Why don’t humans have tails?

Sina science and technology news on November 17, Beijing time, tens of millions of years ago, the common ancestor of humans and other primates had tails. Many modern primates also have tails, such as monkeys and lemurs. However, with the evolution and differentiation of primates, the ancestors of modern humans and the primates closest to us have completely discarded their tails, including apes such as chimpanzees and bonobos.

A6021 rhesus monkeys and other monkeys have tails, while humans and apes do not.

Why do some primates keep their tails, but humans and apes don’t? The disappearance of the tail has been regarded as part of the evolution of human beings into bipedal walking, but how did we lose the tail? Scientists have been trying to figure this out.

However, researchers have recently found a genetic clue that may explain why humans don’t have tails. They found a so-called jumping gene associated with tail growth. Tens of millions of years ago, this gene may have suddenly jumped to another position in the primate genome, and the resulting genetic variation led to the disappearance of the tail.

In fact, humans also have tails, but they are preserved only in the embryonic stage. The feature of tail can be traced back to the first vertebrates on earth, so we will have a tail temporarily in the early stage of embryonic development. This is true not only of humans, but of all other vertebrates. But after eight weeks of development, the tails of most human embryos disappear completely. In an article published in the journal Nature in 2008, scientists pointed out that the disappearance of the tail is realized through apoptosis, which is also known as programmed cell death. It is an inherent function in the development of multicellular organisms.

A660202 chimpanzees are one of our closest relatives. They also have no tails.

After that, the human “tail” is only the tailbone composed of three or four vertebrae.

Human babies are sometimes born with tails, but this is extremely rare. A study published in the Indian society of pediatric surgeons in 2012 pointed out that these residual protrusions are the residual structure of the embryo, usually false tails, not real tails. These false tails are covered with skin and contain muscles, nerves, blood vessels and connective tissue, but they lack bone and cartilage and are not connected to the spinal cord like the real tail.

But how did humans lose their tails? For Shapo, a doctoral student at New York University’s Grossman School of medicine, this question has fascinated him since his childhood. Xia Bo is currently studying the genetic mechanism of human development, disease and evolution. He is also the main author of a study on the disappearance process of human tail. The research findings were published on the paper preprint website biorxiv in September this year and have not been peer reviewed.

“I thought about this problem when I was a child. Because I saw that almost all animals have tails, but I didn’t,” said Shapo. Not long ago, after the caudal vertebra was injured, Shapo decided to further explore this problem, trying to find out how the tails of humans and apes disappeared in the process of evolution.

The earliest known tailless ancestor of humans and apes was a primate called the proto kangshur ape during the Miocene of the Tertiary period (23 million to 5.3 million years ago) Living in Africa, Xia Bo and his co authors wrote in their research report that the disappearance of the tail may have originated a long time ago – about 25 million years ago, when the hominid lineage of humans and apes “parted ways” from the ancient monkey family.

They compared the genetic data of six hominid species with nine monkey species to find out the possible differences related to the tail. They found a DNA fragment called Alu element in the tbxt gene responsible for regulating tail development (this kind of DNA can change its position in the genome and affect protein synthesis) It disappeared. This mutation appeared in the ape and human genomes, but there was no mutation in monkey species, and this fragment was still retained.

Next, the researchers used the gene editing tool CRISPR to copy the gene mutation into the tbxt gene of mice. As a result, the tails of genetically modified mice vary in length, from completely normal to completely disappeared. Although this gene variation does affect the tail, it is not a switch to determine whether the tail exists or not. Scientists realize that other genes play a role in the loss of primates’ tails. However, ether Yanai, director of the Institute of computational medicine at New York University and professor of Biochemistry and molecular pharmacology, pointed out that the emergence of this variation “is likely to be a key event interfering with the formation of tails”.

Advantages and disadvantages of tail

Apes and early humans may have benefited from losing their tails because it helped them transition to upright walking. But Michelle Bezanson, a professor of anthropology at the school of Arts and Sciences at Santa Clara University, points out that primates that retain their tails have no loss because they can perform many functions.

环尾狐猴无法用尾巴抓握,但可以在它们跳跃时维持身体平衡。 ring tailed lemurs cannot grasp with their tails, but they can maintain balance when they jump.

“The tail can stretch during jumping, helping the body maintain balance and prepare for landing,” Besson explained. “They can also maintain body stability during movement, foraging and even sleep. When the animal hangs upside down in the air with its hind legs, the tail can also touch a surface.”

The tail of a primate can sometimes be used as a tool. For example, white headed capuchin monkeys use their tails to suck water from tree holes, and then suck the water from their tail hair dry, just like a sponge. Primates also use their tails as pillows or quilts, and even use them in social behavior.

“I like to watch Little Monkeys grasp their mother’s body or tail with their flexible tail,” Besson said. Monkeys also pull each other’s tails while playing. South American monkeys even wrap their tails around their mating partners to show their love.

The tail can complete so many difficult tasks that humans without a tail will inevitably feel that they are missing something. So, will humans grow tails again one day? Unfortunately, Shapo pointed out that we have lost our tail for too long to grow again. 25 million years ago, it was so long ago that even the emergence of our species, Homo sapiens, was far behind it. During this long period of time, the genes related to tail development in our lineage no longer function, and all the DNA fragments needed to form the tail have long been lost

“Even if we can correct the relevant genetic variation, we may not be able to grow the body structure again,” Shapo pointed out. (leaves)