Unlike most mammals, humans, and the great apes do not have tails. Scientists have now identified the genetic mutation that caused this loss. They also found that it came at an unfortunate cost to our species.
About 20 million years ago, when hominids and the great apes differentiated from other primates, they lost their tails in the process. While many apes and mammals still have tails today, humans have only a tail remnant, a bony shell the coccyx at the end of their backs. In Fact, Bo Xia, a medical doctoral student at New York University, broke his tailbone in 2019 and became interested in why and how humans lose their tails.
He and his colleagues compared genes from six species of tailless monkeys and nine species of monkeys with tails. He noticed that in the middle of a gene called TBXT, which is known to affect the tail length, there was a small insertion in the gene sequence that was present in the tailless monkeys but absent in the others. This sequence, which tends to wander within the gene, disrupts the sequence and affects the configuration of the corresponding protein. When two such sequences are combined, they form a kind of ring, resulting in a shorter-than-usual protein. In humans, there are two versions of the TBXT gene: one with “long” proteins and one with “short” proteins. It is this configuration that would prevent tail growth.
To test their hypothesis, the researchers genetically modified mice with different versions of the gene. Those with two short proteins died, but those with one long and one short protein (like humans and monkeys) developed shorter tails or no tails at all. Although this mutation appears to be the cause of our lack of tails, the researchers note that the tail morphology of the genetically engineered mice is too different to account for our uniformly sized coccyx per se. So, it’s likely that the tail loss is also linked to other genes, the researchers noted in their study, which was pre-published on the bioRxiv server.
Why did humans lose their tails?
The question remains as to why humans lost their tails. Since early hominids were taller than other apes, one would think that the tail would have helped them balance on tree branches. In addition, the researchers found that the mutation had another detrimental effect, sometimes causing spinal abnormalities. Mice carrying the mutation have more malformations, such as spina bifida, which results in a lack of spinal closure. This fissure, which occurs in about one in 1,000 births, causes a sort of pouch in the fetal spine and leads to recurrent meningitis and motor disorders. Its cause remains unidentified. In short, it’s hard to see what benefit we get from tail loss. But it is clear that if this mutation has persisted, it is because it is attractive, perhaps for locomotion or reproduction.
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