We have reason to think that the modern humans who expanded out of Africa some 50,000 years ago had changed in important ways—had, for example, probably acquired sophisticated language abilities. A Neanderthal allele that had not been particularly useful in the genetic context of near-modern humans 100,000 years ago might have been useful to the more advanced people who were expanding out of Africa.
Logically, if admixture occurred at all, it had to happen somewhere in Neanderthal-occupied territory, which means Europe and western Asia. As modern humans expanded their territory, they must have encountered Neanderthal bands again and again. The two kinds of humans coexisted for a few thousand years before the Neanderthals disappeared, at least in some regions. This looks to be the case for the Chatelperronian culture of France and northern Spain, and there are traces of a similar culture in Italy. If there was trade, or if there was enough contact to transmit toolmaking techniques, there was sexual contact as well—depend on it. If in the future we look at very large genetic datasets from huge numbers of individuals, we might find a few traces of neutral Neanderthal genes.14
If we found a few individuals with Neanderthalish mito- chondrial DNA (mtDNA) or Y chromosomes, we might be able to determine whether matings occurred mostly between
Neanderthal males and modern females or modern males and Neanderthal females. We do this type of analysis routinely today and have found, for example, that the maternal ancestry for most Mexicans is Amerindian, whereas most of their paternal ancestry is Spanish (which simply means that male Spanish explorers sometimes mated with Amerindian women). At this point, though we have found no Neanderthal Y chromosomes or mtDNA in modern humans, we cannot rule out significant introgression, because Neanderthal mtDNA and Y chromosomes may well have been neutral or deleterious in modern humans. In either case they would be unlikely to have persisted until today, particularly if the amount of gene flow was small. This does not mean we did not inherit beneficial gene sequences (see section on "Genetic Evidence" later in this chapter).
BUT I DON'T WANT TO BE PART NEANDERTHAL!
There is often a visceral reaction to the idea that we carry some Neanderthal genes. Probably this is due to the general impression that Neanderthals were backward and apelike. Neanderthals weren't really apelike, although they were behind the times—but since it looks, in any case, as if we've absorbed only their best (most useful) traits, we can be happy about our Neanderthal ancestry, proud even. At any rate, it could be worse: We could have picked up genes from a virus. In fact, it is worse: We have.
Most viruses (which are basically just bags full of DNA or RNA) slip into cells and then take over, making copies of themselves and usually killing the host cells in the process. But some RNA viruses (retroviruses, like HIV) copy their RNA into DNA and then, sometimes, integrate that DNA into the host
Stylized rendering of the AIDS retrovirus
cell's genome. If the retrovirus happens to occupy a reproductive cell, one that makes sperm or eggs, the retroviral genes can actually become part of the next generation's genome. This has happened in the past: Humans have many genetic remnants of retroviruses that at one time inserted copies of themselves into the human genome. Most do not seem to have any real function, but a few do. For example, both humans and apes have syncytin, derived from a retroviral envelope protein that our ancestors picked up roughly 30 million years ago. It plays a role in the development of the placenta—in particular, the process that leads to the development of a fused cell layer. Anyone who's overly worried about possible Neanderthal ancestry should remember that we're certainly descended from viruses. As usual, the facts don't care about our feelings.
EXAMPLES OF INTROGRESSION
Introgression as an important evolutionary force is more than just a theory: Geneticists know of many cases in which it has definitely occurred. Most of the examples that are well understood involve domesticated animals and plants, mainly because there are practical economic reasons for undertaking close genetic studies of domesticated species.
Introgression is hardly rare; in fact, it is ubiquitous among domesticated plants. For example, the wheat that produces our daily bread is derived from three different wild grasses. There's evidence of introgression in alfalfa, barley, chili peppers, lettuce, maize (corn), potatoes, rice, rye, sorghum, and soybeans—and that's just a partial list. But since plants are better than animals at tolerating complex genetic events such as changes in chromosome number, introgression in domesticated animals may be a better analogy.
Cows were domesticated at least twice: in the Middle East (humpless taurine cattle) and in India (humped, droopy-eared zebu cattle), and possibly a third time in North Africa. The wild ancestors of taurine and zebu cattle were separated for several hundred thousand years, yet those breeds are interfertile. Zebu genes have been spreading among taurine cattle in Africa and western Asia for the past 4,000 years. It appears that some zebu genes increase tolerance of aridity and heat as well as resistance to rinderpest, a virulent bovine disease. This is very similar to the pattern of introgression we believe must have occurred among moderns and archaics such as Neanderthals.
Evidence of adaptive introgression in wild populations was once rare, but it has become easier to discover and document it
Texas longhorn
in recent years thanks to improved DNA-sequencing techniques. We now have genetic evidence for adaptive introgression in wild organisms such as damselflies, mosquitoes, lake trout, and European hares. One of the most interesting cases of the phenomenon in a (partly) wild population is the recent evolution of honey bees, which is particularly interesting because of some close parallels with human evolution.
