Brain complexity: different human brain evolution (Introduction)

by David Turell , Thursday, August 02, 2018, 18:51 (2305 days ago) @ David Turell

It is not only bigger but differently organized than other animals:

https://www.sapiens.org/evolution/primate-intelligence/

"The downfall of rodents, in other words, is that their brains don’t adapt well to the problems of being big. They don’t compensate efficiently for the communication bottlenecks that emerge as brains increase in size. This constraint has severely limited their capacity for intelligence.

"Primates, on the other hand, do adapt to these challenges. As primate brains become larger from species to species, their blueprints do gradually change—allowing them to circumvent the problem of long-distance communication.

"Kaas thinks that primates managed to keep most of their neurons the same size by shifting the burden of long-distance communication onto a small subset of nerve cells. He points to microscopic studies showing that perhaps 1 percent of neurons do expand in big-brained primates: These are the neurons that gather information from huge numbers of nearby cells and send it to other neurons that are far away. Some of the axons that make these long-distance connections also get thicker; this allows time-sensitive information, such as a visual image of a rapidly moving predator, or prey, to reach its destination without delay. But less-urgent information—that is, most of it—is sent through slower, skinnier axons. So in primates, the average thickness of axons doesn’t increase, and less white matter is needed.

"This pattern of keeping most connections local, and having only a few cells transmit information long-distance, had huge consequences for primate evolution. It didn’t merely allow primate brains to squeeze in more neurons. Kaas thinks that it also had a more profound effect: It actually changed how the brain does its work. Since most cells communicated only with nearby partners, these groups of neurons became cloistered into local neighborhoods. Neurons in each neighborhood worked on a specific task—and only the end result of that work was transmitted to other areas far away. In other words, the primate brain became more compartmentalized. And as these local areas increased in number, this organizational change allowed primates to evolve more and more cognitive abilities.

"All mammal brains are divided into compartments, called “cortical areas,” that each contain a few million neurons. And each cortical area handles a specialized task: The visual system, for example, includes different areas for spotting the simple edges of shapes and for recognizing objects. Rodent brains don’t seem to become more compartmentalized as they get larger, says Kaas. Every rodent from the bite-sized mouse to the Doberman-sized capybara has about the same number of cortical areas—roughly 40. But primate brains are different. Small primates, such as galagos, have around 100 areas; marmosets have about 170, macaques about 270—and humans around 360.

"In primates, some of these new areas took on novel social tasks, such as recognizing faces and the emotions of others, and learning written or spoken language—the very skills that helped to drive the evolution of hominin culture, and, arguably, human intelligence. “Primates with large brains have really superior processing,” says Kaas. “But rodents with larger brains may be processing things almost the same as rodents with smaller brains. They haven’t gained much.”

***

"This story unfolded a little over 60 million years ago, not long after early primates had split off, in quick succession, from three other major groups of mammals that include modern-day rodents, tree shrews, and colugos (a.k.a. “flying lemurs”).

"These early primates were smaller than rats. They crept quietly along tree branches at night, grasping twigs with their prehensile fingers and toes as they hunted insects. They didn’t look like much at all, says Herculano-Houzel.

"But a subtle tweak had already occurred deep in their little brains—a change in the genes that guide how neurons connect to one another during fetal development. This change probably made little difference at first. But over the long run, it would profoundly separate primates from the rodents and other groups that they had parted ways with. This tiny change would keep nerve cells small, even as brains gradually got bigger and bigger. It would bend the arc of evolution for tens of millions of years to come. Without it, humans never would have walked the earth."

Comment: Was this genetic change purposeful from God? Or chance? We have a highly convoluted cortex compared to other animals. The original article is 20 minutes of reading long, and fascinatingly clear as to how the research was conducted in comparative anatomy of rodents and primates from the split 60 million years ago.