Paleoanthropologists used to pray that they would unearth big troves of intact Neandertal skeletons and well-preserved artifacts that they could comb for clues to the origins of the human race. But these days, they can often get as much or more information straight from the DNA in bone fragments.
Case in point: the newly published genome study in Science from Matthias Meyer and Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology and their international team of colleagues. Using a novel DNA sequencing technique that works particularly well with degraded specimens, they examined the genome of a seven-year-old girl who died more than 74,000 years ago, using a surviving sliver from one of her finger bones. That girl's bone fragment was one of the few pieces of evidence that in 2010 revealed the existence of the ancient Denisovan people -- contemporaries of the Neandertals who overlapped with them in eastern Asia.
Yet from that extraordinarily humble source, the Max Planck scientists have drawn a wealth of insights. They learned, for instance, that the Denisovans were probably dark-skinned, unlike the pale Neandertals. Because the girl had two X chromosomes, one from each parent, the scientists were able to infer that the Denisovan population had relatively little genetic diversity. Living natives of Papua New Guinea, Australia, and some southeast Asian islands derived about 6 percent of their genes from the Denisovans, yet the Denisovans seem to have contributed nothing of lasting value to the DNA of people in other parts of the world. Comparison with the Denisovan DNA also allowed the researchers to recognize that Europeans carry somewhat fewer genes from Neandertals than do East Asians and Native Americans.
Such discoveries are endlessly fascinating to some of us. But I can also understand that many people might reasonably question why any of these details matter. After all, Neandertals and our other ancient ancestors have been extinct for 30,000 years or longer. Why should we care so much about their DNA? Is there any practical value to be had from these studies?
I'll argue that there is, and that it might be especially useful in helping us to develop more enlightened attitudes about racial differences and autism. To explain why, it may be useful to start by reviewing some of the major current ideas about how humans evolved in the first place.
Overview of our origins
Fifteen or 20 years ago, it might have been easier to find a rough consensus among paleoanthropologists about this topic than it is today precisely because of the recent bounty of fossil and DNA discoveries. All that information has answered some important questions and filled in a level of detail that might once have seemed inconceivable, but curiously enough, some of the broad strokes in the big picture have become less clear.
Roughly speaking, in Africa 1.7-2 million years ago, the earliest primitive members of the genus Homo appeared. They were small, hairy people who might look a bit apelike by our standards of beauty, but they had bigger brains and more tools than the upright Australopithecus species before them. The Homo erectus people were successful enough to spread out of Africa and migrate across Asia, and are responsible for some of the ancient fossils given names such as "Peking man." Nevertheless, they were probably something of a false start for the spread of humanity as we now it.
The more relevant development came between 400,000 and 800,000 years ago, with a new wave of African emigration into the Middle East and Asia by a group of people with even bigger brains and better tools. They gave rise to the brawny, brow-ridged Neandertal people, Europe's first inhabitants. Yet they also spawned at least one other Asian group, the Denisovans. (It wouldn't be too surprising anymore if still more sibling groups contemporary to the Neandertals and Denisovans turned up elsewhere in Asia.) Meanwhile, humans also continued to prosper and evolve in Africa, and by 80,000 years ago, ones with a fully modern appearance had appeared and started their own exodus into the rest of the Old World.
What happened next is the stuff of archaeologists' heated arguments. The oldest theory is the multiregional hypothesis strongly advocated by Milford Wolpoff of the University of Michigan in Ann Arbor. It claims that as different in appearance as moderns, Neandertals, Denisovans, and even the early Homo erectus might seem, they were all still members of the same human species. Over time, the modern traits predominated but some of the traits in local populations that had adaptive value (such as shorter, thicker bodies in cold climates) were retained and might bear some connection to physical differences seen in populations around the world today.