Honey bees originated in eastern tropical Africa several million years ago and later expanded into Eurasia in two different migrations. One of these led to Western European honey bees and the other to Asian honey bees. Bees living in temperate climates faced fundamentally new problems: more than anything else, cold winters. To a large extent, their adaptation to those new climates was mediated by changes in social behavior.
They needed to choose nest sites that would protect them from the weather, store much more honey, and form a winter cluster—that is, a tightly packed clump of bees that conserves heat. In a 2008 study, Amos Zayed and Charles Whitfield concluded that approximately 10 percent of all protein-coding genes in bees underwent positive selection in that process of adaptation.15 The history of honeybees parallels the history of humans in an interesting way—both involved expansion into a new environment with a drastically different climate followed by strong selection and adaptation.
The parallels don't stop there. Soon after the initial colonization of the Americas, Europeans introduced honey bees, where they did well, swarming many times a year and outrunning colonists. However, they did not do as well in the neotrop- ics, the part of the New World most unlike Europe. Warwick Kerr, a twentieth-century Brazilian geneticist and bee breeder,attempted to develop a strain of bees that would be more productive in the tropics. Using hybridization techniques, he bred Western European bees with African bees. In 1956, twenty-six of his Tanzanian queens escaped and began colonies, and their hybrid descendants have since spread over much of North and South America. These Africanized bees produce more honey than European bees in warm climates, but they are very aggressive and often attack people and animals that come too close to their hives. Once aroused, they may chase an enemy a mile or more. This high level of aggression is adaptive in Africa, where bees have not been domesticated. There, bee colonies are attacked by honey badgers and other predators, and humans raid beehives rather than keeping bees.
Almost all Africanized bee colonies have African mtDNA, but a significant fraction of their nuclear genome is European. That fraction is significantly higher in coding regions of the genome than in noncoding DNA, which indicates that Africanized bees have succeeded in picking up adaptive alleles from European bees while retaining those African alleles that are adaptive in this situation (that is
, most of them). The noncoding regions, presumably neutral or close to neutral, are incorporated at the rate 1 /(2N), while favorable coding genes are incorporated at the 2s rate, as discussed above. It has been suggested that there may be genetic incompatibilities between European mtDNA and the African nuclear genome, which would explain why we find so very few Africanized bees with European mtDNA. The Zayed-Whitfield study provided evidence that "invasive populations can exploit hybridization in an adaptive fashion," which is only reasonable. Just as the Africanized bees have incorporated advantageous genes from the local indigenousbee populations, modern humans, we believe, incorporated advantageous genes from their archaic human precursors, especially from the Neanderthals.
Many instances of adaptive introgression—those, for example, that involve biochemical changes that do not affect appearance—are cryptic and were effectively undetectable before the development of modern molecular research methods. This is worth remembering when we look at the fossil record: The majority of adaptive genetic events do not have noticeable skeletal signs. Some cases of adaptive introgression, though, have readily visible effects, as when genes that increased drought tolerance spread from Utah cliffrose to bitterbrush. The intro- gressed bitterbrush looks more like cliffrose and can survive in places where ordinary bitterbrush cannot.16 In this case, the population with introgressed genes reflects that introgression in its external appearance, but more often the effects of introgression are not readily apparent in the gross anatomy of an organism.
BREEDING EXPERIMENTS
Applied geneticists are always conducting breeding experiments— usually for practical purposes in agriculture, often for research, sometimes for the sheer fun of it. In those experiments, they usually select for some trait (or for the absence of that trait): that is, they breed a new generation from those individuals with especially high (or low) values of a trait. The average value of that trait (or the absence of it) changes over generations and can eventually reach levels that differ greatly from the original population. If you doubt this, consider that the Chihuahua is the product of selection upon wolves. Change gradually slows (atleast in small populations) and the trait plateaus. Sometimes this is because further change is physically impossible, but more often it is because the genetic variety of the population has become exhausted. When several different populations (drawn from the same base population) have undergone such selection and plateaued, sometimes the breeder will take the two best lines and cross them. Some such efforts are unproductive, but others succeed in producing a population with significantly higher trait values.
Life is a breeding experiment, of course. And it looks as if the African-Neanderthal cross has worked out pretty well—so far, anyway.
The key point here is that it would take only a very limited amount of interbreeding for modern humans to have picked up almost every Neanderthal allele with any significant advantage. Limited interbreeding would mean that neutral genes in humans today would look almost entirely African—which they do—while at the same time we might carry a number of functional alleles that originated in Neanderthals. Those alleles would be ones that mattered, ones that made a difference.
This raises the question of just what the Neanderthals might have had to offer. The popular impression is that they were backward, almost bestial—and it's certainly true that moderns had capabilities that Neanderthals lacked. But in archaeological artifacts from as recently as 100,000 years ago, it's hard to see any real differences in the material culture of Neanderthals and the material culture of Africans—so the Neanderthals can't have been all that far behind.
The alleles most obviously worth stealing would be those that implemented adaptations to local conditions in Europe.
