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Big brain or small body?

Scientific discovery and discussion
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Big brain or small body?


Postby XFool » April 28th, 2021, 10:51 pm

‘Big-brained’ mammals may just have small bodies, study suggests

The Guardian

Examination of 1,400 living and extinct species finds evolutionary selection may not be reason for larger brains

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Re: Big brain or small body?


Postby ursaminortaur » May 1st, 2021, 5:49 pm

XFool wrote:‘Big-brained’ mammals may just have small bodies, study suggests

The Guardian

Examination of 1,400 living and extinct species finds evolutionary selection may not be reason for larger brains

Not really surprising since comparisons between species have generally looked at the brain size to body mass ratio (or more recently the more refined Encephalization quotient which is based upon the same variables)

Body size accounts for 80–90% of the variance in brain size, between species

Hence a species in which the body got smaller without any change in brain size would be expected to have a higher EQ just as a species whose body remained the same size but whose brain became larger.

One somewhat surprising fact though is that modern Homo-Sapiens have smaller brains than our ancestors of 20,000 years ago (the Cro-Magnons) - although we are still the same species (and the effect is more pronounced than can be explained by a shrinkage in body mass).

Over the past 20,000 years, the average volume of the human male brain has decreased from 1,500 cubic centimeters to 1,350 cc, losing a chunk the size of a tennis ball. The female brain has shrunk by about the same proportion. “I’d call that major downsizing in an evolutionary eyeblink,” he says. “This happened in China, Europe, Africa — everywhere we look.” If our brain keeps dwindling at that rate over the next 20,000 years, it will start to approach the size of that found in Homo erectus, a relative that lived half a million years ago and had a brain volume of only 1,100 cc.
The Homo sapiens with the biggest brains lived 20,000 to 30,000 years ago in Europe. Called the Cro-Magnons, they had barrel chests and huge, jutting jaws with enormous teeth. Consequently, their large brains have often been attributed to brawniness rather than brilliance. In support of that claim, one widely cited study found that the ratio of brain volume to body mass — commonly referred to as the encephalization quotient, or EQ — was the same for Cro-Magnons as it is for us. On that basis, Stringer says, our ancestors were presumed to have the same raw cognitive horsepower.

Now many anthropologists are rethinking the equation. For one thing, it is no longer clear that EQs flatlined back in the Stone Age. Recent studies of human fossils suggest the brain shrank more quickly than the body in near-modern times. More important, analysis of the genome casts doubt on the notion that modern humans are simply daintier but otherwise identical versions of our ancestors, right down to how we think and feel. Over the very period that the brain shrank, our DNA accumulated numerous adaptive mutations related to brain development and neurotransmitter systems — an indication that even as the organ got smaller, its inner workings changed. The impact of these mutations remains uncertain, but many scientists say it is plausible that our temperament or reasoning abilities shifted as a result.

But the good news is that better nutrition means that the decrease in Homo-Sapien brain size seems to have reversed in the last few hundred years.

After a long, slow retrenchment, human brain size appears to be rising again. When anthropologist Richard Jantz of the University of Tennessee measured the craniums of Americans of European and African descent from colonial times up to the late 20th century, he found that brain volume was once again moving upward.

Since evolution does not happen overnight, one would assume this sudden shift (much like the increase in height and weight) is unrelated to genetic adaptations. Hawks, for instance, says the explanation is “mostly nutrition.” Jantz agrees but still thinks the trend has “an evolutionary component because the forces of natural selection have changed so radically in the last 200 years.” His theory: In earlier periods, when famine was more common, people with unusually large brains would have been at greater peril of starving to death because of gray matter’s prodigious energy requirements. But with the unprecedented abundance of food in more recent times, those selective forces have relaxed, reducing the evolutionary cost of a large brain.

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