Consider an analogy from the history of technology. Somewhere back in late classical times, the use of the camel was perfected—a better saddle was developed, for example, one that allowed camels to carry heavy loads efficiently. Throughoutmost of the Middle East and North Africa, camels were (after those developments) a superior means of land transportation: They were cheaper than ox-drawn wagons and not dependent upon roads. Over a few centuries, people in areas where camels were available abandoned wheeled vehicles and roads almost entirely.19 You can still see the effects in the oldest sections of some cities in the Arab world, where the alleys are far too narrow to have ever passed a cart or wagon. Europeans, not having camels, had to stick with wheeled vehicles, which were clearly more expensive, given the infrastructure they required. But as it turned out, wheeled vehicles—in fact, the whole road/wheeled vehicle system—could be improved. Back then, when camels seemed so much better, who knew that someday there would be horse collars and nailed horseshoes, then improved bridge construction, suspensions that reduced road shock, macadamized roads, steam power, internal combustion engines, and ultimately the nuclear Delorean. The motto here is that sometimes the apparently inferior choice has a better upgrade path: Evolution can't know this, and we aren't particularly good at recognizing it ourselves. On the genetic level, it translates as follows: Natural selection may solve the same problems differently in different populations, and what appears to be the most elegant solution at the time may not in fact turn out to be the one that works best in the long run. The seemingly inferior choice may come out on top down the road. It is easy to think of plausible cases: Imagine, for example, that excess heat production limited the trend toward larger brains in Africa, while in the climate of Europe heat was not much of a problem. Later, as evolution fine-tuned the physiology of large brains, much of the heat problem was solved—and so the new brain could then spread in Africa as well.
Spare Genetic Diversity for the Future
Lastly, we should consider that even a slight degree of Neanderthal admixture would have increased the amount of genetic variation among modern humans, and those imported alleles could have been useful in solving future adaptive problems even if they were not particularly advantageous back in the Upper Paleolithic. We now know that the transition to agriculture posed many challenges that led to strong selective pressures on farmers. Most of the adaptive responses to agriculture were probably the result of new mutations, but some must have made use of preexisting genetic variation, which would have included any alleles that we picked up from Neanderthals or other archaic humans. Think of it this way: Modern humans and Neanderthals were both unsuited to agriculture and civilization, so Africans were not much more likely to carry alleles preadapted to agriculture than Neanderthals were. The solutions to agricultural challenges could have come from either camp.
We're not saying that Neanderthals were competitively superior: After all, we're here, and they're not. But it is highly likely that, out of some 20,000 genes, at least a few of theirs were worth having.
Not only was interbreeding between Neanderthals and moderns likely and potentially important, there is evidence indicating that it actually occurred. That evidence is of two kinds, skeletal and genetic.
SKELETAL EVIDENCE
Neanderthal anatomy differed in a number of ways from that of anatomically modern humans, as we have noted before. There are particular details that are relevant to interbreeding. One isthe occipital bun, a bulge at the back of the skull, which was very common among Neanderthals but is rare among people today. Another is the retromolar space, a gap between the last molar and the back of the mouth. These and other skeletal details characteristic of Neanderthals were unusually common among the modern humans who were their immediate successors but have declined in frequency over time.20 The skeletons we've found of modern humans during the Upper Paleolithic don't have the pulled-forward face that is so distinctive of Neanderthals— which is consistent with the idea that there wasn't a lot of gene flow. Complex craniofacial features probably depend on many genes working together, so such features are unlikely to show up if Neanderthal genes are uncommon in modern humans of the period. There have been claims that certain early but clearly modern human skeletons have several distinctive Neanderthal skeletal features, however, which could indicate recent admixture.21 We think that the skeletal evidence suggests that there was significant Neanderthal admixture, but we also recognize that this evidence is not by itself definitive. Considering the possibility of convergent evolution, the situation is complex. One problem is that skeletal features, like almost everything else, evolved for a reason: They somehow increased fitness for the Neanderthals in their environment. It is therefore possible that some features similar to those of the Neanderthals evolved independently in Cro-Magnons (that is, in anatomically modern humans of the Upper Paleolithic) because they fulfilled the same functions. Moreover, only Neanderthal traits that were adaptive are likely to have introgressed and reached significant frequency today. Fortunately we're not limited to skeletal evidence—we are rapidly acquiring genetic evidence that bears on this question.
GENETIC EVIDENCE
The first investigations of modern humans attempting to detect remnants of ancient lineages looked at mitochondrial DNA and Y chromosomes. Both are of interest because they are inherited from just one parent (the Y chromosome from the father, the mtDNA from the mother) and because they do not recombine. Extensive sampling has shown no evidence of variants that might have existed in archaic human populations such as Nean- derthals.22 The data, in other words, are consistent with low (or zero) gene flow from archaic to modern populations. This pattern might also have arisen if Neanderthal mtDNA and Y chromosomes didn't mesh well with the genetic background of anatomically modern humans and reduced fitness in some way. In that case, they would have dwindled with time and could be rare or nonexistent today even if they had once been moderately common in modern humans.