In the 1980s, however, a starkly opposing theory emerged largely, though not exclusively, from studies of mitochondrial DNA in living populations. (Mitochondria, the organelles in animal cells that create chemical energy, carry their own unique sets of genes, completely separate from the DNA in the nucleus for the rest of the cell's genes.) Those analyses suggested that the maternal bloodlines of everyone alive today converged back on Africa less than 100,000 years ago, with no trace of a genetic contribution from local groups elsewhere. That conclusion spawned the "out of Africa" model, according to which scientists such as Chris Stringer of the Natural History Museum in London argued that when the anatomically modern humans colonized Asia and Europe, they displaced the Neandertals and other ancient residents without breeding with them. Whether the moderns had directly exterminated the ancients or simply outcompeted them for resources was anybody's guess, but interbreeding was effectively nonexistent.
The out-of-Africa model and its mitochondrial DNA evidence proved highly persuasive to many anthropologists. Disagreements remained fierce, but during the 1990s it was often presented as the default explanation for human origins, even though almost everyone acknowledged how counterintuitive it seemed that modern humans would so completely refrain from mixing with creatures that looked so much like them. Mostly, scientists chalked it up to some obscure biological or behavioral speciation barrier.
DNA twists the plot
Ironically, one type of DNA evidence helped put the out-of-Africa model on top but later DNA evidence helped knock it back down. If brief, when Svante Pääbo and other researchers began the painstaking work of recovering nuclear DNA from Neandertal bones and sequencing it, they discovered that on average about 4 percent of living people's genes are derived from Neandertals. (The telling exception was in people of modern African descent, whose genes were generally less than 1 percent Neandertal, which is what one might expect if the mixing would have occurred primarily outside Africa.)
Four percent might not sound like much, but it is substantially more than an out-of-Africa scenario with strict replacement rather than interbreeding would seem to allow. It's remotely possible that this mixture is an artifact of old, unequal mixing of what became Neandertal genes within the ancestral African population (although anthropologist John Hawks has explained on his blog why that situation seems unlikely). The more likely explanation, though, is that some level of interbreeding did occur. For that reason, Stringer and other defenders of the concept now refer to a modified "mostly out of Africa" model that acknowledges some interbreeding but considers it largely trivial in extent and consequences.
That same evidence has, of course, only reinvigorated the multiregional hypothesis (though one might wonder why the percentage of ancient humans' genes in us isn't then higher). It has also nourished a popular new "assimilationist" school of thought that pragmatically splits the difference between multiregionalism and out-of-Africanism. The assimilationist model says that when the anatomically modern humans left Africa 80,000 years ago, they retained their own identity but also mixed to a degree with the older human populations they encountered. Both the modern and ancient groups became locally varying patchworks of physical traits and technologies. In the end, the ancients' societies were too disrupted to survive but some of their genes persist in us.
The question of when and how humans emerged over the past few hundred thousand years is therefore considerably more complicated and less settled than it might have seemed a couple of decades ago. The same can be said for the closely related question about whether Neandertals, for example, represent their own species (Homo neanderthalensis) or just a subspecies (Homo sapiens neanderthalensis) alongside our own (Homo sapiens sapiens) -- or whether, as Wolpoff would have it, virtually all of Homo has been one big species that has varied overtime.
Why we should care
Even if the science of human origins is still a work in progress, the accumulating information about how we got here and indeed what constitutes a member of the human race offers some useful perspectives on matters of scientific and ethical importance.
Perspective on the age of humanity. One small point that studies of the DNA of Neandertals and other ancient people illuminate is just how old or young we humans are as a species. The paleontological record indicates that the mean survival time for a mammalian species is about a million years, though some have lasted ten times that long. If we emerged only within the past 100,000 years or less, then Homo sapiens is indeed an amazingly young and precocious lot. And a loose, handwaving argument might therefore be made that we also probably have a commensurately long future ahead of us.
On the other hand, if Wolpoff is right and we are part of a species that has been around for two million years, then we are much more senior. It might make us look at the extinction rates with a little more sense of urgency.
Perspective on our nonprogressive evolution. The molecular study of our evolution also helps to drive home how unexceptional our biological history has been. Many icons of human evolution unintentionally reinforce a misleading sense of progress -- witness the classic March of Progress illustration by Rudolph Zallinger that shows a modern human leading a Neanderthal and other "less evolved" ancestors.
