Our evolutionary trajectory over the last three million years took us from small-brained walking apes who lived in East African grasslands to modern humans who have colonized just about every type of environment of every major land mass on the planet. So what's next? Are we still evolving? If so, have our culture and our technology changed our evolutionary trajectory? Using new genetic inventories of world populations, researchers are now tracing our recent evolutionary path in remarkable detail. They are discovering that our culture and our general restlessness as a species have had a big impact on our genetic makeup.
A human living in Africa 50,000 years ago wouldn't look out of place groomed and dressed up in a business suit, sipping coffee at a Starbucks in Manhattan. Yet while fully modern humans evolved in Southern Africa, a glance around a Manhattan Starbucks is enough to show you that human evolution has continued since we migrated out of Africa and settled the rest of the world: our stature, skin color, hair, eye color, and other facial features clearly show where in the world at least some of our ancestors lived. Modern humans began branching out into the Near East, Asia, Europe, and Australia by about 40,000 years ago, finally arriving in South America by 12,000 years ago. As our species colonized new environments around the world, we confronted new foods, new pathogens, and other new challenges posed by differences in sunlight, temperature, and altitude. Different populations around the world evolved in response to their unique environmental challenges; as a result, we differ from each other not only in our outward appearance, but also in the inner workings of our bodies. The effects of different evolved adaptations among humans in different parts of world can be seen today in the strong influence our ancestry can have on our health.
By moving around so much, we stir up the human gene pool and alter how evolutionary pressures act on our genes.
To get a better understanding of the changes in our recent evolutionary past, scientists have been looking under the hood at the genetic workings of those evolutionary changes. They're using large genetic inventories of different world populations, such as the Human Genome Diversity Project, to look for mutations that show signs of being actively promoted by evolution. Among the findings are mutations that cause lighter skin color in northern human populations. Lighter colored skin may have evolved in response to the need to maintain sunlight-activated vitamin D synthesis as humans migrated northward. Scientists have discovered different mutations in Europeans and East Asians that are responsible for the lighter skin color in these populations. Other studies have uncovered mutations responsible for straight hair in Asians and blue eyes in Europeans; the evolutionary basis for the short stature of “Pygmy” populations that live in the tropical forests of Africa, Asia, and South America; and the different genetic adaptations of Andean, Tibetan, and Ethiopian high-altitude societies to low oxygen levels that would make the rest of us sick. These changes may seem subtle when you consider what can happen over millions of years, but there is no question that humans have continued to evolve.
There is also no question that we've managed to influence the course of our own evolution. One of the biggest cultural changes we've undergone as a species has been to settle down into villages and cities, and support ourselves by raising crops and livestock. In the process, we've altered the evolution of our immune system and our metabolism. The clearest example of a diet-induced evolutionary change is adult lactose tolerance in dairy-consuming Europeans and African Maasai, a useful trait to have before the availability of Lactaid.
Our species' wanderlust has also had a profound impact on how we've experienced evolutionary change. Much of our genetic makeup is due to what geneticists call founder effects, meaning that our genes reflect the chance membership of the small band of colonists that we've descended from, rather than evolutionary pressure to adapt. The fact that Scots commonly have red hair, while Norwegians have blond hair is likely due to founder effects and not because red hair is better suited to the Scottish climate. Our long tradition of pulling up stakes and seeking our fortunes elsewhere has also had the effect of putting the brakes on natural selection in many cases. One research team studied the fate of seemingly favorable mutations worldwide and concluded that human "populations may be too mobile, or their identities too fluid" for advantageous mutations to spread completely through a population. By moving around so much, we stir up the human gene pool and alter how evolutionary pressures act on our genes.
The recent evolutionary changes studied by scientists all occurred well before a few game-changing developments that include antibiotics, vaccines, mass-produced food, fertility drugs, and online dating services. We've raised the odds that, in most areas of the world, children will live to adulthood and go on to have their own children. Does this mean that we've transcended the messy process of evolution and made ourselves largely immune to natural selection? Not quite—just because our children aren't eaten by predators or don’t succumb to childhood diseases does not mean that evolution has lost its power over our species. For the past 40,000 years, we've been adapting to the local environments that we've colonized; in the future, we will adapt to the social and physical environment we are making for ourselves. We'll face the uncertain new challenges of climate change, but we also continue to confront the questions of how to successfully choose a mate and whether and when to have children. More people are choosing to have children later in life or not at all, a choice that generally wasn't an option for most women not too long ago. The well-being of our children today depends less on the chance occurrence of a famine or epidemic, and more on the choices we make as parents. These kinds of decisions clearly influence whose DNA ends up in the next generation. Our future evolutionary trajectory depends on how billions of people resolve these choices over the next 40,000 years.