factors themselves were largely determined by environmental factors.

Figure 1. Factors underlying the broadest pattern of history. Diamond, Jared, 1997. Guns, Germs, and Steel, The Fates of Human Societies. W. W. Norton and Company, Inc., New York, NY.

That’s the basic theory of Guns, Germs, and Steel. It seems hard to believe that such a simple idea could explain human history, but Diamond marshals an extraordinary number of examples to support his arguments. For example, consider the number of plant species available for domestication around the world. Diamond examines large-seeded grass species, such as wheat and rice, the foundation of agrarian societies. He shows that, at the time of the agricultural revolution, there were 39 large-seeded grass species in Eurasia, 4 in Sub-Saharan Africa, 11 in the Americas, and only 2 in Australia.

Likewise, Diamond examines the large mammals, such as horses and cattle, which were used by early societies for transportation, traction, war, trade, and so on. As above, Eurasia had a great many more available large mammal species compared to other continents. 13 mammals were domesticated in Eurasia, only one in the Americas, and none in Sub-Saharan Africa and Australia. This information is summarized in the tables below. While the global distribution of plants and animals is very different today, because of large-scale migration, it’s clear that early societies in Eurasia enjoyed a distinct advantage relative to their counterparts on other continents.

Large-seeded grass species available for domestication

Geographical area	Number of large-seeded grass species
Eurasia	39
Sub-Saharan Africa	4
Americas	11
Australia	2

Large mammals available for domestication

Geographical area	Possible mammals	Domesticated mammals
Eurasia	72	13
Sub-Saharan Africa	51	0
The Americas	24	1
Australia	1	0

Table 1. Large-seeded grass species, and large mammals, available for domestication at the time of the agricultural revolution. Diamond, Jared, 1997. Guns, Germs, and Steel, The Fates of Human Societies. W. W. Norton and Company, Inc., New York, NY.

Guns, Germs, and Steel has garnered a great deal of praise, but it has also attracted its share of criticism. I think it’s worthwhile to examine the most common criticism of Guns, Germs, and Steel, if only to better understand Diamond’s work.

The most common criticism of Guns, Germs, and Steel involves the truth or one or another specific point. In a book so broad and deep, this is hardly surprising, and perhaps inevitable. Researchers in different fields claim that Diamond ignored the fact that native people in the Americas might have begun to domesticate plants and herbs, that he discounted the potential nutritive value of root crops such as yams and taro, that he misrepresented the relative ease with which plant and animal species spread throughout Eurasia, that he downplayed the possibility that species migrated along north-south corridors, that he overstated the importance of domesticated animals, and so on.

These points challenge Diamond’s ultimate factors, and therefore weaken his argument. Some may have merit, however, even taken together, I do not think that they constitute a substantive refutation of his theory. On the contrary, for the most part they seem trivial. I believe that much of this criticism is simply intellectual jealousy. Academics often object when others trespass in their areas of expertise, or do not pay sufficient homage to the literature. In any case, critics have offered no comprehensive alternative theories, thus Diamond’s work remains relevant and powerful.

A second criticism of Guns, Germs, and Steel is that it is overly Eurocentric. This accusation takes various forms, however, in general, people claim that Diamond focuses exclusively on the success of Western Europe, and disregards the history, experience, and accomplishments of other societies. It’s difficult for me to appreciate this criticism, because the dominance of the Western Europe is precisely the outstanding fact that Diamond is trying to explain. It makes perfect sense that he spends time elaborating this process, although, I would say, he focuses equal or greater attention a vast number of different societies around the world.

A third criticism of Guns, Germs, and Steel is that it is overly deterministic. According to this view, Diamond claims that the environment determined human history in absolute terms, and that events that shaped civilization could not possibly have occurred differently. Above, I used the word determined in much this way: guns, germs and steel determined the fates of societies, these factors were themselves determined by geography. This use of determined is a conventional way to express causality, but Diamond would be the first to argue, and I would be the first to agree, that the environment only determined preliminary conditions, and that exactly how societies developed involved a considerable number of other factors, which may or may not have had anything to do with geography. In this sense, the environment determined the potential for development, and not precise course of human history.

