Smaller gut, bigger brain

Smaller gut, bigger brain

10January 2005


Observations on human evolution by Margaret Cook

Humans are hard-wired to believe themselves, collectively and individually, the greatest. And they like to celebrate their braininess. Even cerebral scientists aspire to spin themselves some mass recognition.

At the University of Chicago, the geneticist Bruce Lahn has been working on the reason why humans’ brains became so much bigger than those of their near- primate relatives. Measuring mutations in genes to assess the pressures of natural selection, he discovered anew that human brains evolved at an astonishingly fast rate. Well, we knew that. He has since (in the journal Cell last month and as widely reported elsewhere over the Christmas/ New Year break) used moral-superior language to conclude that there was a different, privileged, unique order of evolutionary process in human brains. It sounds perilously close to the words used by John Paul II in 1996, trying to theorise how God injected a soul into an animal lineage to create humans, a new sub-divine species.

The large human brain is probably the cause of our gender wars. Women and children have borne the costs: the former in the enormous hazard of giving birth, the latter in being born too soon, dependent and vulnerable, before vital neurological connections are established. Another downside is the metabolic expense: an organ that weighs only 2 per cent of body weight yet requires 18 per cent of the energy budget. These costs must have been matched by huge benefits – otherwise, Homo sapiens would have hit the extinction buffers long since.

The selective advantages to our species in being clever are obvious, and they ensure that few other animals can compete and co-evolve. This is not a thing to be proud of. Yet the fuel that supported the explosion of the brain is seldom acknowledged, because it is humdrum and belongs to the female sphere of influence. I am talking about diet and cooking, no less. Like the Glasgow football team described as “jeelie pieces”, our splendid genetic potential can be realised only if we eat and digest enough to work the vital parts.

Fire may have been in use as long as 1.4 million years ago, when our ancestors were Homo erectus. It allowed humans easily to digest meat and bone marrow, whether these were acquired by hunting or by scavenging. Moreover, tubers and root vegetables, rich sources of calories, may have become more widely eaten after cooking. We may speculate that the accidental discovery of charred tubers – after a forest fire or a lightning strike – were found to be very good to eat. What is certain is that, in the era of Homo erectus, there was a co-evolution of gut and brain. The first shrank as the second burgeoned: a useful economy, as the gut is also metabolically costly. In evolution, a big gut and a big brain are incompatible. In erectus, the archaeological evidence of anatomy – of skull and ribcage – supports the dietary transformation theory.

The cooking revolution paved the way for all those selection pressures – of societal manipulation, memory, language and technical expertise – to be answered.

Lahn and other molecular biologists who peer into their narrow investigative vortex should learn to see the woods as well as the trees.


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