Scientists have Found Out Why Humans no Longer have Tails

About 25 million years ago, an evolutionary divergence happened between our ancestors—the early humans and apes—and monkeys, leading to the loss of tails in our lineage. The specific genetic mutation behind this change, however, has been unknown until recently.

In a recent study published in Nature, scientists revealed a distinct DNA mutation connected to the loss of ancestral tails. This mutation is found in the TBXT gene, which influences tail length in animals with tails.

The path to this discovery began when Bo Xia, the study’s lead author and a graduate student at New York University—now a principal investigator at the Broad Institute—became interested in the evolutionary origins of the tailbone after injuring his own.

Innovative Thinking in Evolutionary Research

Itai Yanai, the scientific director of the Applied Bioinformatics Laboratories at NYU Langone Health and a senior author of the study, commended Xia’s fresh perspective, saying, “Bo is really a genius because he looked at something that thousands of people, at least, must have looked at before — but he saw something different.”

Animal evolution over millions of years is driven by genetic changes, from minor tweaks to complex modifications. One such process involves Alu elements—repetitive DNA sequences unique to primates—that can cause variation by inserting themselves into the genome.

In the latest research, scientists discovered two Alu elements within the TBXT gene that are unique to great apes and absent in monkeys.

Uncovering the Hidden Role of Introns

These elements are located in introns, DNA segments adjacent to exons, which were once thought to be non-functional “dark matter.” However, when the TBXT gene generates RNA, the repetitive nature of the Alu sequences causes them to bind together, leading to the removal of an entire exon during RNA splicing.

When these Alu elements were introduced into mice, the mice lost their tails, mimicking the evolutionary change seen in humans and apes. This finding supports the idea that tail loss was a key factor in the evolution of bipedalism in humans, a significant adaptation.

Additionally, mice with shorter tails showed a higher incidence of spina bifida, a neural tube defect, highlighting potential unintended effects of TBXT deficiency.

Yanai expressed amazement at the broad impact of their findings, stating, “We are now walking on two feet, we have developed large brains, and we use technology—all because of a selfish element inserting itself into a gene’s intron. It’s incredible to me.”

This groundbreaking discovery not only enhances our knowledge of evolutionary biology but also suggests new directions for genomic research, as alternative splicing mechanisms could be responsible for many evolutionary trait changes.

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