But that sense really changes if we and Neandertals are seen as sibling groups, diverging but also sometimes re-merging throughout history. Our evolutionary history looks much less progressive and more like that of other species.
Perspective on race. For centuries (at least), arguments over race have invoked inappropriate biological concepts to make or defend distinctions among peoples -- and distinctions in how they should be treated. They have likened races to subspecies to justify their inherent biological reality, along with some allegedly biological superiority, inferiority, or "otherness."
A simple refutation of that idea has been the proof that the diversity of genetic characteristics within racial groups is greater than the diversity separating them: human races are not well enough defined and different enough to be meaningful biological groups. For that reason, many scientists now argue that race is not a biological concept but rather a social concept that sometimes carries biomedical consequences.
(Here's what that means, if it isn't immediately clear: In a society that mistreats the dark-skinned in general, for instance, black people may be at higher risk for diseases of poverty without having an intrinsic susceptibility to them. But an example that is perhaps less obvious is that of sickle-cell anemia, which is more common in those of black African descent than in those of white European descent. That's because many people whose ancestors lived in regions where malaria was prevalent carry mutations for sickle-cell anemia that offer some protection from the parasite. But not all of those people are racially black and not all blacks carry the mutation. Sickle-cell information campaigns target predominantly black populations because society doesn't accurately group people in terms of "ones whose ancestors had a lot of malaria." In this case, race is a flawed but useful proxy for that nonexistent classification -- but not because of the biological characteristics of the race as such.)
The foregoing is all true only in terms of race as we understand the concept today, however. If the multiregionalists and the assimilationists are right, then the Neandertals, Denisovans, and other ancient people we displaced may not have been separate species of person at all. They may instead have been races so different from modern humanity that they really were akin to other subspecies. Differences in their anatomical, genetic, behavioral, and intellectual traits would surely dwarf any seen in the world today among Homo sapiens. Color me naïve, but I would like to think that these insights might help to strengthen the spirit of color-blind brotherhood we ought to feel for one another.
(And in anticipation of a query I can feel coming: no, hypothetically, I would not be in favor of summarily treating Neandertals as second-class citizens if ever we could use technology to clone one. Neandertals were people and therefore, in my opinion, would deserve to be fully enfranchised. However, the question shows how ethically fraught such high-tech resurrections could be.)
Perspective on neurodiversity. In the course of their recent analysis of the Denisovan DNA, Meyer and Pääbo identified 23 highly conserved areas of the human genome that seem to be unique to our kind. Eight of those contain genes that previous studies have tied to nerve growth and other aspects of brain function. And three of the conserved genes -- ADSL, CBTNAP2, and CNTNAP2 -- have been implicated in some forms of autism.
Those correlations are not entirely surprising. Looking at the artwork and artifacts left by Neandertals, some archaeologists have argued that they seemed to lack a capacity for symbolic thought. Others such as John J. Shea disagree and suggest that the differences between modern and ancient thinking may have been exaggerated. Nevertheless, whatever evolutionary changes marked the emergence of modern humans, it's likely they involved at least some important changes to our cognitive, linguistic, and social abilities. One might expect to find genes for those traits to be altered or absent in older types of humans.
I want to be perfectly clear on this point: this discovery absolutely does not mean that the Denisovans, Neandertals, and other ancients were autistic. Nor does it mean that autistic people exhibit prehistoric thinking. Rather, what it underscores is that normal modes of human thought occupy a broad continuum.
The "neurotypical" way in which most people see the world today is only one way of doing it. As enlightened studies of autism repeatedly drive home, we need to appreciate those variations as part of our human spectrum rather than just labeling them defective or abnormal.
With or without all our cognitive abilities, the Neandertals and Denisovans survived under amazingly hostile conditions for hundreds of thousands of years. Their different ways of thinking may have been dominant throughout long stretches of the past, and might even have had advantages over our own under their circumstances. The lesson that these ancients offer is that we should broaden our minds about how broad minds can be.