The idea of determinism lies at the heart of two related criticisms of Guns, Germs, and Steel.  First, some people believe that Diamond justifies or supports the behavior of Western Europeans. For the sake of argument, assume that environmental factors did precisely determine human history. If this were true, it would absolve humans of responsibility for their actions. The environment made Western Europeans set forth from their homelands and colonize distant continents. The environment made North Americans enslave Africans. Colonialists and slave masters had no say in the matter, and only fulfilled their destiny, determined by the stable climate, free flowing rivers, fertile soils, and rich flora and fauna of Eurasia. Or, alternatively, the native people of the Americas, Africa, and Australia, were powerless to defend themselves, and had no other choice but to succumb to their oppressors.

This frees Western Europeans and North Americans from any feelings of guilt, and allows them to avoid acknowledging their past. It denies the agency of people colonized and enslaved, and precludes their potential to defend themselves, or pursue alternative outcomes. Thus, it both absolves the perpetrators and enfeebles the victims. We could call this the “liberal argument against determinism.”

Others interpret Guns, Germs, and Steel in a nearly opposite manner. They believe that Diamond fails to hold less successful societies responsible for their own failure. Again, for the sake of argument, assume that environmental factors did precisely determine human history. If so, the choices, decisions, mistakes, and deficiencies (including, perhaps, biological deficiencies) of individuals and societies, are irrelevant. In simple terms, the fact that some societies have failed to thrive is not their fault.

This is difficult to accept for people who believe strongly in personal responsibility, and do not readily admit that external factors often play an important part in success and failure. They rather believe that some societies have succeeded because they are superior, or because people from these regions made good decisions, worked hard, or seized the opportunities that were presented to them, while other societies have failed because they are inferior, or because  people from these regions made poor decisions, did not work hard, failed to capitalize on the opportunities presented to them, and so on. We could call this the “conservative argument against determinism.”

As I explained, I do not believe that Guns, Germs, and Steel is deterministic in an absolute sense, and so I do not believe that either of these narratives is true. In the first case, Diamond does not absolve any group from responsibility for their actions, nor deny agency to others. Instead, he confronts and chronicles the injustice of human history as fairly and objectively as possible.

In the second case, the whole point of Guns, Germs, and Steel is to offer an environmental explanation of human history not based on the superiority of a particular group of people and the inferiority of others. That’s the reason that I have taken the time to explain Diamond’s work in my discussion of race and genetics—because he shares the view that there are no fundamental differences between humans around the world.

A final response to Guns, Germs, and Steel is a question, one that my students frequently ask, and that many readers doubtless ask as well. If we assume that the broad middle swath of Eurasia, lying along the same east-west long axis, shared roughly the same stable and hospitable climate, and great number of plant and animal species available for domestication, why did Western Europe ultimately rise to prominence, and not the Middle East or Asia? After all, the first civilizations emerged in the Middle East, and China lead the ancient world in technology, and economic and political development, for centuries. We may reasonably wonder, then, why these societies did not extend their dominance around the world, instead of Western Europe.

Diamond addresses this question with considerable clarity in the epilogue to Guns, Germs, and Steel. Here, again, I can only briefly summarize his arguments.

In the case of the Middle East, the answer is that the land is ecologically sensitive; it has low rainfall relative to its primary productivity. Therefore, the region was particularly vulnerable to deforestation, farming, irrigation, and grazing. Precisely because it was inhabited and exploited for so long—longer and more intensively than any other area on earth—it was gradually transformed from the productive forest and grassland that inspired the name Fertile Crescent, into the arid, barren, desert and steppe, that we associate with the Middle East today. Accordingly, the civilizations that developed there were unable to flourish.

