Microevolution: Part One
At some point, we have all seen the picture that has come to represent human evolution. It usually starts with the Australopithacus. Then comes the Homo Habilus, on to the Homo Erectus, then Homo Neanderthalensis (commonly, Neanderthals), and then us: Homo Sapiens.
The graphic gives human evolution the guise of being neat. In the beginning, we were slightly hunched over, with small brains. Then, our brains grew bigger and we became more upright — a linear progression over hundreds of thousands of years that culminated (note the past tense) in our unique species.
However, we are far from finished products. We are not done evolving.
That’s the idea behind microevolution, at least.
Human microevolution concerns itself with the observable changes that happen to our organism over a span of a few thousand years. (For perspective, macroevolution deals in the millions of years.)
There are certain protocols to genetic evolution. A generation for Homo Sapiens happens once every 25-35 years, a rate that means that it can take up to a thousand years for an advantageous trait to spread throughout the population.
Take lactase persistence, for example. Prehistorically, humans could not digest milk past infancy. Once weaned, we had genes that turned off the milk-digesting enzymes. Then (about 9,000 years ago) we had the idea not only to hunt animals, but also to herd them. Drinking nutrient-rich milk became advantageous for a variety of reasons that evolution does not care about. Evolution cares about surviving.
So what did our genes do? They developed an alteration to produce the enzyme for our entire lives. They passed the gene down from generation to generation at a rate of 10 percent—speedy, by evolutionary standards. A 2011 study shows that in places such as the British Isles, Scandinavia, and parts of West Africa, up to 95 percent of modern-day population have this “LP” enzyme. You scream, I scream, we all scream for ice cream!
In Southeast Asia, the Bajau are people uniquely adapted to their environment. Otherwise known as “sea nomads,” they fish by free-diving at depths up to 230 feet (almost as deep as a football field is long!). They walk, spear in hand, on the sea floor the same way you walk across your living room, smart phone in hand. They hold their breath for minutes at a time.
Blood-work from a 2016 study revealed that the Bajau have a higher concentration of PDE10A, a gene responsible for the production of hormones in the thyroid gland. In rodents, these hormones are associated with an increased size of spleen. And the Bajau, indeed, have an above-average sized spleen — 50 percent larger, in fact, than the land-based Saluan and Han Chinese groups that also took part in the study.
The spleen is a reservoir for oxygenated red blood cells. “When mammals hold their breath, the spleen contracts, expelling those cells and boosting oxygen levels by up to 10 percent,” says Ed Yong of the Atlantic.
This is the interesting part: the larger spleens were found across the Bajau population, even among the non-divers. The spleen size, then, was not the result of a training regimen of certain individuals — it was natural selection at work, picking out useful genes and passing them down through generations.
A Sherpa, climbing Everest. From the BBC
Most people struggle carrying refrigerator-sized objects at sea level. Now imagine doing it at 20,000 feet! Heck, just breathing can become a task at altitude for the unacclimated visitor.