People–A New Animal

In the history of our planet, there has probably never been an animal like us.  I really, really mean never.  You probably think I mean our intelligence, perhaps our ability to envision a deity, something no other animal can really do, I suspect.  On the contrary, I mean that we are the most voracious carnivore to walk, or rather trot, the planet Earth.

Our lineage didn’t start out as carnivores.  Hominids’ ancestors used to be omnivores, tending to eat a mixture of foods.  According to Rick Johnson and collaborators, in the past 30 million years we acquired a gene that made us good at putting on a bit of fat gorging on seasonal fruits and berries as the world become colder, less tropical, and more temperate; fruits became seasonal.  A recent review article by Sayer and Lovejoy illustrates the competing ideas and theories regarding our diet in the past few million years.  However, I think the evidence points to a new predator that arose perhaps 2.6 million years ago.

Genus Homo, starting with Homo erectus, got up on two legs and ran.  Not fast, but far.  We lost the thick hair other mammals have and gained lots of sweat glands.  We acquired the shoulder anatomy to throw hard.  Our brains got better at cognition, while out muscles got weaker, albeit better at sustained endurance efforts.  We became very, very good at mentally modeling the world and communicating– what we call intelligence.  These are strange traits.  What evolutionary purpose do they serve?

Our lineage wasn’t strong or fast.   We didn’t have sharp teeth.  But we could chase any grazing animal, the herbivores, until that creature collapsed from exhaustion.  We could sweat better, trot longer, and imagine further than anything.  The prey might have gotten out of sight momentarily, but we could follow until it can’t move any further.  Then we could get close enough to throw something sharp, drawing blood until the animal died.  We often worked in groups, taking prey much larger and stronger than we were.  When we caught and killed the prey, we ate the high fat portions of the kill first, often from very large carcasses which had proportionally larger fat stores than extant species.  Our diet became more like the big carnivores.  Our metabolism adapted to that regime, using the fat preferentially, since we weren’t getting much carbohydrate from a kill.  We got very good at making the fat from the kill, and our internal stores, into ketone bodies, small molecules that can feed into our ATP-producing energy systems, the mitochondria, just as well as the breakdown products of sugar molecules.  Keto-adaptation is an ancient metabolic state.  In times of starvation, all animals must rely on their fat reserves, going into ketosis to use the fat for normal metabolism in place of glucose.  Researchers have shown that mice can go into ketone adaptation, although I’ve been told the mechanism is different than in humans.  I want someone to study it more carefully in people to learn if we’re better at it than other primates.  If we are, that could be added to the list of persistence hunting adaptations.  Anyone got a lab and maybe primates handy, drop me a line.

When genus Homo appeared, the big carnivores were driven to extinction, out-competed.  When Homo sapiens appeared, we co-existed with other species like Neanderthals and H. erectus for tens of thousands of years, although it looks like we stayed in Africa for most of that time. About 60,000 years ago, we almost went extinct, which caused intense selective pressures on our species.  We came out of that an even more effective predator, spreading across the planet.  One interesting theory is Pat Shipman’s that we teamed with a wolf variant –proto-dogs, who helped us in the colder climates where our amazing heat tolerance wasn’t an advantage relative competing predators, the realm of the Neanderthals, the non-partner wolves, and some large ferocious cat and bear species.  Starting about 50,000 years ago, we expanded out of Arica across Eurasia, driving another cohort of competing predators to extinction, including Neanderthals.  By 15,000 years ago even many of the prey species, like mammoths, were driven to extinction, although that may have been a mix of climate change and human predation.  Over time we adapted to other foods, eventually beginning to scratch the ground and plant things; agriculture.  Jared Diamond has called this the “Worst Mistake in the History of the Human Race,”  but he missed the point that we probably didn’t have a choice.  We got smaller and less healthy, but more numerous.

Here’s a video showing those who still practice persistence hunting–

Although your ancestors may have been eating more plants for the past 10,000-12,000 years, for 2.6 million years before that, their dominant activity was hunting, supplemented with some gathering, punctuated by seasonal gorging on fruits and berries.  I’m sure there’s been some selection for adaptations to a more plant-based diet in at least some human populations.  I’m sure that adaptation process is far from complete.  That’s also not the same thing as the recent appearance of sugar, which your brain and body can respond to by gorging on a grand scale.  Your body still knows to make that sugar into fat, although it gets overloaded in many ways so you can become both fat and sick.  When you stop eating carbohydrates (sugar) and eat fat instead, your body still knows you’re in hunting season, lean and mean.

Where do you want to be?

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