In the case of China, the explanation is also related to the environment. Because of its coastline, its islands, its mountains, and its rivers, China remained geographically, and thus economically and politically, unified for over two thousand years. This unity had important consequences for the development of Chinese society. Throughout history, and despite the numerous technologies that developed there, China repeatedly abandoned innovation, and turned its back on the outside world, behavior that would have been impossible in Europe.

The most obvious example, relevant to the themes discussed here, is the fact that, at one time, China supported vast treasure fleets of ocean-going vessels, with thousands of crewmen, who sailed across the Indian ocean, as far as the east coast of Africa. These fleets, presumably, could have reached Europe. However, in the early Fifteenth century, following a power struggle in the Chinese court, the fleets were dismantled, and ocean going voyages were forbidden. Thus, China never expanded its global reach, and remained isolated.

These examples raise another important question; to what extent do culture factors, or individual people, shape human history? This question is related to a great debate, or divide, in history, between those who believe that history can be largely explained by environmental factors, and those who believe that history is better understood as a series of cultural changes, often driven by individual people.

The preceding discussion should make Diamond’s position clear—he believes that history can be largely explained by environmental factors, however, he does acknowledge that local, temporary, conditions, the actions of individuals, and pure, random chance, have potentially large effects. He calls these “wild cards,” and describes how they can and occasionally did change the course of history. He also points out that the key to quantifying the relative effects of these processes is to investigate historical events that can not be explained by the environment. The implication, of course, is that much can be explained by the environment, as his book demonstrates.

I hope this discussion helps clarify the importance of Guns, Germs, and Steel. I think it’s worth mentioning, again, that I have never encountered a satisfactory refutation of his basic theory, nor any reasonable alternative theories. Diamond brings clarity to topics that previously remained obscure, and his work is substantiated by a large body of evidence.  Finally, it both supports, and is supported by, our understanding of racial equality.

There is one more point I would like to make before moving on. Confronted by Diamond’s work, many people are tempted to propose a selection mechanism that drove the evolution of fundamental characteristics, such as intelligence, in particular groups of people, especially Western Europeans. The argument is the following: as large societies emerged, people who had particular traits, such as greater intelligence, that allowed them navigate and manipulate the various challenges and opportunities of civilization, were better able to survive, and thus pass on their greater intelligence genes to their offspring, relative to other people who were less intelligent. In this way, large societies provided the selective force that drove an increase in intelligence in particular parts of the world.

At first, this idea seems plausible. Something like this almost certainly occurred during the course of human evolution. Ardipithecus ramidus, one of the first upright walking hominins, who lived about 4.4 million years ago, had an average brain volume of about 350 cubic centimeters, equivalent to that of our closest ancestors, chimpanzees. Over the next 4 million years, hominin brains increased in size dramatically. Homo sapiens, or anatomically modern humans, who appeared about 200,000 years ago, had an average brain size of about 1,350 cubic centimeters, four times the size of chimpanzee brains.

The precise mechanism that drove the rapid expansion of our brains is unknown, although it probably involved cultural or behavioral traits, such as tool use and language. To take the latter example, as language developed, individuals with larger brains, that allowed them to employ language, were better able to survive, and thus pass on their larger brain genes to their offspring, relative to other individuals with smaller brains. Language, therefore, provided the selective force for larger brains, which lead to more sophisticated language, which lead to larger brains, and so on, a positive feedback loop, or co-evolution, that drove the rapid increase in brain size, with corresponding changes in genetic composition and fundamental characteristics.

The problem with applying this logic to large societies is that the time span involved is simply too short. If we assume that the agricultural revolution occurred about 10,000 ya, that the first civilizations emerged about 5,000 ya, and that networks of civilizations of sufficient density and complexity to produce the kinds of changes we are trying to explain emerged in the following one or two thousand years, we are left with a incredibly short period of time, a few thousand years, say, from 3,000 ya to the present, for substantive changes to develop. This is not enough time for natural selection to alter fundamental characteristics. Recall that the expansion of our brains, widely described as an “explosion” in brain volume, occurred over a period of at least 4 million years.

