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.