It’s always intrigued me how apparently intractable puzzles emerge in science, then disappear again without any new evidence being found.
What’s changed, instead, is the pattern into which we’re trying to slot the evidence – the ‘organising principle’. It’s an innate human thing: we always look for patterns. And the pattern then becomes reality, often at the expense of evidence that refutes it – a cognitive flaw that includes the so-called ‘clustering illusion’.
Take the issue of the Cro Magnons, for instance. These Ice Age Europeans of around 40,000-odd years ago had brains some 10-15 percent larger than the modern human average. I’ve seen claims that this means we are ‘evolving backwards’. The argument flows from the misconception that for humans, ‘evolution’ is a directional process involving automatic increase in brain size.
Of course it’s nonsensical – it’s like saying that elephants evolve directionally by getting bigger ears, making African elephants more ‘advanced’ than Indian (see what I mean by ‘silly’). Despite spawning a great band name, there is no such thing as ‘de-evolution’. There is only change through time. But the idea of directional human brain-gain was certainly held, even by paleo-anthropologists, in the early twentieth century, and it’s still popular today outside the science community.
Needless to say, there’s no evidence whatsoever that humans automatically got bigger brains as time went on. The whole conceit was an extension of nineteenth century phrenology and the idea that people with bigger heads were probably smarter. That’s just not true. Anatole France (1844-1924) had a very small head with a brain of just 1000 cc, yet was a genius.
So what was happening? Our current shape from the neck down (‘post-cranial morphology’, since you ask…) likely emerged about 1.8 million years ago, when we also began using tools.
The most common humans of that day – Homo erectus – were a widely variable species who had brains of around 750-1250 cc, which is the same average size as Anatole France’s and – at the top end – well within the current modern range. Their brains were much larger than the successful pre-Homo family, Australopithecus, upright-walking Pliocene apes with brains the size of chimps’. Homo erectus – which survived until just 70,000 years ago, alongside other types of human – were typical of a new upright-walking, hairless and very smart group of species which had experienced a substantial one-time jump in average brain size on the back of tools and fire. The current hypothesis is that the shift – also known as ‘punctuated evolution’ – was driven by an adaptive ‘feedback loop’ in which tool use demanded better command-and-control circuitry, coupled with the advent of cooking that altered the food-energy calculation.
Since then, human brain size has edged upwards – a possible outcome of the same process, ongoing – but broadly been in the same league. Don’t be fooled by the sloping foreheads and brow-ridges of classic ‘cave man’ species. If we line ourselves up with our extinct cousins (including H. Sapiens idaltu, which had brow ridges), we’re the odd ones out with our baby faces (‘neotony’).
The evidence actually indicates that our extinct cousins’ noggins fell within overlapping ranges that were largely within the modern spread. Heidelberg Man – thought to be our direct ancestor, had a typical cranial capacity of around 1280 cc, which is about 10 percent smaller but generally within our own range. Our close cousins, the Neandertals (also a descendant of Heidelberg Man), had significantly bigger brains than ours. We haven’t found Denisovan skulls yet, but we know they were genetically closer to Neandertals than we are.
The thing is, we can’t draw generalised conclusions from the tiny number of samples we have. Modern humans have widely varying brain sizes – it’s to do with physical size; smaller people typically have smaller heads, bigger people typically have bigger heads. It doesn’t affect intellect. But we don’t have the luxury of being able to analyse whole populations of extinct humans – all we have are what, statistically, amount to point-samples.
See what I am getting at? That clustering illusion. And Cro Magnons were physically enormous – typically over 6 feet tall, meaning they were going to have proportionately larger heads. Other ‘modern’ human specimens from the same period had smaller brains – but were also physically smaller in general. As we’d expect.
We are now fairly certain that intellect has more to do with internal circuitry than raw capacity – something not preserved in the fossil record even at the broad structural level possible with endocasts of the fossil remains. Once humans had made the jump to brain capacity approaching modern size, intelligence around our level was possible, and I suspect there probably wasn’t too much to choose between the types. Current paleo-anthropological ‘establishment’ thinking is that our survival, alone as the sole human species, wasn’t due to superior intellect. Lest there be any doubt, the ‘Hobbits’ of Flores Island had the cranial capacity of a chimp, but were clearly tool-using, fire-making humans whose heads were proportionate to their diminutive bodies.
All these things stand against the simplistic early twentieth century ‘organising principle’ of human evolution being an automatic brain-scale gain. As usual, when we dig into it, the evidence paints a much more complex – and also more interesting, and very cool – picture than we previously thought. And that’s generally true of a lot about science, not just the human story.
Copyright © Matthew Wright 2016