Following the split of the modern humans' ancestors from those of Neanderthals and Denisovans, their Asian relatives, approximately a hundred amino acids, the building blocks of proteins in cells and tissues, changed in modern humans and spread to nearly all modern humans.
To examine the importance of six changes for neocortex development, a EurekAlert! report specified that the researchers initially introduced the modern human variants in mice.
Essentially, mice are identical to Neanderthals at the said amino acid positions; therefore, such changes made them a model for developing the modern human brain.
Describing the discovery, the study's lead author, Felipe Mora-Bermudez said, they discovered that three modern human amino acids in two of the proteins are causing longer metaphase, a phase where chromosomes are prepared for division of cells, and this leads to fewer errors when the chromosomes are distributed to the neutral stem cells' daughter cells, just like the modern humans.
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A picture taken on September 6, 2021, shows the reconstruction of the face of the oldest Neanderthal found in the Netherlands, nicknamed Krijn, on display at the National Museum of Antiquities in Leiden.
Neanderthal Brain
To verify if the Neanderthal set of amino acids has the opposite impact, the researchers then introduced the ancestral amino acids in human brain organoids, miniature organ-like structures that can be developed from human stem cells in cell culture dishes in the lab and that emulate duplicate aspects of early human brain development.
In this circumstance, metaphase turned shorter, and the study investigators discovered more chromosome distribution errors.
Mora-Bermudez explained that the three modern human amino acid changes in the protein identified as KIF18a and KNL1 are responsible for the fewer chromosome distribution errors spotted in modern humans compared to Neanderthal models and chimpanzees.
He also said that having errors in the number of chromosomes is typically not a good idea for cells, as can be detected in disorders such as trisomies and cancer.
Modern Human Brain
Describing their work published in Science Advances, the lead author said their research indicates that some aspects of the evolution of the modern human brain and function may be independent of the size of the brain since Neanderthals and modern humans have similar-sized brains.
The findings suggested, too, that brain function in Neanderthals may have been more affected by chromosome mistakes than that of modern humans, Wieland Huttner, who co-supervised the research, summarized.
Meanwhile, Svante Paabo, who also co-supervised the research, added that future studies are needed the investigation whether the reduced error rate impacts modern human traits linked to brain function.
The Difference in Brain Sizes
Many modern humans have a little Neanderthal gene in them. More so, modern humans of Asian or European descent inherited somewhere between one and four percent of the genes from this hominid that had gone extinct 30,000 years back.
Neanderthals and modern humans coexisted, seemingly more than coexisted, with them for as many as over 5,000 years, although they died out, and the latter-mentioned remained, a How Stuff Works report said.
Both were two very similar hominid species, and it is difficult to pinpoint the advantage Homo sapiens of the time had over the Neanderthals; modern humans appeared to thrive and grow their populations during the last age, for example.
Moreover, Neanderthals had larger brains than modern humans and appeared to have done extremely "human things," like burying their dead, making tools and adornments, and cooking.
Lastly, even though the average Neanderthal had a larger brain than present-day humans, it was, perhaps, comparable in size to the Homo sapiens' brain of the time.
Related information about the difference between modern humans and Neanderthals is shown on Shomprakash Shukla's YouTube video below:
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