We must therefore return to our previous conclusion; there is no plausible selection mechanism that would have created differences in fundamental characteristics between people around the world including, we can now add, the development of civilizations.

Epigenetics is an emerging field that promises to change our understanding of biology. It is widely recognized as one of the most important discoveries since the structure of DNA, and the basic process of information storage and protein synthesis, were articulated in the latter half of the twentieth century. Below, I will do my best to explain the basic science of epigenetics, and how it may relate to race and genetics.

The name epigenetics comes from the Greek epi or above, because is describes processes that operate above or in addition to the genetic code and conventional inheritance. The first modern studies of epigenetics appeared in the 1990’s. To understand these studies, we have to explore the way that DNA is organized, and how its activity is regulated.

A common misconception is that DNA consists of a single continuous strand. In fact, DNA is broken into discrete sections of different lengths, called chromosomes. Humans have 46 chromosomes; we inherit 23 from our fathers, and 23 from our mothers. Sunflowers have 34 chromosomes, chimpanzees have 48, some insects have only 2, and one type of fern has at least 1400. The number is not important; it bears no relation to the complexity of the organism, and is an accidental relic of evolutionary history.

Chromosomes, in turn, consist of long sections of DNA that appear to have little or no function, interspersed with sequences of bases that contain the information to make specific proteins. These sequences of bases are called genes. A common rule is that one gene contains the information to make one protein. In fact, the process is more complex; one gene contains the information to make several different proteins. The important point is that chromosomes are broken into genes, which you can think of as the functional units of DNA.

Gene expression, or how genes are regulated, is one of the most important areas of genetics. Consider the fact that all of the cells in your body contain the same DNA. How does a cylindrical light sensitive cell in the retina of your eye become an eye cell, or a globular gastric acid producing cell in the lining of your stomach become a stomach cell, or a planar keratin producing cell in the outermost layer of your skin become a skin cell? In each type of cell a different set of genes is activated, or expressed: in eye cells the genes to make light sensitive molecules are expressed, in stomach cells the genes to make gastric acid are expressed, in skin cells the genes to make keratin are expressed, and so on. What makes cells different is not the DNA that they contain, but the particular genes that are expressed, and the proteins that are produced.

Cells control not only which genes are active and which are silent, but also the rate and timing of gene expression, and the interactions between different sets of genes. Many gene products have important regulatory roles. There are also DNA sequences that do not necessarily produce proteins, but which act as on or off switches, damping or stimulating the activity of other genes, as well as numerous additional molecules that contribute to these processes.

One method of gene regulation is related to DNA storage. If all the DNA in a single cell were placed end to end, its combined length would be nearly two meters. The average diameter of human cell nuclei, on the other hand, is only 6 micrometers. How does DNA fit into cell nuclei? DNA is stored in a way that maximizes the amount of space it can occupy. First DNA is wrapped around a class of proteins, called histones, in much the same way that you wrap string around a spool. Then DNA is folded into loops and coils to form dense structures. These structures determine which genes are exposed to or concealed from protein synthesis machinery.

Epigenetics is based on the following principle. A set of very simple molecules, called epigenetic markers, bind directly to DNA, or to histone proteins, and change the way that DNA is stored. One way to think about epigenetic markers is to imagine that they determine how tightly or loosely DNA is coiled: if DNA is tightly coiled, particular genes will be hidden on the inside of the larger structure, and if DNA is loosely coiled, the same genes will be visible on the outside, and can thus be turned into proteins.

The most common epigenetic markers are methyl groups (a carbon atom bonded to three hydrogen atoms), acetyl groups (two carbon atoms bonded to an oxygen atom and three hydrogen atoms) or phosphorous atoms. More markers almost certainly exist. The figure below shows two main components of the epigenetic code.

