Why did Humans lose their Tails?

A new study published on February 28th, 2024 in the journal Nature has identified the specific genetic mutation responsible for ancestral humans and apes losing their tails around 25 million years ago.

The Tailless Ancestor Mystery

While monkeys possess tails, an ancestor species that humans share with apes underwent a key genetic divergence resulting in tail loss over the course of evolution. However, the actual genetic drivers behind this dramatic physiological change were unknown until now.

Serendipitous Discovery

The study’s lead author Bo Xia, currently with the Broad Institute, got intrigued by the evolutionary puzzle after injuring his own tailbone. Along with teams from New York University (NYU) Langone Health and Applied Bioinformatics Labs, his curiosity-driven investigation pinpointed unique jumping gene activity that deactivated the tail-growth gene TBXT.

The Role of Jumping Genes

Over generations, DNA accumulates changes enabling species adaptation through evolution. The study found older repetitive genetic sequences called Alu elements that jumped into strategic introns of the TBXT gene.

Introns are non-coding DNA portions that get sliced out before the gene sequence is converted into proteins. The intron-inserting DNA ‘jumping genes’ disrupted normal protein formation by the tail-regulating TBXT gene.

This genetic mutation was spotted in apes but not monkeys, coinciding with ancestral tail disappearance in the former group after both diverged from a common monkey group ancestor.

Alternative Splicing and Multiple Proteins

The Alu element insertion caused the TBXT gene to undergo alternative splicing and generate multiple proteins variants instead of one form coded by monkeys. This indicates more complex downstream impacts compared to straightforward gene disabling.

Researchers confirmed through lab experiments that inserting the exact Alu sequences into mice TBXT gene also led to truncated tails in mice besides increasing risk of spinal defects.

Evolutionary Significance

The study illustrates how small non-coding DNA changes can profoundly reshape physiology over thousands of generations to enable evolutionary adaptation.

Loss of stabilizing tails may have enabled ancestral apes to adopt bipedal motion crucial for later human development. The mutation likely occurred randomly without an initial adaptive benefit.

However, it conferred survival value once interplay between taillessness and walking upright offered mobility advantages within forest habitats.

Future Impact

Beyond solving the longstanding tail evolutionary mystery, the pathbreaking discovery promises to accelerate genetics research on non-coding DNA and complex alternative gene splicing effects.

Intron sequences dismissed previously as ‘junk DNA’ now open up new appraisal of their hidden role in driving evolutionary changes to anatomy over time.

Deeper analysis can reveal if similar jumping gene insertions underlie other evolutionary divergences between ancestral primates and humans.


The study underscores how small-scale genetic changes can catalyze sweeping physiological adaptations central to a species’ evolutionary history. Shedding light on humanity’s tailless past sets the stage for fresh investigation into other attribute transformations during ancestral primate evolution over millions of years until modern humans emerged.



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