It's no secret that human brains are significantly larger compared with other species, but even the best scientists in the world don't know why.

Now, three almost identical genes may have just given us the answer. They could explain how 0.54 liters of gray matter in early human ancestors became this 1.4-liter organ that has made human species so distinct from others.

These genes are descendants of an ancient development gene that went through several changes during evolution. They add to the expanding list of DNA related to why a human brain is bigger than others' brains.

Why Our Brains Are So Large Compared With Others

A couple of new studies published in the Cell journal now show that this trio of genes boosts the number of potential nerve cells in brain tissue. One team was even able to identify the protein interactions potentially responsible for such a phenomenon.

The most important thing to note is that these genes are exclusive to humans. They're not present at all in monkeys or orangutans, according to the researchers. They are, however, present in gorillas and chimpanzees — the primates we share much of our genome with — but inactive.

The genes, dubbed NOTCH2NL, work by slowing down the development of cortical stem cells into neurons, which results into more neurons. The researchers discovered that they're abundant in the neural stem cells of the cerebral cortex.

"Given the relatively fast evolution of the human brain, it is tempting to speculate that newly evolved, human-specific genes may help shape our brain in a species-specific way," says Pierre Vanderhaeghen, a developmental biologist who is part of the team.

The Evolution Of The Human Brain

Understanding how the human brain has evolved from the beginning of time to the present day is crucial because it will help us determine how we, as humans, have adapted to the demands of a modern, intellectual civilization. Brain development has helped us think better, solve more complex problems, and develop society and culture to a degree that the rest of our neighbors in the animal kingdom can't attain.

To identify the genes specific to humans, the researchers developed a new way to analyze the human RNA, crucial to gene expression and coding. Focusing on the NOTCH2NL gene, they performed a series of tests on mice embryos and human pluripotent cells and discovered what exactly NOTCH2NL does.

"From one stem cell, you can either regenerate two progenitor cells, generate two neurons, or generate one progenitor stem cell and one neuron," says Vanderhaeghen.

NOTCH2NL veers that decision slightly toward progenitor regeneration, which can later lead to formation of more neurons. By deleting this gene from human stem cells, the researchers found that while stem cells differentiated faster into neurons, the overall stem cell pool depleted, resulting in a smaller patch of cortex tissue.

Though the findings are astonishing, further study needs to be conducted to know more about the mechanisms that NOTCH2NL genes use. The researchers also aim to conduct longer experiments on larger samples to observe the phenomenon more up-close.

"That's really compelling biological data," says evolutionary genomicist James Noonan. "In other studies of genes involved in human evolution, it's been very difficult to draw a line from the genetic difference to the phenotype to a biochemical mechanism that's responsible."

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