Scientists who were experimenting on mice have discovered that crucial components of the modern human brain take longer to develop than those of our long-lost cousin, the Neanderthal.
Slow and steady is the winner here, as is the hare and the tortoise. Protein differences appear to also reduce chromosome errors, ultimately resulting in a healthier, more robust population.
The findings of the study suggest that this step in the development of our neocortex (the wrinkled outer layer responsible for higher order thinking) plays a role in protecting us from disease, something that Neanderthals appear to be missing.
Advances in genetics have enabled scientists to sequence DNA extracted from ancient remains, providing additional information on the Neanderthal genome's similarities and differences with ours.
For example, we know of around 100 amino acids the structures that make up proteins that changed when modern humans diverged from the branch that gave rise to the Neanderthals and another close cousin, the Denisovans.
The use of amino acids can have significant effects, but it was unclear how these modifications differed between humans and Neanderthals.
Six of the identified substitutions exist in proteins that are already known to play a role in the distribution of chromosomes during cell division. So a group of researchers from Germany studied whether or not they could determine how these amino acid changes might affect the development of the neocortex.
Laboratory mice were chosen as their natural subject, because they have six amino acids similar to Neanderthals (and apes) in the relevant proteins. Using CRISPR Cas-9, the researchers substituted those amino acids for those found in modern humans.
They also used modern human amino acids in the opposite direction. Neanderthal/mouse/ape variants were developed. Organoids of human brains from embryonic stem cells lumps of brain tissue that are not alive or sentient were also grown.
The results were striking and instructive.
"We discovered that three modern human amino acids in two proteins lead to a longer metaphase, a period where chromosomes are prepared for cell division," Mora-Bermudez explained, "and this results in fewer errors when the chromosomes are distributed to the daughter cells of neural stem cells, just like in modern humans."
In addition, the metaphase in Neanderthalized human organoids was shorter, resulting in twice the number of chromosome separation errors compared to the control organoids. This suggests that three modern human amino acid substitutions are responsible for fewer chromosome distribution errors than Neanderthals.
The findings suggest that changes in the number of chromosomes, also known as polysomies, may have been beneficial to modern humans. They also suggest that chromosomal troubles might have a larger impact on brain function in Neanderthals than we see today.
In their research, the authors conclude that modern humans might have a lower probability of any such detrimental effects as Neanderthals, Denisovans, and apes.
"Further research is required to investigate the potential of these interactions for modern humans."
The research has been published in Science.