Figure 15. The two main components of the epigenetic code. Qiu J. 2006.

This is all well and good, but hardly revolutionary. So far, epigenetics is just another regulatory mechanism. The remarkable thing about epigenetics is that particular patterns of epigenetic markers, and particular patterns of gene expression, can be passed down from generation to generation. Furthermore, epigenetic markers can be modified by the environment. Therefore, epigenetics is a system of inheritance that acts in parallel to genes, as well as a powerful link between genes and the environment.

Two examples illustrate the importance of epigenetics. The first is a simple laboratory experiment involving mice. The second is a harrowing chapter in human history.

Two American biologists, Robert Waterland and Randy Jirtle, bred mice with a modified version of a widespread gene, called the Agouti gene. The mice, called Agouti mice, had yellow coats, and a constellation of health problems, including obesity, diabetes, and heart disease. The offspring of Agouti mice had similar characteristics, as expected, because they inherited the modified Agouti gene from their parents.

Waterland and Jirtle then fed female Agouti mice a diet high in nutrients known to promote health. The condition of the mice improved markedly. More important, their offspring were thin and brown, and did not suffer from health problems. They showed that diet changed the pattern of methylation on DNA, and that this methylation pattern effected health, and was passed down to offspring.

A similar natural experiment occurred in the Netherlands during World War Two. In the winter of 1944-45, German forces occupying the Netherlands established a blockade that prevented food and fuel from reaching densely populated western regions. This blockade created a famine, known as the Hongerwinter, or Hunger Winter, that killed an estimated 22,000 people. The famine was alleviated by several missions to transport food to the area, conducted by Sweden, the United Kingdom, Canada and the United States, with the tacit approval of Germany. The blockade was eventually dismantled when the Allies liberated the area in May 1945.

In the decades that followed, the Dutch Famine Birth Cohort Study, a collaboration between researchers in the Netherlands and Britain, found that the children of women who were pregnant during the famine were more susceptible to diabetes, obesity, cardiovascular disease, and other health problems, including schizophrenia and neurological defects. In addition, their children were smaller than average at birth, and their children were also smaller. In other words, the effects of the famine persisted for at least two generations. The most likely explanation is that nutrient deprivation during pregnancy changed the methylation pattern of DNA, and this methylation pattern was passed on to subsequent generations, with corresponding health effects.

What does this have to do with race and genetics? Epigenetics introduces the possibility that the environment could produce differences in fundamental characteristics, not by changing the genetic code, but by changing the epigenetic code, not in one generation, but over several generations.

Consider two groups of people, one that has flourished over the last several hundred years, and enjoys considerable wealth and prosperity, and another that has stagnated, and suffers entrenched poverty and deprivation. Epigenetics could explain these differences. For example, people in one group could carry epigenetic markers that allow them to succeed, while the other group could carry epigenetic markers that prevent them from reaching their potential.

This represents a radical shift in our understanding of race and genetics, because it contradicts our conclusion that there are no mechanisms that could have produced differences in fundamental characteristics. That said, I do not think that epigenetics threatens our understanding of racial equality.

To begin with, a great deal of research is needed before we use epigenetics to explain the course of human history. Also, epigenetic effects, if any exist, are likely very small compared to the effects of individual variation and the environment. Finally, and most important, the epigenetic code, unlike the genetic code, can change rapidly. If the environment changes, the epigenetic code changes. Therefore, epigenetics does not establish the foundation for a new type of racism. Rather, it offers hope for the future.

Sports performance may seem like a strange topic to include in this discussion. After all, as I explained in the main text, when we talk about race and genetics, we are usually concerned with fundamental characteristics, such as intelligence or aggression, and not how far a particular person can throw a ball, or how high they can jump. Nonetheless, sports are such a popular and visible part of society, that they function as a kind of open field, where erroneous beliefs and racist myths are projected and disseminated. There are also some aspects of sports performance that cry out for explanation, and can potentially shed light on a larger discussion of race and genetics.

