Evolution Mostly Driven by Brawn, Not Brains, Analysis Finds

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The most common measure of intelligence in animals, brain size relative to body size, may not be as dependent on evolutionary selection on the brain as previously thought, according to a new analysis by scientists.


Brain size relative to body size has been used by generations of scientists to predict an animal’s intelligence. For example, although the human brain is not the largest in the animal kingdom in terms of volume or mass, it is exceptionally large considering our moderate body mass.

Now, a study by a team of scientists at UCL, the University of Konstanz, and the Max Planck Institute of Ornithology has found that the relationship between the two traits is driven by different evolutionary mechanisms in different animals.

Crucially, researchers have found that the most significant factor in determining relative brain size is often evolutionary pressure on body size, and not brain size. For example, the evolutionary history of bats reveals they decreased body size much faster than brain size, leading to an increase in relative brain size. As a result, small bats were able to evolve improved flying maneuvrability while maintaining the brainpower to handle foraging in cluttered environments.

 

Written By: ScienceDaily
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8 COMMENTS

  1.   For example, the evolutionary history of bats reveals they decreased body size much faster than brain size, leading to an increase in relative brain size. As a result, small bats were able to evolve improved flying maneuvrability while maintaining the brainpower to
    handle foraging in cluttered environments.

    This should really be no surprise, as there is considerable selection pressure on flying animals to reduce weight to surface area ratios.

    While lightening bones and reducing muscles which are not required for flight, should be possible with reduced growth of these, brains and sight, are crucial to avoiding fatal collisions and achieving safe landings, so any clumsy adjustments to these, would be injurious or fatal.

  2. I’ve heard that there’s also a reverse case: the Koala in Australia, which has a brain that is way too small to fit its cranium.

    I wonder what the relevance to human evolution might be.

    There may have been considerable selection pressure on early humans. There must have been at some point because humans are so very much different to all other primates. (If only because humans exterminated all near rivals that more closely resembled humans.)

    If, like bats, humans brains are relatively large because they’ve been stranded by a more rapidly downsizing body then there might be fossil remnants indicating distant ancestors with significantly larger bodies compared to similar brain capacities.

    One thing that might rapidly downsize body size in humans might be a change in diet involving much more travel. As in hunting and scavenging for meat over a wide range instead of settling for whatever is less challenging and immediately available in the surrounding bush.

    This might link with the selection drivers as for upright posture, gracility, loss of body fur, division of labour, and language. Evolving a lighter body would be better for endurance performance for distance coverage. Same issue with Olympic sprinters versus distance athletes.

    In present day humans distance athletes have relatively larger brains than sprinters, owing to the difference in body mass. Same brains, but lighter bodies. Having less body mass to operate means that distance runners’ spare brain capacity may enable them to navigate complex city environments and traffic during marathon events, find drink stops etc. While sprinters only have to show up and somehow barely make to the nearest 100m mark.

  3. The trouble with this analysis based on bats shrinking in body size while brain size remains the same is that it can’t be transferred directly over to humans. Brain size in humans is not an artefact of our body growth.

    Our brains evolved via a definitive increase from 500 cc (cubic centimetres) to roughly 1500 cc, whereas body mass has not increased nearly as much.  Australopithecines tended to reach heights (when standing upright) between four and five feet, whereas the most recent Homonids reached six feet.  Since chimpanzees have roughly the same brain size as Australopithecines do, and making a direct comparison between chimps and humans, the facts categorically show that humans have grown bigger in body size and still have a dramatic difference in brain size.  If the article applied to us, the body size of our lineage should have shrunk even as the brain increased.

    The other problem is that the Encephalization Quotient is, and always has been, an approximate measure of intelligence.  Brain size is a side effect of whatever tools or mental gadgets were recruited for an animal’s way of life — bigger isn’t always better.  To give an example, Tyrannosaur skulls show that they had brains much bigger than ours and nearing the upper limit of theropod intelligence according to the EQ, but a huge chunk of it was devoted to olfactory bulbs and association cortices, giving them an excellent sense of smell rivalling that of a turkey vulture.

    The article is fair enough in pointing out that brains and bodies respond to independent pressures, so EQ is not foolproof, but that’s also why the EQ is so useful.  In general, most lineages will show a constant average of EQ.  Human and bat brain size, for instance, deviates enough to tell us that something interesting is going on.  If a lineage has a larger or smaller EQ than usual relative to cousin lineages (as in other mammals), that’s a hint there’s something worth looking into.

  4. >For example, although the human brain is not the largest in the animal kingdom in terms of volume or mass, it is exceptionally large considering our moderate body mass.

    I think this is bogus.  You don’t need a big brain to control a big body. Consider how small a dinosaur brain was.  Consider that a whale has no more complex musculature than a mouse.  Consider than a dolphin uses a large hunks of its brain for auditory processing of echoes. Humans use large parts of their brains to manage balancing on two legs. This has nothing to do with intelligence.

  5. “Humans use large parts of their brains to manage balancing on two legs. This has nothing to do with intelligence.”

    Yet ostrichs of similar hight and body mass manage the same with a tiny brain. (and their legs bend the other way). Surely the main reasons humans are (for want of a better word) intelligent, is because they are trained/educated, but that could only happen if there is a lot of brain mass to start with. Or am I wrong?

  6. Now we have ways of studying the brain, we know a lot more about what each hunk is used for. Gross brain weight is a rather crude tool for comparing different species. We should be looking at the relative size of various portions.

    A really good balance, auditory, olfactory, or visual processor takes up space, but does not necessarily increase what we are so proud of — ability to reason. Some animals use distributed processing or hardware to solve problems we solve with the brain.

    I have been debating these matters for many decades.  I get the feeling the true intent of the argument is partly religious, to prove that man is “best” in some sense, and hence in justified in maltreating other species.

  7. “There may have been considerable selection pressure on early humans.
    There must have been at some point because humans are so very much
    different to all other primates. (If only because humans exterminated
    all near rivals that more closely resembled humans.)”

    “Crucially, researchers have found that the most significant factor in
    determining relative brain size is often evolutionary pressure on body
    size, and not brain size.”

    From my limited understanding, the reduced jaw size in humans greatly impacted human evolution and brain development. (?)

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