The story of how and when humans lost their tails

Imagine a world where humans sported tails, swinging through trees like their primate relatives. While that might be a fun thought experiment, a new study reveals the reason why we lack these appendages.

Researchers from NYU Langone Health and NYU Grossman School Of Medicine have identified a specific genetic tweak responsible for the loss of this unique feature.

Importance of tails

Throughout the animal kingdom, tails have played a crucial role. They help creatures of all kinds maintain balance as they move across different environments. 

Tails also act as a communication tool, allowing animals to send signals to each other during social interactions. In some species, tails are even essential for regulating body temperature or defending themselves against threats. 

However, humans and apes are unique among mammals in that we lack tails. This evolutionary change is believed to be linked to our ancestors’ transition to walking on two legs (bipedalism) and their move from forest habitats to more diverse landscapes. 

The new study suggests that a specific change in a gene called TBXT might be responsible for the lack of tails. The change is caused by a small piece of DNA called an AluY element that got inserted into the TBXT gene millions of years ago. The DNA doesn’t directly alter the gene’s code, but it does affect how the gene works.

Surprising findings about human tails

To understand the impact of this genetic modification, the researchers engineered mice to carry the same TBXT gene alteration. The results were striking: some mice had shortened tails, while others were born entirely tailless. 

These variations provide concrete evidence that the AluY insertion in the TBXT gene directly contributes to tail development and its eventual evolutionary loss in certain primates. The experiment demonstrated the real-world effects of the genetic alteration observed in humans and apes.

“This finding is remarkable because most human introns carry copies of repetitive, jumping DNAs without any effect on gene expression, but this particular AluY insertion did something as obvious as determine tail length,” explained study co-author Jef D. Boeke.

Alternative splicing

The scientists explained that the extra DNA affects “alternative splicing,” which is a process that allows a single gene to create multiple different proteins.  

In this case, the genetic change affects a gene involved in tail development. Through alternative splicing, it now produces a unique protein version, different from the one typically present in tailed animals. This altered protein disrupts the normal course of tail development. 

The study highlights the significant impact of even minor changes in our DNA. This alteration, although seemingly small, ultimately contributed to the loss of tails in our evolutionary past.

Potential drawbacks of human tails

The experts also discovered a potential downside to the genetic change. While the exact reasons for tail loss remain under debate, the research suggests a possible link between this evolutionary change and certain health problems.

The mice with altered genes experienced a higher rate of birth defects related to the closing of the neural tube, which forms the spinal cord and brain.

This suggests that while losing tails may have provided some benefits, it may have also increased the risk of specific health issues.

“Future experiments will test the theory that, in an ancient evolutionary trade-off, the loss of a tail in humans contributed to the neural tube birth defects, like those involved in spinal bifida, which are seen today in one in a thousand human neonates,” said Itai Yanai from the Institute for Systems Genetics at NYU Langone Health.

When exactly did humans lose their tails?

Scientists pinpoint the disappearance of the external tail in the human lineage to approximately 25 million years ago.

This period corresponds to the emergence of the first apes, setting the stage for the evolution of bipedalism and other traits distinct to humans and their closest relatives.

The loss of the tail is not just a matter of physical appearance but signifies a deeper evolutionary strategy, adapting to new environments and ways of moving.

Genetic Clues and Fossil Evidence

The tale of the lost tail is not merely speculative; it is grounded in genetic and fossil evidence. Genetic analyses reveal specific changes in the DNA of humans and apes, which differ from that of tailed primates.

These genetic differences indicate alterations in the development processes that lead to tail formation.

Fossil records, albeit sparse, provide crucial insights into the morphology of early human ancestors and their primate cousins, showing a clear absence of the external tail in apes and humans alike.

Benefits of a Tailless Existence

The transition to a tailless form had significant implications for early human ancestors. Freed from the constraints of a tail, our ancestors developed enhanced balance and mobility, crucial for bipedalism.

This adaptation facilitated a more upright posture, allowing for the use of hands in tool-making, foraging, and social communication.

The evolutionary success of humans and apes without tails underscores the advantages of this trait (or lack thereof) in navigating the terrestrial and arboreal environments they inhabited.

Study implications and future research

This research sheds light on how evolution affects both our body features and our risk of certain health problems.

The study demonstrates that evolution is not just about physical changes but also shapes our susceptibility to health issues. 

We must consider evolutionary changes from a wider perspective, thinking about both the benefits they might bring and the potential health risks they might carry. 

This new way of thinking opens doors for further research into the genes and evolutionary history of diseases, helping us better understand how our evolutionary past influences our health today. 

A tail of evolutionary adaptation

In summary, the story of how and when humans lost their tails is a fascinating chapter in the saga of evolution. It illustrates the dynamic nature of evolutionary change, driven by the interplay of genetics, environment, and adaptation.

The loss of the tail is a testament to the complex journey of human evolution, highlighting how seemingly minor physical changes can have profound impacts on the trajectory of a species.

As we continue to uncover the mysteries of our past, the tale of the lost tail remains a pivotal piece of the puzzle in understanding what makes us uniquely human.

Next time you spot a monkey’s long, graceful tail, ponder this: evolution may have swapped ours for another crucial adaptation, shaping us into the beings we are today.

The study is published in Nature.

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