A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Has human evolution stopped?

November 4, 2016

Dear Cecil:

Thinking about current events, I often wonder: Is it possible that our species has entered a stage of devolution, or at least that we stopped evolving thousands of years ago?

Cecil replies:

I know it’s easy to see humanity in an unflattering light these days, now that the most pea-brained amongst us can impulsively jab dim musings into their phones to share with all the virtual world. But you’re hardly the first to suspect that our species is slaloming downhill into a genetic sewage tank. Barely had the scientific community accepted evolution in the first place when some of its leading lights started worrying that natural selection might cease to affect humans, or even throw us into reverse gear. Their concern, though, was needless — just like yours is.

Let’s back up to review Darwinian theory at its most basic. If you, an organism, are the lucky possessor of some inheritable trait that boosts your relative chances of thriving in the environment you occupy, that trait will tend to be passed along to your fortunate offspring, and to theirs and to theirs. But, the Lees of the world have long worried, what if humans have made our environment so uniquely cozy for ourselves that basically everyone thrives? What if, thanks to advanced medicine and other forms of coddling, all the negative traits that once led to genetic dead ends no longer lower our likelihood of surviving and spawning? Surely that points to a future of sluggish dullards communicating solely in emoji, right?

Hardly. Natural selection is still affecting human development — very slowly. We mammals take our own sweet time evolving compared to fish or lizards, and humans average a leisurely 20 years between generations, Still, even within recent history (evolutionarily speaking), our genes have adapted to our changing circumstances, particularly to the advent of agriculture and animal husbandry, not to mention the discovery of fire. In the past ten millennia, our skulls have rounded, our facial features have thinned, and our jaws, adjusting to the softer food we eat, have shrunk. There have been downsides — the changes in our jaw and larynx structure beginning 300,000 years ago may have led to sleep apnea. But if you can drink a milkshake without doubling over in gut pain, thank natural selection — lactose tolerance is a late addition to humanity’s bag of digestive tricks.

Our brains, it seems, continue to evolve: key variants of two genes that influence brain size, MCPH1 and ASPM, showed up in our pool only about 37,000 and 5,800 years ago respectively, and they continue to spread through humanity. And though “Should I eat this berry?” is hardly the life-or-death question it used to be, other environmental factors remain in play, particularly among specific populations: Tibetans’ lungs and blood have adapted to the low-oxygen atmosphere of the Himalayas, while a genetic resistance to malaria may be developing in sub-Saharan Africa.

In fact, more than two dozen human genes — including ones linked to speech, cognition, and defense against disease<>have been identified as still evolving today. Humans may already be developing resistance to HIV and other viruses. And women may be evolving more significantly than men. Working from almost 60 years of data from a major multigenerational study of cardiovascular disease, the authors of a 2009 paper project that the next generation of women in the study population will be slightly shorter and stouter on average than the preceding cohort, with lower cholesterol levels and systolic blood pressure, and an increased period of fertility — starting about a half a month earlier and ending a month later. Not as flashy as growing wings or tusks, certainly, but remarkable nonetheless.

Our environment hasn't stopped changing either — much of this our own doing, of course — and it's sure to pitch us a curveball or two in the coming millennia. Beyond whatever we’ll have to adapt to on a hotter earth, attempts to survive in space or colonize another planet could amp up the evolutionary process. Travelers on space flights are exposed to heightened levels of chromosome-damaging radiation, and without some serious shielding future dwellers on the lunar or Martian surface would receive doses dozens of times greater than the terrestrial going rate. Off-earth life could gradually transform our bodies in other ways too. Despite regular workouts while aloft, astronauts returning from the International Space Station have shown significant bone loss in their femurs; it may be that long-term existence in zero gravity would cause our legs to dwindle.

Evolution isn’t the only force at work on how humans develop, though. We’re not just a species that reshapes its environment — through medical science, we’ve also become a species that controls how it adapts to that environment. If we haven’t quite conquered death, we’ve lowered infant mortality rates drastically and continue to extend age expectancy. And every year researchers redraw the frontiers of prosthetic and implant technology: the average healthy denizen of 2316 could well be tricked out with so many nifty cyborg accessories that our current conception of the human body may no longer apply. But I’m confident that doomsayers will still find cause to complain that this new generation of post-humans is the dumbest bunch yet.

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