Why Did the Ancestors of Humans Lose Their Tails Around 20 to 25 Million Years Ago? — Here’s What Researchers Discovered

Long ago, many species of primates had tails. It helped them in their natural habitats, which were thought to be forestry areas. Tails provide balance, an additional climbing limb, and just overall make moving in trees more efficient, safer, and better. But somewhere along the line of the hominoid or ape species lineage, tais disappeared.

Some of the primates today still retain their tails, while humans and apes, who are both part of the hominoid superfamily, lost theirs a long time ago. It’s not just us, the current species, but also our ancestors millions of years ago did not have a tail anymore. So, did our lineage really have a tail at one point, and if so, when did our ancestors lose it and for what reason? Let’s dive deeper into these questions.

The Genetic Discovery: A Jumping Piece of DNA in the TBXT Gene

First, let’s look at some of the latest findings in modern science. A study published in Nature provided a compelling explanation and experiment for how our species and its Old World ancestors may have lost their tails that they probably had at one point.

Looking at the human anatomy, scientists found the coccyx, or tailbone, which often serves as an evolutionary evidence that the human lineage once had a tail, or at least our distant ancestors. So, in the recent study, researchers examined the ape lineage and found a piece of DNA called the Alu element that is inserted in a gene known as TBXT, which has significance in tail development.

The insertion did not directly contain the instruction whether the species would have a tail or not, but it influenced how that gene could be read and interpreted. This led to the shortening of tail growth in embryos, and sometimes a complete loss. So, despite having the gene for a tail, if that gene is processed in a way that will limit its application, the appearance of a tail could be suppressed.

To test this, scientists tested the insertion of an engineered gene from the apes into mice. Then those mice developed very short tails, and in other cases, they did not grow any tail at all. It simply showed that those specific genetic changes could have influenced a tail loss in a species.

What this implies is that this single but powerful mutation could be what caused the loss of the tails in ape lineage a long time ago. But, how exactly did this mutation occur? If we now have an idea how our tails could have disappeared, then the next questions to answer is why our ancestors mutated, when it happened, and what made them lose their tails over time?

Read more: Nature.com

Timing and the Fossil Record: When in Evolution Did Tails Disappear?

How Humans Lost Their Tails — HHMI BioInteractive Video

Scientists discovered fossilized ancestors of both monkeys and apes, like Aegyptopithecus zeuxis, from around 33 million years ago. This species still has its tail connected to its tailbones or coccyx. However, around 20 to 25 million years ago, an evolutionary split happened, and the ape lineage diverged from the other primates. This is also the timeline, fossils and molecular evidence point to when our ancestors likely lost their tails.

So, old world monkeys retained their tails, while the ape lineage, which includes hominins such as modern humans, did not. A split that is observed in fossils and genetic lineages of different species. But, of course, we do not have fossil evidence for each timeline, and it is incomplete, which is why the estimated ages of when we lost our tails have a wider range.

However, the genetic studies estimate that the timing of the mutation aligns with whatever fossils we have discovered that showcase an ape lineage not having a tail anymore, around 20 to 25 million years ago. But, thinking of this timeline, it occurred long before bipedalism and walking upright evolved around 6 to 7 million years ago, so is the loss of the tail evolutionary advantageous in other ways instead? Let’s look at some prominent theories.

Read more: DNA Science

Evolutionary Theories: Why Losing the Tail Might Help

Finding the mutation that could have caused the tail loss is a great discovery, but it does not explain why that mutation occurred in the first place. It could certainly be a random event caused by an error in DNA replication or other internal processes. However, the fact that it persisted for millions of years suggests that there could be an evolutionary reason that happened gradually instead of just one quick DNA error.

One of the theories is for locomotion and posture. When the ape lineage is believed to have spent reduced time in trees, or above branches and forestry environments where a tail would be beneficial, they may have needed to adapt a more upright and vertical movement. This could be because of open areas like grasslands expanding and forests receding. Climate change and environmental shifts may have caused it, leading some species to adapt to these new habitats.

The removal of the tail may have freed the lower spine to be more vertical, and adapt a different posture or walking position that is advantageous in the new environment. It could also be that the disappearance of the tail is just the first evolutionary step to make way for upright walking. If no tail swings behind, the lower back and spine could have finally anatomically adapted to allow the prolonged standing up of apes.

Another theory could be that the tail loss is just slightly advantageous or completely neutral. However, if it became less critical and needed for survival, then our lineage may have gradually lost it without any heavy negative consequences, so the disappearance of tails was retained even after millions of years.

Some more theory refers to the energy expenditure of having, developing, and moving a tail. So, if it was not as critical for survival anymore because of moving to the ground or at least under the branches of the trees or brachiation, and its absence meant more energy saving, then our ancestors could have just gradually evolved to lose it.

Read more: Smithsonian

What Remains: Open Questions and Vestigial Remnants

Why Don't Humans Have Tails?

Even with the new genetic studies and theories supported by fossil evidence, not everything is solved. Some of the questions still remaining are whether the mutation or the loss of tail occurred just once in a common ancestor, or did it happen in multiple different species and lineages.

Additionally, humans still carry a vestigial tail structure, the tailbone or the coccyx. This was a part of us that is made up of fused vertebrae that would have been the skeleton of the tail if we still had it. However, we still retained it after millions of years as a remnant or leftover of our ancestors. It now just supports different muscles and ligaments as an anchoring point.

Author's Final Thoughts

Losing a tail is not just any evolutionary change in our lineage; it may have resulted in much greater significance later down the line in our evolution, after 20 to 25 million years ago. It could have been the first step that led to our ancestors standing upright, walking with two legs, and finally freeing their hands for even more revolutionary changes like the development of tools.

In other words, our taillessness could be one of many traits that separated us from other primates and ultimately shaped the path of human evolution. People still often ask why we don’t have any tails anymore, but maybe this is part of the countless adaptations that set the stage for our eventual emergence as an intelligent species capable of pondering such a question in the first place.

Read next: How Did Humans Contribute to the Megafauna Extinction Around 10,000 Years Ago? — Here’s What Researchers Found Out

References & Further Reading

Xia, B., et al. (2024). On the genetic basis of tail-loss evolution in humans and apes. Nature. https://www.nature.com/articles/s41586-024-07095-8

Tojima, S., & Yamada, S. (2021). A comprehensive understanding of the “human tail.” PMC / NCBI. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8128518/

Almécija, S., et al. (2021). Fossil apes and human evolution. Science. https://www.science.org/doi/10.1126/science.abb4363

Smithsonian Magazine (2024). Why don’t humans have tails? An old genetic mutation could explain it. Smithsonian Magazine. https://www.smithsonianmag.com/smart-news/why-dont-humans-have-tails-an-old-genetic-mutation-could-explain-why-monkeys-but-not-apes-have-the-extra-appendage-180978764/

ScienceNews (2024). A genetic parasite may explain why humans and other apes lack tails. ScienceNews. https://www.sciencenews.org/article/genetic-parasite-humans-apes-tail-loss-evolution