Our very ancient animal ancestors had tails. Why don't we? Somewhere around 20 million or 25 million years ago, when apes diverged from monkeys, our branch of the tree of life shed its tail. From Darwin's time, scientists have wondered why—and how—this happened. And as an article in Nature explains, Bo Xia wondered the same thing while getting over a tailbone injury suffered during a cab ride. He also happened to be getting his PhD at New York University, and set out to explore the question. Now, Xia and other researchers have identified at least one of the key genetic tweaks that led to this change, reports the AP.
"We found a single mutation in a very important gene," said Xia, a geneticist at MIT's Broad Institute and co-author of a study published Wednesday in the journal Nature. The researchers compared the genomes of tailless apes and humans with the genomes of 15 species of monkeys with tails to pinpoint key differences between the groups. As Cosmos reports, "The research revealed a DNA insertion in a gene called TBXT present in apes and humans, but not in monkeys." They tested their theory that the insertion was the cause by using the gene-editing tool CRISPR to tweak the same spot in mouse embryos. Those mice were born without tails.
Xia cautioned that other genetic changes may also play a role in losing tails. As the Nature article notes, "The researchers analyzed 140 genes involved in tail development and identified thousands of genetic changes unique to apes that might also have played a part in tail loss." Another mystery: Did having no tails actually help these ape ancestors—and eventually, humans—survive? Or was it just a chance mutation in a population that thrived for other reasons?
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Rick Potts, who directs the Smithsonian Institution's Human Origins Project and was not involved in the research, suggests being tailless may have been a first step toward some apes adopting a vertical body posture, even before they left the trees. Not all apes live on the ground today. Orangutans and gibbons are tailless apes that still live in trees. New York University biologist Itai Yanai, a co-author of the study, said that losing our tails was clearly a major transition. But the only way to certainly know the reason "would be to invent a time machine," he said. (More discoveries stories.)
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