Why the human brain evolved as it did never has been plausibly explained. Apparently, not since the first life-form billions of years ago did a single species gain dominance over all others – until we came along. Now, in a groundbreaking paper, two Israeli researchers propose that our anomalous evolution was propelled by the very mass extinctions we helped cause. Or: As we sawed off the culinary branches from which we swung, we had to get ever more inventive in order to survive.
As ambling, slow-to-reproduce large animals diminished and gradually went extinct, we were forced to resort to smaller, nimbler animals that flee as a strategy to escape predation. To catch them, we had to get smarter, nimbler and faster, according to the universal theory of human evolution proposed by researchers Miki Ben-Dor and Prof. Ran Barkai of Tel Aviv University, in a paper published in the journal Quaternary.
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In fact, the great African megafauna began to decline about 4.6 million years ago. But our story begins with Homo habilis, which lived about 2.6 million years ago and apparently used crude stone tools to help it eat flesh, and with Homo erectus, which thronged Africa and expanded to Eurasia about 2 million years ago. The thing is, erectus wasn’t an omnivore: it was a carnivore, Ben-Dor explains to Haaretz.
“Eighty percent of mammals are omnivores but still specialize in a narrow food range. If anything, it seems Homo erectus was a hyper-carnivore,” he observes.
And in the last couple of million years, our brains grew threefold to a maximum capacity of about 1,500 cranial capacity, a size achieved about 300,000 years ago. We also gradually but consistently ramped up in technology and culture – until the Neolithic revolution and advent of the sedentary lifestyle, when our brains shrank to about 1,400 to 1,300cc, but more on that anomaly later.
The hypothesis suggested by Ben-Dor and Barkai – that we ate our way to our present physical, cultural and ecological state – is an original unifying explanation for the behavioral, physiological and cultural evolution of the human species.
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Out of chaos
Evolution is chaotic. Charles Darwin came up with the theory of the survival of the fittest, and nobody has a better suggestion yet, but mutations aren’t “planned.” Bodies aren’t “designed,” if we leave genetic engineering out of it. The point is, evolution isn’t linear but chaotic, and that should theoretically apply to humans too.
Hence, it is strange that certain changes in the course of millions of years of human history, including the expansion of our brain, tool manufacture techniques and use of fire, for example, were uncharacteristically progressive, say Ben-Dor and Barkai.
“Uncharacteristically progressive” means that certain traits such as brain size, or cultural developments such as fire usage, evolved in one direction over a long time, in the direction of escalation. That isn’t what chaos is expected to produce over vast spans of time, Barkai explains to Haaretz: it is bizarre. Very few parameters behave like that.
So, their discovery of correlation between contraction of the average weight of African animals, the extinction of megafauna and the development of the human brain is intriguing.
From mammoth marrow to joint of rat
To be clear, just this month a new paper posited that the late Quaternary extinction of megafauna, in the last few tens of thousands of years, wasn’t entirely the fault of humanity. In North America specifically, it was due primarily to climate change, with the late-arriving humans apparently providing the coup de grâce to some species.
In the Old World, however, a human role is clearer. African megafauna apparently began to decline 4.6 million years ago, but during the Pleistocene (2.6 million to 11,600 years ago) the size of African animals trended sharply down, in what the authors term an abrupt reversal from a continuous growth trend of 65 million years (i.e., since the dinosaurs almost died out).
When Homo erectus the carnivore began to roam Africa around 2 million years ago, land mammals averaged nearly 500 kilograms. Barkai’s team and others have demonstrated that hominins ate elephants and large animals when they could. In fact, originally Africa had six elephant species (today there are two: the bush elephant and forest elephant). By the end of the Pleistocene, by which time all hominins other than modern humans were extinct too, that average weight of the African animal had shrunk by more than 90 percent.
And during the Pleistocene, as the African animals shrank, the Homo genus grew taller and more gracile, and our stone tool technology improved (which in no way diminished our affection for archaic implements like the hand ax or chopper, both of which remained in use for more than a million years, even as more sophisticated technologies were developed).
