What kind of experience is it for humans to grow tails?

Please answer quickly in 3 seconds: compared with most primates, what parts are missing in your body?

See? you don't have a tail.

Where's the tail?

In theory, we all had a tail before we were born.

When we were embryos, we would temporarily grow a "short tail" at about the 6th week. The tail has more than a dozen vertebrae, but within two weeks, half of the bones will be reabsorbed and the other half fused to form a caudal vertebra.

Nature loves diversity, and for some reason, very few people (mainly men) retain the tip of this embryonic tail when they are born. The tail has blood vessels and, in some cases, can even control the movement of the muscles on the tail, but no vertebrae can be found.

Compared with protuberances caused by various lesions or cancer, these residual "real tails" are very rare and are rarely seen in the medical literature.

Fortunately, most of these tails are easily removed surgically after childbirth-because they are too small to be useful and attract unnecessary attention.

If we all had a long tail, the situation would be completely different.

Why don't we have tails?

About 66 million years ago, an impact may have contributed to the extinction of dinosaurs, and less than 10 million years later, small primates appeared.

Most of them, like monkeys, lemurs, lemurs and bushy monkeys today, have a long tail that may help them keep their balance as they move quickly from crown to crown.

Over time, some of these tails evolved to be very flexible, like an extra thumb. In fact, such a tail is so convenient that it has evolved in two separate primate branches.

Unfortunately, about 20 million years ago, a group of tailless primates appeared. Their offspring include gorillas, chimpanzees, bonobos and orangutans. And, of course, us humans.

A recent study has determined why we lose our tails: inserting a sequence called Alu into a gene that is critical to forming a tail, interfering with the ability of the original gene to work.

This was discovered by Xia Bo, a stem cell biologist at New York University. Xia Bo compared the genomes of six species of tailless apes and nine species of tailed monkeys to find out which mutations made apes lose their tails.

In this study, he stumbled upon a gene called TBXT (T-box transcription factor T). Nearly a century ago, Russian geneticist Nadezhda Dobrovoskaya-Zawakaya (Nadezhda Dobrovolskaya-Zavadskaya) proved in experiments that mutations in this gene can shorten the tail of mice.

By taking a closer look at the Alu fragment in the TBXT gene, the researchers thought it was unlikely to work. But Xia Bo noticed that there are actually two Alu fragments very close to each other.

A single Alu sequence does not have any "destructive power", but the combination of the two can affect the expression of the TBXT gene. In the end, this mutation made apes like us lose their tails. Over time, our ancestors evolved other mutations that helped consolidate this change: the outline of the buttocks became obvious and the coccyx gradually flattened, making it ideal for sitting, squatting and striding on two legs.

While this may explain how this mutation works, it does not tell us why accidental disruption of the TBXT gene in ape ancestors years ago spread widely among a range of Homo species.

What if human ancestors had tails?

We can only speculate on why the loss of tails brings survival advantages to our ancestors.

Surprisingly, losing a tail brings additional health risks: after losing a tail, babies are more likely to develop neural tube defects and expose the spinal cord (such as spina bifida) after birth. Therefore, losing a tail must have another important advantage.

The tail is not only suspended at the end of the spine and extends out of the body, but its roots are also fixed in some important anatomical structures around the buttocks. These bones, ligaments and muscles can coordinate, maintain balance, and grasp and move objects (in some monkeys).

So if humans have tails, we also need buttocks and muscles to control them. Otherwise, the tail is not much better than a bulky sausage that drags around on the floor-no one wants such a thing.

This will be a big change. Primates with tails tend to have longer spines and more vertebrae to allow them to move flexibly between branches.

Tailless apes, on the other hand, have more developed skeletal muscles, shorter waists and harder spines. This gives them stronger backs that can better withstand falls or jumps, and may facilitate larger bodies or quick jumps to the bottom branches or the ground.

As for those tail muscles, a previous assumption was that they now provide support for our abdominal organs such as the bladder and intestines.

This structure allows us to put more pressure on the intestines while still keeping all the tissues in place and preventing the inadvertent excretion of urine and feces.

With this in mind, if humans have a monkey-like tail, the rest of our body must make some adjustments: we need to put the pelvic floor muscles back in place to connect and support the extra muscles and bones on the tail. If you have to keep walking on two feet, this may increase the risk of hernia and may lead to incontinence.

How important the disappearance of the tail played in the process that our ancestors evolved to stand and walk on two feet is highly controversial. But if our ancestors had not given up this tail, humans might not have evolved to what they are today.