Introducing the brain: why so big? (Introduction)

by David Turell , Thursday, July 11, 2024, 21:43 (133 days ago) @ David Turell

A huge article tries to explain its size in Darwin theory terms:

https://www.newscientist.com/article/mg26334991-100-why-did-humans-evolve-big-brains-a-...

"Over the past two decades, we have learned that small-brained hominin species survived on Earth long after big-brained ones appeared. Moreover, evidence is growing that they were behaviourally sophisticated. Some, for instance, made complex stone tools that could probably only have been fashioned by individuals with language.

"These discoveries turn the question of human brain evolution on its head. “Why would selection favour big brains when small-brained humans can survive on the landscape?” says DeSilva. Neural tissue consumes lots of energy, so big brains must surely have brought benefits to the few species that evolved them. But what?

***

"Surprisingly, the sorts of changes that drove this expansion could also explain a more recent 10 per cent reduction in human brain size. What’s more, this suggests our brains may shrink further still.

***

“'But we do need another explanation because, at some point, you see that body mass is not really increasing in ancient humans, but brain size is increasing a lot.”

***

"Researchers rarely questioned the idea that big-brained humans were smarter and evolutionarily superior to their small-brained ancestors.

"Then came some extraordinary discoveries that undermined this assumption. It began 20 years ago when researchers in Indonesia discovered Homo floresiensis, a 1.1-metre-tall human with a tiny, 425-cubic-centimetre brain. It survived until astonishingly recently – about 50,000 years ago, according to current estimates. This means that long after our species appeared more than 300,000 years ago, there were small-brained humans on Earth. And H. floresiensis wasn’t alone. A further discovery around five years ago revealed another small hominin, Homo luzonensis, survived in the Philippines until around the same time. Then there is the diminutive Homo naledi. Discovered in 2013, it had a brain volume of no more than 550 cubic centimetres – literally pint-sized – but lived in southern Africa alongside our big-brained species until at least 235,000 years ago.

***

"If these discoveries weren’t remarkable enough, we now have signs of something even more astonishing: small-brained hominins seem to have been capable of sophisticated behaviour. A striking example of this comes from a site called Gona in Ethiopia. H. erectus living there about 1.6 million years ago produced “Acheulean” stone tools. These artefacts, which include teardrop-shaped hand axes, are so difficult to make that experiments suggest it would have required at least rudimentary language to teach and learn the skill. Given that, you might assume the individuals at Gona had large brains. But a 2020 study revealed that at least some of them had brains that were less than half the size of those of people today. (my bold)

"Last year brought a similar revelation. Before hominins made Acheulean tools, they made simpler – but still quite complex – “Oldowan” tools. It has long been thought these were largely the handiwork of H. habilis, a species with a brain volume of around 550 cubic centimetres. But at a site called Nyayanga in Kenya, researchers reported finding Oldowan tools up to 3 million years old in association with fossils of Paranthropus, a hominin with a brain volume as low as 450 cubic centimetres. Despite its small brain, says Beaudet, Paranthropus may have made Oldowan tools.

***

"H. naledi, they argued, had used torchlight to carry dead individuals into a deep burial chamber, the walls of which they had decorated with etchings. For most researchers, this is a step too far. “I don’t believe naledi could bury its dead or make engravings in caves,” says Gerhard Weber at the University of Vienna in Austria. Nevertheless, the finds at Gona and Nyayanga suggest that hominins didn’t require an overabundance of neural tissue to behave in complex ways. This makes the evolution of large brains a bit of a head-scratcher.

"Adding to the puzzle is the fact that big brains carry some clear disadvantages. They are demanding to run: ours consume around 20 per cent of our daily energy intake despite accounting for just 2 per cent of our body mass. Moreover, a baby with a large brain is tricky to deliver and to raise. “Childbirth is difficult,” says DeSilva. “And when you’re trying to feed this infant with its growing brain, it’s an energetically exhausting endeavour.”

"Nonetheless, hominin brains did evolve to be larger over time. At a conference of the European Society for the Study of Human Evolution in Denmark last year, Thomas Püschel at the University of Oxford and his colleagues showed that this trend was mostly driven by changes within particular species. For example, when H. erectus first appeared in the fossil record about 2 million years ago, its brain volume was as little as 550 cubic centimetres. By the time the last H. erectus were walking Earth, some 108,000 years ago, that volume had doubled.

Comment: See Part two