If we started some 3.3 million years ago with large, crude stone hammers that may have been used to bang big animals on the head or break bones to get at the marrow, over the epochs we invented the spear for remote slaughter. By about 80,000 years ago, the bow and arrow was making its appearance, which was more suitable for bringing down small fry like small deer and birds. Over a million years ago, we began to use fire, and later achieved better control of it, meaning the ability to ignite it at will. Later we domesticated the dog from the wolf, and it would help us hunt smaller, fleet animals.
Why did the earliest humans hunt large animals anyway? Wouldn’t a peeved elephant be more dangerous than a rat? Arguably, but catching one elephant is easier than catching a large number of rats. And megafauna had more fat.
A modern human can only derive up to about 50 percent of calories from lean meat (protein): past a certain point, our livers can’t digest more protein. We need energy from carbs or fat, but before developing agriculture about 10,000 years ago, a key source of calories had to be animal fat.
Big animals have a lot of fat. Small animals don’t. In Africa and Europe, and in Israel too, the researchers found a significant decline in the prevalence of animals weighing over 200 kilograms correlated to an increase in the volume of the human brain. Thus, Ben-Dor and Barkai deduce that the declining availability of large prey seems to have been a key element in the natural selection from Homo erectus onward. Catching one elephant is more efficient than catching 1,000 rabbits, but if we must catch 1,000 rabbits, improved cunning, planning and tools are in order.
Say it with fat
Our changing hunting habits would have had cultural impacts too, Ben-Dor and Barkai posit. “Cultural evolution in archaeology usually refers to objects, such as stone tools,” Ben-Dor tells Haaretz. But cultural evolution also refers to learned behavior, such as our choice of which animals to hunt, and how.
Thus, they posit, our hunting conundrum may have also been a key element to that enigmatic human characteristic: complex language. When language began, with what ancestor of Homo sapiens, if any before us, is hotly debated.
Ben-Dor, an economist by training prior to obtaining a Ph.D. in archaeology, believes it began early. “We just need to follow the money. When speaking of evolution, one must follow the energy. Language is energetically costly. Speaking requires devotion of part of the brain, which is costly. Our brain consumes huge amounts of energy. It’s an investment, and language has to produce enough benefit to make it worthwhile. What did language bring us? It had to be more energetically efficient hunting.”
Domestication of the dog also requires resources and, therefore, also had to bring sufficient compensation in the form of more efficient hunting of smaller animals, he points out. That may help explain the fact that Neolithic humans not only embraced the dog but ate it too, going by archaeological evidence of butchered dogs.
At the end of the day, wherever we went, humans devastated the local ecologies, given enough time.
There is a lot of thinking about the Neolithic agricultural revolution. Some think grain farming was driven by the desire to make beer. Given residue analysis indicating that it’s been around for over 10,000 years, that theory isn’t as far-fetched as one might think. Ben-Dor and Barkai suggest that once we could grow our own food and husband herbivores, the megafauna almost entirely gone, hunting for them became too energy-costly. So we had to use our large brains to develop agriculture.
And as the hunter-gathering lifestyle gave way to permanent settlement, our brain size decreased.
Note, Ben-Dor adds, that the brains of wolves which have to hunt to survive are larger than the brain of the domesticated wolf, i.e., dogs. We did promise more on that. That was it. Also: The chimpanzee brain has remained stable for 7 million years, since the split with the Homo line, Barkai points out.
“Why does any of this matter?” Ben-Dor asks. “People think humans reached this condition because it was ‘meant to be.’ But in the Earth’s 4.5 billion years, there have been billions of species. They rose and fell. What’s the probability that we would take over the world? It’s an accident of nature. It never happened before that one species achieved dominance over all, and now it’s all over. How did that happen? This is the answer: A non-carnivore entered the niche of carnivore, and ate out its niche. We can’t eat that much protein: we need fat too. Because we needed the fat, we began with the big animals. We hunted the prime adult animals which have more fat than the kiddies and the old. We wiped out the prime adults who were crucial to survival of species. Because of our need for fat, we wiped out the animals we depended on. And this required us to keep getting smarter and smarter, and thus we took over the world.”