Among biologists and other scientists, the debate about evolution doesn't center on whether we descended from apes. They're agreed on that—now, they've focused on the finer points of how we might still be changing as a species.
When we think of evolution, it's usually survival of the fittest, the law Charles Darwin outlined in his 1859 tome Origin of the Species. Of course, that doesn't mean the winner of the CrossFit games is crowned king. Rather, it refers to genetic fitness, the fact that people with the genes best suited to a given environment live long enough to have children and pass their DNA along, points out Sharad Paul, an evolutionary biologist and author of The Genetics of Health.
Right now, we are in a period of relative genetic stability, says Nathan Lents, a molecular biologist at the John Jay College of Criminal Justice and the author of Not So Different: Finding Human Nature in Animals. Most people live long enough to have as many kids as they want, and those who die early typically do so from non-genetic causes like accidents.
But that doesn't mean we aren't still changing, even if it's so slow we don't realize it. "We are continuing to evolve, albeit at a slower pace than we notice," Paul says. "Over the past 50,000 years our migration and diets have shaped our stature, skin color, and more. It only seems we are not evolving because we tend to look at this concept as fitting within our lifespans."
What's more, natural selection isn't the only way evolution can occur—it's just the best known. Random changes that don't provide a survival advantage also play a big role. It's a phenomenon called genetic drift, and it looms even larger when populations shrink to a smaller size following a dramatic event—say, nuclear warfare, natural disaster, or a dramatic climate change. Survival in these cases has more to do with luck or geography than genes, and whatever traits these fortunate few happen to possess will then be passed on to future generations, Lents says.
Given current events, our rate of evolution might pick back up sooner rather than later, he says. In the meantime, scientists point to these signs we're still shifting, if subtly.
We're beating diseases.
Tweaks to our DNA may play a role in reducing the risk for infectious diseases. For instance, researchers have identified dozens of variants that help confer resistance to malaria—which kills many children—that are becoming more common in African populations.
We're also using genetic testing to detect, and eliminate, certain genetic conditions. For instance, testing for a serious condition called thalassaemia, which affects levels of hemoglobin in the blood and causes a type of anemia, has cut the number of new cases by as much as 95 percent in some regions. Eventually, testing could have a big impact on diseases like Huntington's disease, as more people make reproductive decisions with knowledge of whether they carry certain mutations, Lents says.
In certain native populations, such as Pacific Islanders in Papua New Guinea, people may also be evolving to resist type 2 diabetes. Rates soared when tribes that previously had little exposure to outside contact met Western-like calorie-rich diets and sedentary occupations. "People died before reproductive age, or at reproductive age, and took their genes with them when they did it," Lents says.
Meanwhile, those who had a genetic advantage—for instance, having a certain mutation that affects metabolism—lived longer and passed those advantages on. "These situations are few and isolated—but these special situations are interesting from a population point of view, because they show us how quickly evolution works," he says.
We can thrive at high altitudes.
The higher the climb, the harder it is to breathe, thanks to lower levels of oxygen in the air. Your body's immediate reaction is to increase your breathing and heart rates (or develop altitude sickness). After a few days or weeks, you produce more red blood cells, which transport precious oxygen.
That's not evolution, which requires an actual change to your genetic code, Lents points out. And it also may not confer a survival advantage in the long term. Too many red blood cells thicken the blood so much it causes problems for pregnant women, slowing the growth of developing fetuses and increasing the risk babies born in the mountains will die soon after birth.
However, scientists have identified differences in the DNA of Tibetans, who live at altitudes topping 13,000 feet, that they believe do represent evolutionary shifts. A snippet of genetic code called EPAS1 encodes a protein that helps the body detect oxygen and better regulate the production of red blood cells. And it's much more common in Tibetans than in Han Chinese, close relatives who live in the lowlands.
We're getting nicer.
As of yet, the part of our gray matter that remembers phone numbers and directions hasn't completely atrophied—believe it or not, it is actually possible to survive and even reproduce without an iPhone. But our brains actually have been shrinking since we started walking upright and gained consciousness—to the tune of about 10 percent over the past 20,000 years.
Though no one's sure exactly why, Lents says brains are energy-hungry—about 20 percent of your daily calories actually go to your noggin. If genetic mutations could produce brains that were smaller and more efficient but still worked well enough to master the skills of survival, it makes sense that those who had them would live longer in the times of scarcity and famine characteristic of our not-so-distant past.
Brain size isn't 100-percent correlated with intelligence (if so, sperm whales would rule the world). But they do have one perk: They seem to make us less aggressive and more cooperative. Researchers at Duke University, among other places, have compared the brains of domesticated animals to those of their wild relatives and found a 10 to 15 percent reduction. In fact, some believe we've essentially domesticated ourselves, creating evolutionary pressure to communicate and collaborate that's helped civilization thrive.
We're different colors.
The fact that populations with roots in different parts of the world have different skin tones represents a highly visible sign of evolution, Paul says. As our ancestors marched from Africa to Europe, their skin lightened as a way of boosting vitamin D production in less-than-tropical climates.
When man (and woman) traveled south to sunnier lands, their skin darkened. This may have occurred because UV rays break down folate, an essential nutrient that prevents birth defects and help our body's cells divide, Paul says. Recent analyses comparing genes of prehistoric skeletons with those of Ukrainian people living today found evidence mutations that favor lighter skin—including variants of a gene called TYR—are about eight times as common in modern times as they were 5,000 years ago.
We're becoming intelligent designers.
We're not yet manipulating our genes with enough dexterity to affect the next generation. But thanks to technologies like the gene-editing technique CRISPR, we might soon be able to create a form of "unnatural selection" that could do everything from eliminate genetic diseases to create designer babies to halt the aging process, Lents says.
Whether this truly represents evolution might be a matter of debate—Paul says he's not sure it does, though he fears the implications of us creating artificial intelligence that surpasses our own. But Lents feels differently. "If we go in there and slice and dice our genome a little, that will change the genetic complement of the next generation, so yeah, it's evolution, as far as I'm concerned," Lents says. He's hopeful that doing so can bring on more beneficial changes, such as an even lower risk of genetic diseases, provided we continue to discuss and adhere to ethical guidelines along the way.
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