25-Million-Year-Old DNA Explains Why Humans and Apes Don’t Have Tails

While many primate species have tails, humans and their ape cousins do not. For many years scientists have debated the reasons for this curious tail loss variation, trying to understand the reasons behind this difference.

A team of geneticists, affiliated with NYU Langone Health in New York City, published a new paper in the journal Nature. Their research suggests that a seemingly random piece of DNA inserted into the genome of a human ancestor is responsible for the loss of the tail in modern humans and apes. The addition of this foreign element would have occurred approximately 25 million years ago, and its impact on the subsequent evolution of our species and its ancestors has been profound.

Tail phenotypes across the primate phylogenetic tree provides a visual representation of which primates have and have not experienced tail loss. (Xia, B. et. al. / CC BY 4.0 DEED)

A True Tale about Tail Loss in Humans

It is specifically the category of primate species known as hominoids, which includes humans, chimpanzees, gorillas, orangutans and gibbons, that do not have tails. Other types of primates have them and it is known that an evolutionary split took place at some point that explains why this is the case.

Relying on the latest technologies employed in genetic research, the team of scientists examined 140 genes linked to the development of tails in vertebrates (animals with backbones). They were looking for DNA variations between tail growers and non-tail growers, to see if they could figure out how tail development might have been halted in the ancestors of modern-day hominoids.

Eventually, the researchers zeroed in on changes in a gene known as TBXT, which is highly influential in tail growth in all animals that have tails. Inside the hominoid version of the gene they identified a unique element known as AluY, which was apparently inserted into the DNA sequence of TBXT millions of years ago, during the Oligocene epoch.

Genetic testing on mice and its impact in relation to tail loss. (Xia, B. et. al. / CC BY 4.0 DEED)

To test their theory that AluY and the changes it caused could interfere with tail development, the researchers inserted TBXT genes that mimicked the AluY-infected versions into the genomes of mice. This meant the mice possessed their own TBXT genes, plus the newly added variety that was designed to match the shape and performance of TBXT genes found in primates that don’t grow tails. 

Notably, all the mice that had the new gene added either failed to develop a tail or grew one that was much shorter than normal. This shows that the TBXT gene with the AluY element added would have had a strong inhibitory effect on tail growth in any species where it was present.

During their research the genetic scientists discovered something else that was quite surprising. They found that mice given the altered TBXT genes sometimes developed neural tube defects, a condition that is present in about one out of every 1,000 newborn human babies.

Neural tube defects cause deficits in brain, spine or spinal cord development in fetuses. They are often associated with severe disabilities that can be life-altering or even terminal. Based on the results of their work with mice, the genetic researchers from NYU Langone Health suspect that the AluY element in the human genome is somehow linked to the onset of neural tube defects (although further research will be needed to evaluate this hypothesis).

Model for tail loss evolution in early hominoids. (Xia, B. et. al. / CC BY 4.0 DEED)

The Evolutionary Legacy of Tail Loss: Could Humans Someday Grow Tails Again?

The discovery that the insertion of the AluY DNA sequence may have caused tail loss in our ancient ancestors raises an interesting question: if the offending genetic material were removed, could humans start growing tails again?

According to the researchers involved in the new genetic study, the answer to this question is “no.” They suggest that additional changes in the human genome likely stabilized the no-tail pattern, reinforcing the impact of the added AluY genetic element.

Because of these stabilizing factors “a change to the AluY element in modern hominoids would be unlikely to result in the appearance of the tail,” the researchers confirmed in their Nature article. This observation highlights the fact that modern humans are distinct from their distant ancestors in many ways, and that it would not be possible to reverse the course of human evolution by simply making a few changes in the genome here or there.

In evolutionary terms, an alteration in DNA is not enough to explain the development of a new characteristic, or the loss of an old one. There must be some survival-related advantage connected to the change in order to make sure its effects are lasting and eventually passed on to the entire species.

“The specific evolutionary pressures relating to the loss of the tail in hominoids are not clear,” the researchers wrote, “although they are probably involved in enhanced locomotion in the transition to a non-arboreal lifestyle.”

In other words, tails may have been useful when ancient hominoids were living in and climbing around in trees, but less necessary when they began living on the ground. Tails may have even been a hindrance in this living environment, which would have sped up their disappearance.

Whatever the reason for the extinction of the hominoid tail, “the selective advantage must have been strong because the loss of the tail may have included an evolutionary trade-off of neural tube defects,” the genetic researchers noted. The researchers believe that, 25 million years after the ancient human gene pool experienced tail loss, these defects are still evident in modern humans. This suggests that the evolutionary trade-off continues to impact human health today.

Top image: Why is it that hominoids have experienced tail loss, while other primates have not? Source: v_blinov / Adobe Stock

By Nathan Falde