That might mean, for example, acquiring the ability to tolerate cold weather, resist local diseases, or adjust to big swings in the length of the day over the course of a year (in contrast to the tropics, where the length of the day does not vary much). These kinds of adaptations, along with the more sophisticated, technological solutions to cold characteristic of modern humans, such as building shelters and so on, may have been important in human settlement of the far north, and eventually of the Americas.
These sorts of changes were important—adaptation to local climates and pathogens was obviously necessary for humans to succeed in northern climates. But on the whole, they are not all that interesting. Obviously, even penguins are better adapted to cold than humans. If that is all the Neanderthals had to offer, the question of interbreeding with them would not matter so much. The most interesting genetic changes are surely those that change minds rather than bodies. And there are several lines of argument that suggest that the Neanderthals may have had something to contribute along those lines as well.
Changing Minds
Neanderthals had developed larger brains during their time in Europe, just as modern human ancestors had in Africa (and to some extent as the archaic populations in Asia had as well). Those large brains paid off in increased fitness in both populations, else they'd never have come into existence, but there may have been functional differences.
There were deep differences between Homo sapiens and Homo neanderthalensis in way of life, with Neanderthals being high-risk, highly cooperative hunters, rather like wolves, while anatomically modern humans in Africa probably had a mixeddiet and were more like modern hunter-gatherers. Those differences could mean that big Neanderthal brains were solving different sorts of problems than big African brains. As a purely hypothetical example, Neanderthals, facing high risks as ambush hunters of big game, might have benefited from an ability to imagine and anticipate the reactions of their prey—call it "theory of animal mind." Neanderthals were strong and heavily built, but their hunting success depended on brainpower to a far higher degree than that of lions or wolves. Their intelligence made their way of life possible via the use of tools and weapons, but there must have been other ways in which their big brains aided survival. Improved accuracy in guessing just how a wounded bison would react could well have kept a Neanderthal from having his ribs kicked in, for example. There's a precedent for this notion of one species having a theory of mind in dealing with another species: Wolves can't take a hint, but dogs have an evolved ability to read people.17
And yet, European Neanderthals probably faced many of the same life problems that African humans did. To some degree, big brains may have been solving the same problems in both populations. Even if that is the case, though, we can be certain that those problems were not solved in exactly the same way. Examples demonstrating how natural selection works can shed light on this concept. Let's look again at human adaptation to malaria. We see hemoglobin variants in both Africa and Southeast Asia (sickle cells and Hemoglobin E [HbE]), but they're not the same variants. Although both alleles protect against malaria, there's no reason to assume that one works exactly like the other (or even as well as the other). HbE, for example, definitely has fewer negative side effects than sickle cells.
We see a similar pattern in human adaptation to high altitude. On one hand, the Amerindians of the high Andes have barrel chests and blood crammed with red cells; the Tibetans, on the other hand, have much lower levels of hemoglobin but breathe faster to take in more oxygen. Both peoples are far better adapted to high altitude than flatlanders, but the Tibetan adaptation is apparently more effective, since their babies are plumper and healthier. Adaptation depends on the supply of favorable mutations, which are generated randomly; thus, two populations facing the same problem may well find different solutions, and those solutions need not be equally efficient. Neanderthals and the anatomically modern humans of Africa faced some of the same conditions and adapted to them, but they did not necessarily do so in the same way or with the same degree of efficiency.
As we mention elsewhere in this work, sometimes variation in human personality is best explained as a genetically based alternative behavioral strategy that works well when rare but whose advantage dissipates as bearers of that strategy
become more common. For example, many suspect that human sociopaths, individuals who are well-designed "cheaters," like con artists, can prosper when they are rare but suffer fitness loss as they become more common and others become more aware of them.18
There are many possible alternative strategies, and it is possible that the Neanderthals had some that never came into existence among the modern humans in Africa—and yet could succeed among them, particularly since increased innovation had shaken up society. So, it could be the case that as the modern humans moved north and came into contact with Neanderthals, they picked up alternative strategies for solving variousproblems—strategies with a genetic basis that came to them not by observation but through introgression and natural selection, and which depended upon new mental functions and cognitive processes.
Paths on Fitness Landscapes
Another point: Ongoing natural selection in two populations can allow evolutionary events to occur that would be impossible in a single well-mixed population, since it allows for simultaneous exploration of divergent paths. Natural selection is shortsighted: Alleles increase in frequency because of their current advantage, not because they might someday be useful. Think of various possible solutions of some problem as hills, with higher hills corresponding to better solutions. Natural selection climbs up the first hill it chances upon; it can't see that another solution has greater possibilities in the long run. Not only that: Since the environmental conditions of Europe and Africa were significantly different, evolution could try solutions in Europe that couldn't be explored in Africa, because the initial step along that path had negative payoffs in Africa. In Europe, for example, you had to worry about staying warm enough, whereas Africans faced heat stress: These issues were important considerations in the evolution of larger brains. It may be that the relative unimportance of heat stress in Europe opened up some evolutionary pathways that had greater long-term possibilities than the ones that developed in Africa.
The 10,000 Year Explosion Page 5