Human evolution; our neocortex appears do novo (Introduction)

by David Turell , Thursday, February 16, 2023, 00:43 (644 days ago) @ David Turell
edited by David Turell, Thursday, February 16, 2023, 00:49

There is no past anticendent form:

https://www.quantamagazine.org/gene-expression-in-neurons-solves-a-brain-evolution-puzz...

"The neocortex stands out as a stunning achievement of biological evolution. All mammals have this swath of tissue covering their brain, and the six layers of densely packed neurons within it handle the sophisticated computations and associations that produce cognitive prowess. Since no animals other than mammals have a neocortex, scientists have wondered how such a complex brain region evolved.

"For more than 50 years, some evolutionary neuroscientists have argued that the neocortex and the DVR were both derived from a more primitive feature in an ancestor shared by mammals and reptiles.

"Now, however, by analyzing molecular details invisible to the human eye, scientists have refuted that view. By looking at patterns of gene expression in individual brain cells, researchers showed that despite the anatomical similarities, the neocortex in mammals and the DVR in reptiles are unrelated. Instead, mammals seem to have evolved the neocortex as an entirely new brain region, one built without a trace of what came before it. The neocortex is composed of new types of neurons that seem to have no precedent in ancestral animals. (my bold)

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" The discovery that gene expression can reveal these kinds of important distinctions between neurons is also prompting researchers to rethink how they define some brain regions and to reassess whether some animals might have more complex brains than they thought.

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"...brain regions do not evolve neatly one on top of another. Instead, the brain evolves as a whole, with older parts undergoing modifications to adapt to the addition of new parts, explained Paul Cisek, a cognitive neuroscientist . “It’s not like upgrading your iPhone, where you load up a new app,” he said.

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"The debate over the origins of the neocortex and DVR stretched out over decades. Now, however, a recently developed technique is helping to break the stalemate. Single-cell RNA sequencing enables scientists to read out which genes are being transcribed in a single cell. From these gene expression profiles, evolutionary neuroscientists can identify a wealth of detailed differences between individual neurons. They can use those differences to determine how evolutionarily similar the neurons are.

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"...differences in gene expression suggested that the reptilian DVR and the mammalian neocortex evolved independently from different regions of the brain.

“'The 2018 paper was really a landmark paper in that it was the first really comprehensive molecular characterization of neural types between mammals and reptiles,” said Bradley Colquitt, a molecular neuroscientist at the University of California, Santa Cruz.

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"Mammals, however, were a different story. Salamander neurons didn’t match anything in the mammalian neocortex, although they did resemble cells in parts of the mammalian brain outside the neocortex.

"Moreover, several kinds of cells in the neocortex — specifically, the types of pyramidal neurons that make up the majority of neurons in the structure — didn’t match with cells in the reptiles either. Tosches and her colleagues therefore suggested that these neurons evolved solely in mammals. They aren’t the first researchers to propose that origin for the cells, but they are the first to produce evidence for it using the powerful resolution of single-cell RNA sequencing.

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"Tosches and her team propose that essentially all of the mammalian neocortex is an evolutionary innovation. So while at least part of the reptilian DVR was adapted from the brain region of an ancestral creature, the mammalian neocortex evolved as a new brain region burgeoning with novel cell types. Their answer to the decades of debate is that the mammalian neocortex and the reptile DVR are not homologous because they don’t have a common origin.

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"The new answer from Tosches’ team doesn’t mean that the neocortex in mammals evolved to sit neatly atop older brain regions, as the triune brain theory proposed. Instead, as the neocortex expanded and new types of pyramidal neurons were born within it, other brain regions kept evolving in concert with it. They didn’t just hang on as an ancient “lizard brain” underneath. It’s even possible that the complexity emerging in the neocortex pushed other brain regions to evolve — or vice versa.

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"First they compared the full array of neural cell types in each species to find the ones that they shared, which must have been passed down from a common ancestor. Then they looked for neural cell types that differed between the species.

"Their results showed that both conserved and novel neural cell types are found all over the brain — not just in the brain regions that appeared more recently. The entire brain is a “mosaic” of old and new cell types, said Justus Kebschull, an evolutionary neuroscientist at Johns Hopkins University."

Comment: No surprise. If the Cambrian explosion had no precursors, so did the specialized pyramidal neurons of our cortex. A designer can add anything new He wishes. This is a 'neuron gap' as significant as the Cambrian. The new cells will obviously push around the placement of old ones, so the whole brain continues to evolve. dhw take notice.