Around 20-30 million years ago, the branch of apes that would eventually evolve into humans evolved away from having tails. A team at New York University identified a particularly interesting mutation that likely played an important part. To find it, they collected mutations that were only found in humans and other primates without tails, then restricted only to mutations that were in genes that affect tail development in mice. Among all the mutations found, they examined a very interesting mutation in a gene named TBXT. Unlike most mutations which happen from a small change to one’s DNA, this mutation was caused by a parasitic chunk of DNA that can copy and paste itself into new spots in the genome, and it turned out to have a pretty drastic impact on TBXT and tails.

Using human cells, they found that this mutation results in a chunk of TBXT sometimes getting removed, leading to a shortened version of the gene. After recreating the shortened form of TBXT in mice, they saw tails that were much shorter or absent altogether. However, they had to use a slightly different mutation, since the original mutation didn’t make the shortened TBXT as often as in human cells, which highlights the difficulty of studying these kinds of human evolution questions in mice. The team also found a potential health consequence from this mutation. Mice with only the shortened TBXT form died in development and had abnormal spinal cord development, similar to some human birth defects. So, it’s possible that this mutation, which helped cause tail loss, also placed humans at increased risk for these kinds of disorders.

It would be impossible to say definitively when and what mutations led to the loss of tails for the human evolutionary branch without time travel. However, this study presents a convincing case for how the mutation they found contributed to that loss and maybe how humans’ upright and tail-less position might have some biological trade-offs.

This study was led by Bo Xia, formerly a PhD student at NYU in the research groups of Jef Boeke and Itai Yanai, now an investigator at the Broad Institute of MIT and Harvard.

Corresponding Author: Alex Yenkin

Original Article:  “On the genetic basis of tail-loss evolution in humans and apes,” Nature

Press Article: “A single genetic tweak caused human ancestors to lose their tails 25 million years ago — but it came at a cost,” Business Insider

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