Natural selection Still acting on humans

Because humans can alter the environment to suit ourselves, we sometimes assume that we've stopped evolving, but we haven't. Natural selection has continued to act, increasing the frequency of advantageous genes. In fact, the very changes we make can select for evolutionary changes. For some examples, read on.

Of lice and men

If you always suspected that humans were special in some way, here's another bit of evidence to prove that you're absolutely right: Although most species of mammals have at most one kind of louse, the human species has three! This fact actually says quite a bit about human development.

Humans have head lice, adapted to hanging onto hair; body lice, adapted to hanging onto clothing; and pubic lice which like the nether regions. An analysis of the amount of genetic difference between the head lice and the body lice suggests that they diverged approximately 100,000 years ago, which gives scientists an approximate date of origin for tight-fitting fabric clothing — the sort body lice adapted to attach to. In the absence of clothing, lice were confined to hair, but when humans began to wear clothes, lice were able to spread to other regions of the body. Because the selective forces were different in each environment — head versus body — speciation between head and body lice eventually occurred.

So what about the third type of human lice: pubic lice? As it turns out, human pubic lice are closely related to gorilla body lice. They're divergent enough from gorilla lice, however, for scientists to be able to say that the transfer from gorillas to humans predated the evolution of body lice (and, therefore, the development of clothing). This finding suggests that humans lost their body hair (and, hence, had separate head and pubic-hair regions that different species of lice could colonize) long before they developed clothing.

Bottom line? Lice tell a story. If you have hair all over your body, you have only one kind of lice. Separate the hair into patches, and you can get lice adapted to each different patch. Cover some of the hairless area with a substance like clothing, and you can get diversification and speciation as one of the lice species (it just happened to be the one on the human head) radiates into this new habitat.

Oh, and by the way, lice are continuing to evolve. If you've ever had to deal with head lice and couldn't get rid of the buggers, you already know that many of them are now resistant to the chemicals we use to eradicate them.

The evolution of lactose digestion

None of our primate relatives can digest lactose — the sugar found in milk — as adults. Infant mammals need to be able to digest milk, but historically, they haven't needed this ability after weaning. By domesticating dairy animals, however, humans altered the environment in a way that selected for evolution of lactose-digesting ability in adults.

Human lineages that are associated with dairy farming have a much lower level of lactose intolerance than do lineages that aren't. In Africa, for example, the Nigerian Yoruba, an ethnic group in Nigeria, are 99 percent lactose intolerant, whereas cattle herders in the southern Sudan are less than 20 percent lactose intolerant.

Human disease and population density

The evolution of human disease is strongly affected by human population size. And large human populations became possible with advances in food production, namely the advent of farming. As human population density increased, human pests and parasites thrived, and humans evolved increased resistance to them. So in large populations, humans have developed built-in protection against the organisms that would do them harm.

But not all humans developed this resistance — just the ones living in high-density environments. Human populations living in low-density environments are relatively free of virulent pathogens. Small populations can't sustain infection, because everyone can be infected at the same time, and survivors of the disease become immune. The result? The pathogen goes extinct. If no one in the small group survives (in small groups, less chance exists that resistant individuals are present), the pathogen still goes extinct. As a result of the lack of constant exposure, small populations don't evolve resistance to disease. In a large population, on the other hand, new sensitive individuals are always being born.

When humans from large populations come into contact with humans who have traditionally lived in small groups, those from the large populations pass along diseases that can be far more serious for the people in the small populations. Tragic examples are the debilitating effects of disease on native populations in the Americas after contact with European explorers. The risk continues to this day when people from large populations come into contact with small groups in the Brazilian rain forest and other isolated locations.

Think about smallpox. Back in the day when smallpox was rampant, somebody always had it. (If at any instant no one was sick with smallpox, the virus would've gone extinct.) In a large population, new susceptible individuals were always being born, so the virus always had a refuge of new people to infect. Although smallpox was a dreaded disease, people in Europe, Asia, and

Africa were relatively resistant to it. Only 25 percent of people who contracted it died — a big number, true, but 75 percent of infected people survived. Compare that outcome with the experience of smaller populations, particularly in the New World.

The Americas were colonized via the Bering land bridge at a time preceding the development of agriculture and large population sizes. Movement to the New World would have been in small groups of hunter-gatherers — groups too small to maintain infectious diseases such as smallpox. As a result of the relative freedom from European diseases in the New World (an accident of history), human populations in the Americas didn't evolve resistance to these diseases and were devastated by them upon coming into contact with European explorers and colonists.

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