Yet another scientific myth has been deflated: No, human teeth did not become smaller because our brains grew bigger, we learned to cook and didn’t have to chew on raw hippo anymore. It is true that throughout our evolution, our brains grew bigger and our molars grew smaller. But our brains and teeth did not co-evolve, dental anthropologists have now demonstrated.
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The popular theory had been that as the human brain evolved and we invented stone tools and cooking, we expanded and improved our diet. Thus we did not need to chew so hard, so our teeth co-evolved, diminishing in size.
But now it emerges that the increase in brain size and contraction of tooth size differed in pace and extent from species to species, as scientists show in a seminal paper, entitled, “Brain enlargement and dental reduction were not linked in hominin evolution,” published in PNAS, the official scientific journal of the U.S. National Academy of Sciences. Even within a given species, the brain and the post-canine teeth evolved at different rates, the researchers explain.
Among the human ancestors that George Washington University Prof. Aida Gómez-Robles and colleagues studied were species of Australopithecus, Paranthropus and Homo.
In early Homo, the brain increased in size before the teeth began to grow smaller, they found.
In Neanderthals (studied by others), it went the opposite way: Substantial dental reduction preceded the expanded brain possessed by “late” Neanderthals (the "early" ones are believed to have split from Homo somewhere between 700,000 to one million years ago; “late” refers to Neanderthals shortly before their extinction).
In fact, sustained rapid evolution in brain size started before the Homo and Paranthropus australopithecines separated over 3 million years ago, Gómez-Robles and her team found in their study.
Small brain, big tool
The phenomena of big brains or small teeth are not necessarily associated with advances in tool technology either. Other research has shown that stone tool manufacture and use significantly predated the increase in brain size observed in early Homo.
In 2015, crude but unmistakable knapped stone tools dating back 3.3 million years were found in Kenya, well before modern humans were a gleam in some ape’s eye. This is not some aberration: A massive field of such artifacts was found. Also, tools clearly worked by hand, dating 2.6 million years, have also been found in Ethiopia, although cutting tools and axes would take some more time to emerge; the earliest known ones are about 1.8 million years old.
That said, tool technology began to seriously accelerate with the advent of Homo sapiens some 200,000 years ago. (These prehistoric stone tools are not comparable to tools used by capuchin monkeys or chimps or crows, for instance. Animals are known to have done some refining of their tools, but not to the extent that the primitive people did.)
Yet the evolution of our dentition did not correlate with our technological prowess. Tooth size evolved at a relatively consistent neutral rate across hominin species – which means that the changes were not necessarily driven by natural selection, but by genetic drift.
On the other hand, first of all, brain size evolved at different rates in different hominin species, a heterogeneity that cannot be explained by a “neutral model.” Put otherwise, the changes in brain size cannot be explained by genetic drift.
The bottom line is that it ostensibly made sense that our consumption of softer, cooked food – which we figured out how to prepare thanks to our bigger brains – led to diminishment of tooth size. Recent research results indicate that different ecological and behavioral factors influenced the evolution of hominin teeth and brains, the scientists explain.
While about it, the team looked at the evolution of brain organization – meaning, the evolution of different parts of the brain. Their findings support the long-standing theory that brain organization (as inferred from skull shape) evolved independently of brain size.
Based on statistical tests, in most species, changes in skull shape were governed by genetic drift – and the kind of rapid change that isn’t statistically random was seen only along the evolutionary branch leading to modern humans, from our last common ancestor with Neanderthals.