Scientists have discovered a flaw in our DNA that may have helped distinguish the minds of our ancestors from those of Neanderthals and other extinct relatives.
The mutation, which has arisen over the past few hundred thousand years, spurs the development of more neurons in the part of the brain we use for our most complex forms of thinking, according to a new study published in Science on Thursday.
“What we found is a gene that certainly helps make us human,” said Wieland Huttner, a neuroscientist at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, and one of the study’s authors.
The human brain allows us to do things that other living species cannot, such as B. using a mature language and making complicated plans for the future. For decades, scientists have compared our brain anatomy to that of other mammals to understand how these sophisticated abilities evolved.
The most obvious characteristic of the human brain is its size – four times the size of chimpanzees, our closest living relatives.
Our brain also has distinctive anatomical features. The region of the cortex just behind our eyes, known as the frontal lobe, is essential for some of our most complex thoughts. According to a 2018 study, the human frontal lobe has far more neurons than the same region in chimpanzees.
But comparing humans to living apes has a serious flaw: Our most recent common ancestor, with chimpanzees, lived about 7 million years ago. To supplement what has happened since then, scientists have had to turn to fossils of our more recent ancestors known as hominins.
Studying hominin skulls, paleoanthropologists have found that the brains of our ancestors increased dramatically in size about 2 million years ago. They reached the size of living humans about 600,000 years ago. Neanderthals, one of our closest extinct hominin relatives, had brains the size of ours.
But Neanderthal brains were elongated, while humans have a more spherical shape. Scientists cannot say what accounts for these differences. One possibility is that different regions of our ancestors’ brains changed size.
In recent years, neuroscientists have begun studying ancient brains with a new source of information: bits of DNA preserved in hominin fossils. Geneticists have reconstructed entire genomes of Neanderthals and their eastern cousins, the Denisovans.
Scientists have pinpointed potentially crucial differences between our genome and the genomes of Neanderthals and Denisovans. Human DNA contains about 19,000 genes. The proteins encoded by these genes are mostly identical to those of Neanderthals and Denisovans. But researchers have found 96 human-specific mutations that changed a protein’s structure.
In 2017, Anneline Pinson, a researcher in Huttner’s lab, looked at this list of mutations and noticed one that changed a gene called TKTL1. Scientists know that TKTL1 becomes active in the developing human cortex, particularly in the frontal lobe.
“We know that the frontal lobe is important for cognitive function,” Pinson said. “So that was a good indication that it could be an interesting candidate.”
Pinson and her colleagues conducted initial experiments with TKTL1 in mice and ferrets. After they injected the human version of the gene into the animals’ developing brains, they found that the mice and ferrets made more neurons as a result.
Next, the researchers conducted experiments on human cells, using pieces of fetal brain tissue obtained through the consent of women who had had abortions in a Dresden hospital. Pinson used molecular scissors to excise the TKTL1 gene from the cells in the tissue samples. Without them, human brain tissue produces fewer so-called progenitor cells, from which neurons develop.
For their final experiment, the researchers set out to create a miniature Neanderthal-like brain. They started with a human embryonic stem cell and edited its TKTL1 gene so that it no longer had the human mutation. Instead, it carried the mutation found in our relatives, including Neanderthals, chimpanzees, and other mammals.
Then they placed the stem cell in a bath of chemicals that caused it to turn into a lump of developing brain tissue called a brain organoid. It created progenitor brain cells, which then produced a miniature cortex made up of layers of neurons.
The Neanderthal-like brain organoid formed fewer neurons than organoids with the human version of TKTL1. This suggests that our ancestors could produce extra neurons in the frontal lobe by mutating the TKTL1 gene. While this change didn’t increase our brain’s overall size, it may have reorganized its wiring.
“It’s really a feat,” said Laurent Nguyen, a neuroscientist at the University of Liège in Belgium, who was not involved in the study.
The new finding doesn’t mean that TKTL1 alone reveals the secret of what makes us human. Other researchers are also looking at the list of 96 protein-changing mutations and conducting their own organoid experiments.
Other members of Huttner’s lab reported in July that two other mutations change the rate at which developing brain cells divide. Last year, a team of researchers from the University of California, San Diego found that another mutation appears to alter the number of connections that connect human neurons.
Other mutations could also prove important for our brains. For example, as the cortex develops, individual neurons must migrate to find their proper place. Nguyen observed that some of the 96 mutations unique to humans altered genes likely involved in cell migration. He speculates that our mutations could cause our neurons to move differently than neurons in a Neanderthal brain.
“I don’t think that’s the end of the story,” he said. “I think more work is needed to understand what makes us human in terms of brain development.”
This article originally appeared in The New York Times.
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