Several years ago, one of my students asked me whether or not it was true that black athletes “add muscle mass” faster than white athletes. This student was intelligent, sincere, and not racist. He was simply trying to explain the observation that black athletes appear to be more successful than white athletes in some sports. He assumed, falsely though reasonably, that black athletes are different, that their bodies or brains make them particularly suited to competition.

I answered that, while one individual may well develop muscle mass faster than another in response to training, there is no evidence that black athletes as a group add muscle mass faster than white athletes. Nor is there any truth to other common theories that explain the success of black athletes, such as notion that black athletes have higher levels of testosterone, making them more competitive and aggressive, or that African Americans, largely descendant from slaves, represent the “fittest of the fit,” the strongest individuals who survived the brutality of the slave trade in the past, and therefore dominate sports today.

Why, then, are black athletes disproportionately successful in some sports? I will try to answer this question using the same approach I have followed throughout this website, namely, by arguing that there is no basis to distinguish human races, but that our history has produced unique variation that is evident in several areas, including sports performance.

To begin with, we must determine that black athletes are more successful than white athletes. In North America, black athletes dominate two of the most popular professional sports, basketball and football. Baseball is more diverse, and white and Latino athletes enjoy considerable success. If we consider other sports that require similar skills, such as rugby, lacrosse, or volleyball, we find a surprising lack of black athletes. And that only involves field sports with teams and a ball. If we consider sports such as swimming, gymnastics, tennis, golf, rowing or sailing, a small number of notable exceptions notwithstanding, black athletes are hardly represented at all.

If we expand our view to include international sports, the patterns change. European basketball is played mostly by white athletes. Soccer, the most popular professional sport in the world, is quite diverse, and it would not be fair to say that black athletes dominate, although they may appear to in countries that are predominantly white. Cycling is almost entirely white. For over one-hundred years, the winners of the Tour de France, the flagship event of professional road cycling, second only to the Olympics and World Cup soccer in numbers of viewers, have been predominantly white. There have been a handful of competitive Latino cyclists, fewer Asian cyclists, and still fewer black cyclists. Recently, an African cyclist won the Tour de France for the first time—but he is white, and grew up in the United Kingdom. Sports popular in India and Pakistan, such as cricket, and sports popular in Asia, such as table tennis and badminton, are dominated, not surprisingly, by people from these regions.

If we consider winter sports, the patterns change again. Black athletes are almost completely absent from sports such as ice hockey, and various ski disciplines. Finally, if we consider female sports, the patterns are similar, with some variation. For example, the most successful male soccer teams are from Brazil, while the most successful female soccer teams are from North America.

This survey demonstrates that black athletes are not necessarily more successful than white athletes, or any other athletes, in all sports all of the time. It should be clear that sports success is determined largely by culture. Some important factors include: history, tradition, income, poverty, social class, economic incentives, marketing, climate, geography, role models, infrastructure, including physical resources and human resources, and, in some cases, political forces. These factors explain why black athletes participate in sports such as football and basketball, while white athletes participate in sports like golf and sailing (these sports are associated with income and social class). They also explain why black athletes are absent from winter sports (countries with large black populations do not have cold winters), why black athletes have not won the Tour de France (countries with large black populations do not have the required infrastructure), and why black athletes do not dominate sports such as cricket or table tennis (these sports are not popular in countries with large black populations). Very few people would disagree with these assertions.

At the same time, I don’t think it’s enough to say, “Success in sports is determined only by culture.” This is not a satisfactory explanation, and leads to more doubt than certainty. Can culture really explain sports success? Are black athletes really the same as white athletes?

