Scientists think they finally know why humans don’t have tails

For years, scientists have pondered the question: Why is our species devoid of a tail, especially when our primate ancestors once possessed one?

Researchers believe they have finally figured out the genetic mechanism responsible for our tailless condition and that of our ape ancestors.

It appears to be linked to a mutation in a key gene.

The tail has been a predominant feature in most vertebrates for more than half a billion years, and its disappearance may have brought advantages as our ancestors moved from arboreal habitats to terrestrial environments, scientists say.

The researchers compared the DNA of two groups of primates: apes, which have tails, and hominoids—humans and apes—which do not. They found a mutation in a gene called TBXT that was present in humans and monkeys but absent in monkeys. To test the effects of this mutation, the researchers genetically modified laboratory mice to have this trait. These mice end up with either a reduced tail or none at all.

“For the first time, we propose a plausible scenario for the genetic mechanism that led to the loss of the tail in our ancestors. It’s surprising that such a large anatomical change can be caused by such a small genetic change,” said NYU Langone Health geneticist and systems biologist Itai Yanai, who helped lead the study, published in the journal Nature.

The lack of a tail may have better balanced the body for orthograde — upright — locomotion and possibly bipedalism, said geneticist and systems biologist Bo Xia of Harvard University and the Broad Institute, the study’s lead author.

The mutation that led to the loss of the tail, according to the researchers, occurred about 25 million years ago, when the first apes evolved from ape ancestors. Our species, Homo sapiens, appeared about 300,000 years ago.

The evolutionary line that gave rise to apes and humans diverged from the line that gave rise to today’s Old World monkeys, a family that includes baboons and macaques. Hominoids today include humans, the “great apes” – chimpanzees, bonobos, gorillas, orangutans – and the “lesser apes” – gibbons. The earliest known hominoid, called Proconsul, was tailless.

Hominoids evolved the formation of fewer caudal vertebrae, losing an external tail. Traces of tail remain in humans. The bone at the base of the spine, called the coccyx or coccyx, is formed from the fused remnants of tail vertebrae.

For many vertebrates, the tail has aided functions such as locomotion—think propulsion by fish and whales—and defense—as in dinosaurs, which twirled tails with clubs or spikes. Some monkeys and some other animals have prehensile tails that can grasp objects such as tree branches.

“A tail can be useful when you live among trees. But as soon as you move to land, it can be more of a liability,” Yanai said.

The benefits of going queue-free seem to come with a price. Because genes can contribute to multiple functions in the body, mutations that confer an advantage in one area can be detrimental in another.

In this case, the modified mice showed a slight increase in severe birth defects, called neural tube defects, of the spinal cord, resembling spina bifida in humans.

“This suggests that the evolutionary pressure to lose the tail was so great that despite creating the potential for this condition (neural tube defects), we still lost the tail,” Yanai said.

An interesting thought experiment is to consider whether humans evolved with tails.