Have you ever wondered why we humans don't have tails like many other animals? In fact, the loss of the tail during evolution is a distinguishing feature that distinguishes humans from other primates. Scientists have long speculated that the disappearance of the tail may be related to human upright walking, but the genetic mechanism behind this has remained a mystery.
In March 2024, the journal Nature reported a groundbreaking study in which scientists from Harvard University, New York University, and other institutions found that the "inaccession" of a genetic element called Alu may be a key factor in causing human ancestors to lose their tails.
The study was featured on the cover of Nature (Image: Nature)
A scientist's curiosity stems from childhood questions
This study of the mystery of the disappearance of the human tail also has an interesting "prequel". The first author and co-corresponding author of the article, Xia Bo, is a student in the 2009 life science experimental class of China Agricultural University. Years ago, as a young man, he once pondered a question: Why don't I have a tail? This seemingly childish question became the starting point of his future scientific research path.
In the summer of 2019, an accident resurfaced Xia Bo's childhood doubts. At that time, he accidentally injured the soft tissue of the coccyx while giving up his seat to the passenger who was back in the taxi. For many, an injury may be just an unfortunate experience, but for a scientist, it can be a rare opportunity to research. Xia Bo is such a person who is good at turning "misfortune" into motivation. He decided to take advantage of the accident to revisit the problem of his childhood. Thus, a study on the genetic mechanism of human tail loss began.
What stole the human tail?
You know, in the large family of primates, the absence of a tail is a unique "hallmark" of humans and apes. It is speculated that this change occurred about 25 million years ago. At that time, the ancestors of humans had not completely separated from the other apes, but they had already begun to quietly "say goodbye" to the tail.
Several species of apes and human tailbones (Image source: Wikipedia)
Over the course of a long evolutionary process, the tail of hominin gradually degenerated, and finally only 3 to 5 tail vertebrae remained, forming what we now have as the tailbone. So, what caused this change? The secret is in our genome.
Thanks to advances in primate genome sequencing technology in recent years, scientists have been able to look for clues in vast genetic sequences, linking changes in certain genes to the characteristics of an organism.
In order to find the "culprit" behind the loss of human tails, Xia Bo and his colleagues compared and analyzed the genomic data of a large number of primates. They focused on genes that are closely related to tail development to see how they differ from species to species. These genes are like "switches" that control the growth of the tail, and if something goes wrong with them, the tail may not grow.
Tails exist during human embryonic development (Image source: Wikipedia)
The hard work paid off, and the research team eventually locked on a gene called TBXT. This gene is crucial in vertebrate embryonic development, and its mutations can cause the tail to shorten or even disappear altogether in model organisms such as mice. And to the excitement of the scientists, they found that in the TBXT gene of all great apes, there is a unique Alu element inserted that other monkeys do not.
What exactly is this ALU element? Simply put, it's a small piece of DNA sequence scattered throughout our genome, like a mischievous "uninvited guest." Normally, they sit quietly and do not have any effect on gene function. But sometimes, they "jump" into certain genes, changing the way genes are expressed, which in turn affects the traits of the organism.
In this study, scientists speculate that about 25 million years ago, when our ancestors had not yet "parted ways" with other monkeys, an Alu element "uninvited" inserted into the TBXT gene.
Mouse experiments have successfully demonstrated that the insertion of the Alu element into the TBXT causes a portion of the gene to be spliced off during transcription, resulting in a deletion version of the TBXT protein. And this mutilated protein is the culprit that causes the tail of mice to shorten or even disappear completely.
Tail shortening or even disappearing in mice caused by TBXT gene mutation (Image source: Reference 1)
As a result, scientists speculate that similar genetic alterations may have occurred in our ancestors. When the Alu element invaded the TBXT gene, affecting its normal expression, the tail of ancient humans began to degenerate until it disappeared completely. Curiously, there are a number of other consequences that can be triggered by this change. Mice carrying the mutated TBXT gene were also found to be prone to neural tube defects, one of the most common birth anomalies in humans, with a probability of up to 1 in 1000.
This suggests that the disappearance of the human tail may not be without cost, and that the probability of birth defects is increasing at the same time.
Explore the mysteries of human evolution
This study reveals the molecular mechanisms behind the major evolutionary event of human tail loss, and it expands our understanding of our own evolutionary process. At the same time, it shows how seemingly inconsequential elements of the genome can quietly change the fate of humanity. The insertion of the ALU element, although a "fortuitous event" in evolutionary history, has had a profound impact on our posture, behavior, and even health.
Xia Bo's story also tells us that the seeds of science are often buried in our childhood curiosity. A chance opportunity, an accidental injury, can be a turning point for us to regain our curiosity and pursue the truth.
Of course, the disappearance of the tail is only a fragment in the long history of human evolution. Upright walking, brain development, language skills...... There are complex genetic mechanisms at work behind the formation of every trait. How these mechanisms evolve and how they interact with each other have shaped us today, and there are still many unsolved mysteries waiting to be explored.
The evolutionary process of the missing tail of the ape (Image source: Reference 1)
This study of humans losing their tails is like a small step towards that puzzle. In the future, there will be more such steps, leading us to uncover the mystery of human evolution little by little.
bibliography
[1] On the genetic basis of tail-loss evolution in humans and apes, Bo Xia et. al
[2] The Biological World: Why Don't Humans Have Tails? Xia Bo et al. published a cover paper in Nature, revealing that this skipping gene erased the human tail and posed additional risks
Planning and production
Produced by丨Popular Science China
Author丨Li Jinjuan
Producer丨China Science Expo
Editor-in-charge丨Yinuo
Reviewer丨Xu Lai, Lin Lin