Introducing the brain: neuron density evolution (Introduction)

by David Turell , Wednesday, March 23, 2022, 15:04 (763 days ago) @ dhw

New views about neurons in studying brain evolution:

https://www.the-scientist.com/news-opinion/reptiles-are-the-real-bird-brains-69818?_hsm...

A research group argues that a species’ number of neurons, rather than brain volume, should serve as indicator of cognitive capacity when studying brain evolution, but some experts voice doubts.

Mammals and birds have dramatically more neurons in their forebrain and cerebellum than reptiles, and neuron numbers have scaled up significantly only four times in more than 300 million years of brain evolution in the clade that includes reptiles, birds, and mammals, according to a study published in PNAS on March 7. Instead of brain volume, which has long been used as a proxy for brain complexity, the study’s authors used the number of neurons typically found in species’ brains as an indicator of smarts. (my bold)

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Němec adds that it would be even better to combine neuronal number with the number of synapses to estimate complexity, “but we currently don’t have a tool to measure the number of synapses precisely, and definitely not across many species.” (my bold)
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With a phylogenetic analysis in the current study, the researchers show that the relationship between neuron number and brain size changed in a major way only four times in the evolution of land vertebrates. “With the appearance of birds and mammals, brains not only enlarged, but also density increased a lot,” says Němec. Within mammals, previous studies had established that primates have higher neuronal density. Within birds, the new study finds that so-called core land birds, a group that includes woodpeckers, falcons, and parrots, also have relatively large brain sizes and densities in the brain. “The appearance of birds and mammals, and within these groups independently the two crown groups [core land birds and primates] . . . increased processing power significantly,” says Němec. “One highly surprising finding is that it was actually very rare, such occasions. We expected that it would be changing within evolution, going up and down all the time. This is partly true, but these really big changes are extremely rare.” (my bold)

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“Neuron number is certainly an important variable, but my guess is that it will be most useful if combined with other variables, such as relative brain size,” Font writes. Němec agrees that additional variables would be useful. “You might have different sizes of neurons, different numbers of cortical connections. But this data is simply not available and certainly not available for many species.” In studying vertebrate brain evolution on a large scale, neuron number would be a good proxy, he argues. “If an animal has billions of neurons, it’s definitely more clever than an animal that has millions of neurons. But I would not say that it is a very tight correlation.”

Comment: this study is in the same point of view when I point out the very special arrangement of the human five layer prefrontal cortex. Neuronal density with synaptic connectivity is the key to high mental process ability. We and birds (clever corvids) have it. Also note my bold regarding its evolution as most unusually "sudden and rare." Not Darwinian in any way but highly supportive of the actions of a designer.