Wednesday, August 3, 2016

What evolutionary pressure drove human evolution?

Humans evolved a number of characteristics that differentiate them from other apes (less hair, shorter gut, weaker muscles, complex language, complex tool use, walking upright, dexterous hands, cooperation capable of larger tribes). What was the evolutionary pressure driving our unique evolution? Giraffes feed on leaves too high for other animals to reach, driving their long necks.  And thanks to the pressure of using dams/ponds for protection, the beaver's teeth wear easier in the back than the front, and are constantly growing, which means their chisel-like teeth self-sharpen with heavy use cutting down trees. Can we point to a similar niche that could explain the multitude of unique human adaptations?

To answer this question, let's start with determining the core evolutionary breakthroughs that led to modern humans?
It would seem to come down to two principal innovations: Complex tool use and Complex Language. I suspect most of the other adaptations depend on these two innovations. For instance, we can afford to have short/weak intestines because we mastered fire (tool use) which provides food that's easier to digest. We don't require (lice-prone) hair to cover our whole body because we can use animal skins and shelters for warmth (tool use). We are weaker and slower than other apes our size (and require ~50% less calories than Neanderthals) because we use tools, language, and cooperation for defense and hunting and can therefore get by with weaker bodies. We walk on two legs presumably so we can carry tools with our hands. And we form much larger groups than other primates, and are rather good at cooperating within our tribe, which is also predicated on language (without language you can't have such large groups- gossip, reputation, rules, religion, agreements, are necessary to motivate individuals to correct behavior and maintain cohesion in large groups, and all these depend on language). So I think to understand what was the driving force behind human evolution, we can focus on what sort of niche drove language and tool use.

To answer this question, note the following facts:
-Homo Erectus and Neanderthals hunted meat, including large mammals for food (but also ate cooked vegetables based on isotope analysis of bits of food on Neanderthal teeth!)
-Neanderthals were present in Europe for ~600,000 years and went extinct around the time of the arrival of humans in Europe 40,000 years ago 
-The quaternary extinction event is the widespread extinction of appx 173 large mammals (over 70% of large mammal species outside of Africa) that began around the time humans arrived on each continent (it was less dramatic in Africa likely because the mammals had time to evolve fear of humans as humans' hunting skills progressed). 
-In Europe, large mammals survived alongside Neanderthals for 550,000 years, but many quickly succumbed to the arrival of humans
-Thus humans were apparently quite skilled at hunting large mammals, apparently way better at it than the Neanderthals: In fact, if you accept that humans were the cause of the quaternary extinction event, you'd have to conclude that humans are the best hunters on Earth. I doubt any other predator in the history of Earth has caused so many extinctions.
-What makes us successful hunters? Tool use (i.e. spears, arrows (sometimes with poison-tips), traps, dogs, etc.) and complex cooperation and planning for the hunt (i.e. let's start a brush fire that pushes animals off that cliff, or let's all surround this herd of horses and on my signal slowly enclose them, spearing them as they escape).  I'd guess a group of 50 well-coordinated spear-wielding humans can kill pretty much any mammal they please. 

So, in conclusion, I'd guess that much of our unique characteristics evolved as a result of hunting.

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