A number of recent reports, however, provide evidence that people do retain some autosomal alleles from archaic humans.23 Some of these reports have detected odd patterns in our genome as a whole, whereas others have looked closely at particular unusual genes.
V. Plagnol and J. D. Wall found that the pattern of linkage disequilibrium—that is, of the history of chromosomes having broken and reformed—among SNPs (single nucleotide polymorphisms, or single base differences between chromosomes) in the human genome was inconsistent with an unstructured ancient population, estimating that 5 percent of genetic variation among both Europeans and sub-Saharan Africans originated in archaic humans such as the Neanderthals.24 This is interesting, in thatevidence for introgression is nearly as strong among Africans as it is among Europeans. This is what one would expect to happen if many of the alleles picked up from Neanderthals or eastern archaic humans were generally advantageous and spread very widely. There may also have been significant archaic populations somewhere in Africa: There are some apparently archaic variants found only in Pygmies, which suggests an African origin. Conditions for fossilization are poor in much of Africa west of the Rift (for example, chimpanzees have almost no fossil record)—and there may well have been hominid populations other than anatomically modern humans in the blank spots in those Africa maps. Since some of these alleles are found at high frequencies in people today, and since the overall level of admixture was probably low, they probably gave a fitness advantage— in other words, were adaptive.
P. D. Evans and his colleagues at the University of Chicago looked at microcephalin (MCPH1), a very unusual gene that regulates brain size.25 They found that most people today carry a version that is quite uniform, suggesting that it originated recently. At the same time, it is very different from other, more varied versions found at the same locus in humans today, all of which have many single-nucleotide differences among them. More than that, when there are several different versions of a gene at some locus, we normally find some intermediate versions created by recombination, that is,
by chromosomes occasionally breaking and recombining. In the case of the unusual gene (called D for "derived") at the microcephalin locus, such recombinants are very rare: It is as if the common, highly uniform version of microcephalin simply hasn't been in the human race all that long in spite of the high frequency of the new version in many human populations. The researchers estimated that it appeared about 37,000 years ago (plus or minus a few tens of thousands of years). And if it did show up then, Neanderthals are a reasonable, indeed likely, source.
Another interesting possibility involves FOXP2, a gene that plays a role in speech that was replaced by a new variant some 42,000 years ago.26 This is very recent in evolutionary terms, and there is evidence that the same version of that gene existed in Neanderthals.27 If the new FOXP2 allele is really that recent in modern humans, it is likely that the migrating humans picked it up from Neanderthals, since that's about the time they encountered them in their expansion out of Africa. The idea that we might have acquired some of our speech capabilities from Neanderthals may be surprising, but it is not impossible. The timing of the acquisition is certainly consistent with the creative explosion. If it is true that we gained the gene by means of introgression, then the version of FOXP2 in the Neanderthals is likely to be older and have more variation than it does in modern humans. Further sequencing efforts on the skeletal remains of Neanderthals should eventually confirm or refute this possibility.
If FOXP2 is indeed a "language gene" and responsible, perhaps, for some of the creative explosion of modern humans in Europe and northern Asia, it would explain a major puzzle about modern human origins. There were at least two streams out of Africa 50,000 years ago, one northward into Europe and central Asia, and another eastward around the Indian Ocean to Australia, New Guinea, and parts of Oceania. There is no trace of any creative explosion in populations derived from the southern Indian Ocean movement, who brought and retained Neanderthal-grade technology and culture.28
CONCLUSION
A burst of innovation followed the expansion of modern humans out of Africa. Signs of that change existed in Africa before the expansion, but the pattern became much stronger in Europe some 20,000 years later, after anatomically modern humans had encountered and displaced the Neanderthals. That transition to full behavioral modernity—as seen in the archaeological record—occurred patchily and finished later in other parts of Eurasia. We argue that even limited gene flow from Neanderthals (and perhaps other archaic humans) would have allowed anatomically modern humans to acquire most of their favorable alleles. We believe that this sudden influx of adaptive alle- les contributed to the growth of the capabilities that made up the "human revolution," and we believe that this introgression from archaic human populations will prove central to the story of modern human origins.
So by 40,000 years ago, humans had become both anatomically and behaviorally modern (which is not to say they were exactly like people today). They had vastly greater powers of innovation than their ancestors, likely owing in part to genes stolen from their Neanderthal cousins. The speed of cultural change increased by tens of times, and when the glaciers retreated and new opportunities arose, it accelerated further.
