Scientists experimenting with mice have found evidence that key parts of the modern human brain took longer to develop than those of our long-extinct cousins, the Neanderthals.
Like the hare and the tortoise, slow and steady is the winner here. The extra time is caused by protein differences that also appear to reduce chromosome aberrations, resulting in healthier, more robust populations.
The study results show that this stage of development of our neocortex (the wrinkled outer layer responsible for higher-order thinking) plays a role in protecting us from disease, a feature Neanderthals seem to have lacked.
In recent years, advances in genetics have allowed scientists to sequence DNA extracted from ancient remains, revealing detailed information about how the Neanderthal genome compared and contrasted with our own.
For example, we know about 100 amino acids — the compounds that make up proteins — that changed when modern humans diverged from the branch that gave rise to Neanderthals and another close cousin, the Denisovans.
Amino acid substitutions may have important effects, but it was unclear which functions these substitutions changed between humans and Neanderthals.
Six of the identified substitutions are in proteins already known to play a role in chromosome segregation during cell division. So a group of specialists drove by geneticist Felipe Mora Bermudez of the Max Planck Institute of Molecular Cell Biology and Genetics in Germany directed tests to check whether these amino corrosive changes could assume a part in the improvement of the neocortex.
The natural subjects were laboratory rats, which share six amino acids within the corresponding protein with Neanderthals (and monkeys). Using CRISPR Cas-9, the researchers replaced these amino acids found in modern humans.
He also took research in the opposite direction. They created human brain organoids from embryonic stem cells – lumps of brain tissue that are not living or sentient – and replaced the modern human amino acids with Neanderthal/mouse/monkey variants.
The results were surprising and fascinating.
“We found that three modern human amino acids in two proteins lead to a longer metaphase, a stage where chromosomes are prepared for cell division,” explained Maura Bermudez, “and consequently less Mistakes occur when chromosomes are distributed in daughter. Neural stem cells cells, just like modern humans.”
In addition, metaphase was shorter in Neanderthalized human organoids, resulting in twice the number of chromosome segregation errors compared to control organoids. This suggests that the three modern human amino acid substitutions are responsible for fewer chromosome segmentation errors than Neanderthals.
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Because errors in the number of chromosomes, called polysomy, can lead to serious diseases as well as cancers such as leukemia and carcinoma, the results suggest that this change was beneficial to modern humans. They also suggest that brain function in Neanderthals may have been affected by chromosomal disorders at a higher rate than we see in modern humans.
“The present data suggest that any such deleterious effects of chromosomal mis-segregation may be less likely in modern humans than in Neanderthals, Denisovans, and apes,” the researchers wrote in their paper.
“Further work is needed to resolve the significance of these effects for characterizing modern humans.”