To answer these questions, we need to find a sport that 1) is popular around the world, 2) is practiced by many people regardless of history, nationality, income, and social class, and 3) requires very little infrastructure. Fortunately, a strong candidate exists: running. Of course running is influenced by culture, but much less so than other sports. Therefore, it provides an ideal context to compare the performance of different athletes.

In the past, famous runners came from all over the world, including the Americas, Africa, Europe, the Middle East, India and Asia. There were legendary Andean distance runners, British milers, Moroccan middle distance record holders, Indian sprinters, and Japanese marathoners. However, since the late 1980s and early 1990s, black athletes have dominated running events at major international competitions, such as the World Championships and the Olympics.

Sprinting events, like the one hundred meter dash, are usually won by large powerful athletes from West African nations, such as Cameroon or the Ivory Coast, or from former colonies populated by slaves from these nations, such as Jamaica and the United States. Long distance events, like the marathon, are usually won by small light athletes from East African countries, such as Kenya and Ethiopia. Middle distance events are won by athletes from throughout Africa, especially North Africa. This is true of both male and female competitions. Occasionally, white athletes reach the finals, but the winners are almost always black.

The success of black runners demands explanation. Culture must play some role, even a large role. Cultural factors often work in circular fashion, that is, as runners gain recognition and wealth, their countries gradually develop support systems and infrastructure, and more successful athletes emerge. However, these factors cannot fully explain the overwhelming success of black runners. A satisfactory explanation must go further.

The answer may be related to processes I discussed above, namely, genetic bottlenecks, and the founder effect. Recall that the humans who left Africa about 65,000 years ago were a sub-population of a larger ancestral group. This migration, and all subsequent migrations, represent a series of founder events. Humans who left Africa carried with them only a fraction of the genetic diversity that remained behind. Alternatively, humans who remained in Africa had more genetic diversity that those who left.

The American biologist Sarah Tishkoff and her colleagues examined over one thousand genetic markers from 121 African populations, four African American populations, and 60 non-African populations. The figure below shows the results of their analysis.

The axes (gray lines) represents different statistical variables. The data (colored spheres) are distributed along the axes, and provide a three-dimensional representation of genetic distance. If the spheres are farther apart, there is greater genetic distance between the populations, and if the spheres are closer together, there is less genetic distance between the populations.

In the first picture (A) you can see that the genetic distance between populations in Africa, such as the Hazda and Saharan Africans, is greater than the genetic distance between populations in the rest of the world, such as Eurasians and Oceanians. In the second picture (B) you can see the genetic diversity in Africa. For example, Hazda and Pygmy populations, and some members Saharan African and Southern African populations, are separated by substantial genetic distance.

This study is noteworthy because, historically, Africans were treated as a relatively homogeneous group. We can now see that there is far more genetic diversity in Africa than anywhere else in the world.

Figure 13. Human genetic variation in Africa. Tischkoff et al. 2009.

Because Africa has greater genetic diversity than anywhere else in the world, we expect to find the fastest sprinters and the fastest marathon runners there. For the same reason, we also expect to find the slowest sprinters and the slowest marathon runners. Imagine the results if a West Africa runner competed in the marathon, or an East African runner competed in the one hundred meter dash. Following the same line of reasoning, we expect to find the extremes of most characteristics in Africa. We can return once again to height. Do we find the tallest and shortest people in Africa? Because height has an environmental component, it’s difficult to compare countries directly, but there are definitely very tall and very short populations in Africa, for example, the Massai, an extremely tall and slender people from Kenya, and the Mbuti, one of several indigenous groups of pygmies from Central Africa.

This is one explanation for the disproportionate success of black athletes in some sports. It acknowledges that success is determined in large part by culture, but asserts that there must be a genetic component as well. It offers a plausible mechanism to explain genetic variation, and it is supported by evidence. Furthermore, it does not support any commonly held racist beliefs. At the very least, this is the kind of explanation that we need to explore to understand human variation.