Lone Survivors (43 page)

When I give public lectures, I am invariably asked where evolution will lead us, and what humans will look like in the future, and equally invariably I try to avoid answering such tricky questions. I have, though, taken a different view in public from Gould and my geneticist friend Steve Jones over whether human evolution is over. Jones suggests that modern culture and its benefits like medical care removed the power of natural selection to affect humans, since virtually everyone now reaches reproductive age. I disagree because, first, changes in our genome are occurring all the time, whether we can detect them or not; some calculations suggest that each of us could have about fifty new mutations compared with our parents' DNA. Second, life in the developed world has its own differential costs in terms of reproduction and health, with the general availability of contraception, but also of junk food, alcohol, and drugs. Third, and even more significant, at least a quarter of the world's population is still denied the benefits of decent health care and the necessities of healthy living conditions and diets. Thus selection is operating strongly on those billions of people, and I cannot see that stopping any time soon. From my perspective, evolution is certainly still working away on
Homo sapiens
, and there is even evidence that its effects have accelerated rather than diminished over the last 10,000 years, as we will see below.

Science fiction images of humans of the future often show us with huge brains but, as we saw, big brains are not necessarily the best brains—witness the extinct Neanderthals—and if anything our brains have actually shrunk in size over the last 20,000 years. In practical terms, unless the process of birth is bypassed, our brain size is already at the limit at which the female pelvis can cope with delivery. Then there is the sheer cost in energy of running a big brain, and the evidence that larger brains are not necessarily as efficient at some tasks. And anyway, so much of our memorizing and thinking is done externally now—in the brains of other people or in the processors of our computers. All of these factors could be responsible for our shrinking brains, as well as more mundane factors like an overall reduction of body size compared with our Paleolithic ancestors.

More realistically for our future evolution, there is the prospect of genetic engineering, which is already happening on a small scale. Genetic counseling is available to advise potential parents about harmful DNA mutations that could be passed on to their children, and to give them the choice about whether to proceed. As this becomes more common and wider in its reach, future gene pools will be affected. Even more ambitiously, gene therapy could be applied to a faulty organ in the body, and germ-line therapy could plant a permanent change in the genome of an unborn baby. There are formidable ethical questions to be addressed here, not to mention the scientific ones. For example, we know that the actions of genes are often interrelated, and that a single gene may perform more than one function. So great care would be needed to ensure that the targeted change in DNA achieved only what was intended. And the social consequences of even giving people the simple choice of a male or a female child are enormous, let alone providing opportunities to enhance that child's beauty, talents, or intelligence.

Most of that is still science fiction, and perhaps it is for the best that some elements will always remain so. But for the last 10,000 years, selection seems to have been engineering people to cope with massive lifestyle changes. When humans expanded into novel environments over the last 50,000 years, including into rain forests in Africa and new habitats in Eurasia, Australasia, and the Americas, they encountered fresh challenges and had to adapt both physically and culturally. The physical adaptations ranged from gross changes in body size or shape, down to immunological responses to a host of new pathogens. And over the last 20,000 years these have also included distinct mutations in Europe and Asia for depigmentation, to assuage the lower levels of sunlight, as well as the spread of blue eyes in western Eurasia—although this latter change might equally have come from cultural selection.

Culture, rather than slowing changes in our DNA, may well have provided the means to speed them up. This is the view of a growing number of geneticists and anthropologists, including Henry Harpending, Gregory Cochran, John Hawks, Anna Di Rienzo, Pardis Sabeti, Sharon Grossman, Ilya Shylakhter, and Kevin Laland. They argue that profound changes in human lifestyles over the last 10,000 years—with the moves to pastoralism, agriculture, and urbanization—would have had equally profound evolutionary effects. With the consequent huge increase in human numbers, there are clear parallels with the relationship between demography and innovation: a larger population will not only have more mutations, and more beneficial mutations, but also provide better chances for them to be conserved and disseminated. And the fact that farming also entailed self-induced changes in societies, diets, and environments (not all of them beneficial to everyone) would have ensured that selection remained a powerful force for evolutionary change.

Ten thousand years ago, as agriculture was taking off in its west and east Asian cradles, the hunter-gatherer populations of the world probably numbered only a few million people, and in many areas they must have been thinly scattered. The estimated figure only 8,000 years later was over 200 million, and following the industrial revolution and the advent of measures like vaccination, our numbers are now soaring toward 10 billion. The huge increase between 10,000 and 2,000 years ago would have ensured a proportionate increase in mutations, including potentially favorable ones, and, provided population density was high (which it was in many agricultural and subsequent urban communities), any genetic changes had the potential to spread rapidly. As people acquired stable food supplies through farming, they settled down in increasingly large communities, but with this change there were also many downsides. Unsanitary living conditions and densely packed communities were ripe for parasites and epidemic diseases like smallpox, cholera, and yellow fever, while the clearance of forests and the use of irrigation led to the spread of malaria across much of the tropics and subtropics. Overreliance on one or two staple foods also meant that the benefits of a broader hunter-gatherer diet were lost, and, for many, hard labor in the fields wore out bodies prematurely. Societies and technologies had to keep up with the changes too. People were thrown together and socializing in larger numbers, with the growth of task specializations, and disparities in wealth, status, and, no doubt, reproductive success.

An evolutionary tree showing the geographic distribution of humans and humanlike relatives from the last 2 million years. Note the complexity of relationships now implied by the latest genetic data.