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AGRICULTURE: THE BIG CHANGE
Favorable mutations are rare, and many of those that do occur are lost by chance. In the small human populations of the Old Stone Age, establishing such mutations typically took hundreds of thousands of years. It's not that it took that long for favorable mutations to spread—the problem was generating them in the first place.
But as human population sizes increased, particularly with the advent of agriculture, favorable mutations occurred more and more often. Sixty thousand years ago, before the expansion out of Africa, there were something like a quarter of a million modern humans. By the Bronze Age, 3,000 years ago, that number was roughly 60 million. Favorable mutations that had previously occurred every 100,000 years or so were now showing up every 400 years.
One might think that it would take much longer for a favorable mutation to spread through such a large population than it would for one to spread through a population as small as the one that existed in the Old Stone Age. But since the frequency of an advantageous allele increases exponentially with time in a well-mixed population, rather like the flu, it takes only twice as long to spread through a population of 100 million as it does to spread through a population of 10,000.
Agriculture imposed a new way of life (new diets, new diseases, new societies, new benefits to long-term planning) to which humans, with their long history as foragers, were poorly adapted. At the same time it led to a vast population expansion that greatly increased the production of adaptive mutations.1 So agriculture created many new problems, but it created even more new solutions. Earlier innovations had also helped to increase population size and thus had speeded up human evolution, but agriculture had a far greater effect and is in a class of its own.
Naturally, increased population size had a similar impact on the generation of new ideas. All else equal, a large population will produce many more new ideas than a small population, and new ideas can spread rapidly even in large populations. In Guns, Germs, and Steel, Jared Diamond observed: "A larger area or population means more potential inventors, more competing societies, more innovations available to adopt—and more pressure to adopt and retain innovations, because societies failing to do well will be eliminated by competing societies."2 We take this observation a step further: There are also more genetic innovations in that larger population.
This is a new picture of recent human evolution. It implies that humans have changed not just culturally, but genetically, over the course of recorded history, and that we must allow forsuch changes when we try to understand historical events. The implications of this contention are vast: If correct, it means that peoples in different parts of the world have changed in varying ways, since they adopted different forms of agriculture at different times—or in some cases not at all.
Since genetic change wasn't uniform, discrete populations came to differ genetically from one another, and sometimes those genetic differences conferred competitive advantages. We believe that such genetic advantages have played a role in migrations and population expansions—and thus are important in explaining the current distribution of languages and peoples. In fact, history looks more and more like a science fiction novel in which mutants repeatedly arise and displace normal humans—sometimes quietly, simply by surviving, sometimes as a conquering horde.
It's probable that the evolutionary response to farming also affected the distribution of cognitive and personality traits, and that these changes played a crucial role in the development of civilization and the birth of the scientific and industrial revolutions.
SETTING THE STAGE
When the Ice Age ended around 10,000 BC, the world became warmer and wetter, and the climate became more stable. Carbon dioxide levels increased, which increased plant productivity. The stage for agriculture was now set—and this time the actors were ready as well.
Although there had been other interglacial periods in the past, early humans had never developed agriculture then. We suspect that increases in intelligence made agriculture possible,but the route may have been indirect. For example, the invention of better weapons and hunting techniques, combined with other technologies that let humans make better use of plant foods, could have led to lower numbers, or even extinction, of key game animals—which would have eliminated an attractive alternative to farming.
Farming appeared first in the Fertile Crescent of Southwest Asia. By 9500 BC, we see the first signs of domesticated plants: first wheat and barley, then legumes such as peas and lentils.3 From there farming spread in all directions, showing up in Egypt and western India by 7000 BC and gradually moving into Europe and India. Around 7000 BC, rice and foxtail millet were domesticated in China. Animals were domesticated on a similar timeline, with the Middle East in the lead. Goats were tamed around 10,000 BC in Iran, sheep about 1
,000 years later in Iraq. Both the taurine cattle we're familiar with in the Middle East and the humped zebu cattle in India were domesticated around 6000 BC.
Agriculture came later to the rest of the world. In some cases it spread by a geographic expansion of farmers, in others through hunter-gatherers adopting already-existing methods of agriculture, and in yet others by hunter-gatherers independently inventing their own forms of agriculture. In Europe, agriculture was spread by Middle Eastern immigrants and by native Europeans learning to grow Middle Eastern crops such as wheat and barley. In sub-Saharan Africa, geographic barriers and climatic differences blocked adoption of most Middle Eastern crops and domesticated animals. There, agriculture appeared around 2000 BC and was based on locally domesticated crops such as sorghum and yams. The story is similar in the Americas, where the Amerindians were almost entirely cut off from therest of the world and had to domesticate their own crops. (Some of those, such as maize and potatoes, are among the most important crops in the world today.)
The 10,000 Year Explosion Page 6