All of these upheavals should have provided fertile ground for evolution to operate, and so many groups of geneticists have been combing the human genome for signs of this. The methods are known as
genome-wide
or
whole-genome association studies
, where a correlation is sought between genes and particular traits, whether these are physical, such as skin color or height, or physiological, such as susceptibility to a disease. Of course such studies must take into account environmental influences, as well as the complexity of gene expression, since a particular end result may come from the interaction of several different genes, rather than just one. A major source for association studies has been the International Haplotype Map, which has provided data on millions of SNPs in 270 people of European, Nigerian, Chinese, and Japanese descent. These single letter mutations are inherited within larger sequences of DNA, and segments break down over time as a result of the remixing of the DNA on our chromosomes with every new generation. New mutations can be spotted, and their age can be estimated by the amount of mixing that has occurred around them.

Sure enough, the signals of recent selection not only were there but were also very strong, acting on perhaps 20 percent of our genes. Some could be directly related to the changes induced by farming, linked to new diets, such as the gene for lactase. This is an enzyme that allows infants to digest lactose (milk sugar) when nursing, but it usually switches off during childhood, so that many adults are lactose-intolerant. However, in the last 10,000 years, separate genetic changes occurred in East Africa and regions of western Eurasia that prevented the lactase gene switching off, meaning that adults (about 80 percent in the case of Europeans) can comfortably digest milk from livestock. Populations elsewhere who lack the mutations, such as East Asians and Native Australians and Americans, are still only able to drink milk comfortably as babies. Meanwhile, mutations have evolved to allow the digestion of other “new” carbohydrates in the diet in West Africa (for the sugar mannose) and East Asia (mannose and sucrose). And there have also been changes in a gene that codes for salivary amylase (which helps to digest starch), both in its structure and in the number of copies of the gene in many individuals. Examples of recent selection in human genes have been known for many years in connection with protection from malaria, and at least twenty-five different examples have now been detected. Because the malarial parasite is transmitted in the bloodstream, many human defenses originate in the blood, such as mutations in the hemoglobin gene, which carries oxygen, or in the enzyme G6PD. And blood groups have responded too, with an entirely new one—Duffy—seemingly selected specifically to combat the disease. Many further changes seem to be related to resistance to infectious diseases such as tuberculosis, and 10 percent of Europeans have been fortuitous in carrying mutations that have apparently been selected to resist smallpox; they also seem to confer resistance to HIV.

Other recent changes may be related to the changing social conditions brought by agricultural life. In chapter 6, we mentioned mutations in the gene for the cholesterol-transporting apolipoprotein E that seem to lower the risk of many age-related conditions such as coronary disease, and there are at least fourteen other recently mutated genes that are linked with conditions most expressed in the old, such as cancers and Alzheimer's. Considering the crucial importance of extended families for both hunter-gatherers and farmers, selection seems to have been working on the survival of people past reproductive age as well, given the consequent social benefits. But a possible downside for social harmony from higher population densities is the greater potential for adultery, and this may be reflected in widespread but regionally distinct mutations controlling the quantity and vigor of human sperm—perhaps indicative of “sperm competition,” caused when a woman partners more than one man within a day or so. Perhaps some of the one hundred or so recent mutations in brain neurotransmitters concerned with mood and demeanor have correspondingly been selected to cope with the social consequences of our large population numbers and the possible resultant tensions.

Those neurotransmitters are only a part of our changing genome as far as the brain and senses are concerned. Although this is a highly controversial area, it is likely that selection has favored different behaviors and cognitive abilities as modern humans have diversified in different environments and social complexities. With the development of specialized occupations and their associated skills, selection may have increasingly come into play. For example, the need to work out stocks of cereals or animals, followed by the rise of trading and the arrival of money, would all have encouraged selection for mathematical abilities. And the increasing complexity of communication in small or ever-larger groups may be marked by recent mutations in genes that produce proteins for the cilia of our inner ears and the membrane that coats them, as well as one that helps to build the actual bones of the middle ear, which transmits sonic frequencies. The fact that different mutations are found when comparing Chinese and Japanese, Europeans and Africans suggests that selection might even have been tracking the evolution of different languages and their most characteristic sounds. Sight, too, may have been under recent selection in East Asia—mutations in the protocadherin-15 gene there affect the workings of both inner ear cells and photoreceptors in the retina.

But to return to the question posed earlier, it appears that human evolution, at least in terms of changes in individual DNA sequences, has accelerated rather than slowed or stopped over the last 10,000 years. Indeed, some calculations suggest that it is now happening a hundred times faster than it did since we split from the lineage of chimpanzees, probably more than 6 million years ago. About 7 percent of human genes seem to have mutated recently in some populations, the majority within the last 40,000 years, and particularly within the last 10,000 years. Some caution has to be injected here, since geneticists like Sarah Tishkoff and Mark Stoneking have pointed out that the expansion of human populations might have increased rare variants by chance alone, so the functional benefit of the genetic change needs to be properly demonstrated—as it can be in many cases. Additionally, and perhaps more seriously, the constant loss and overwriting of changes in our DNA mean that some ancient signals of genetic change—during the Middle Stone Age, for example—have been lost or are difficult to detect now. Thus we have a biased signal for the last 10,000 years or so, because this is the very period when we have the most chance of recognizing novel mutations.