FOr half a billion years or so, our ancestors grew tails. Like fish, they used their tails to swim through the Cambrian Sea. Much later, when they evolved into primates, their tails helped them stay balanced as they ran from branch to branch through the Eocene forests. But then, about 25 million years ago, the tail disappeared.
Charles Darwin was the first to recognize this profound change in our ancient anatomy. But how and why this happened remains a mystery.
Now a team of scientists in New York says they have pinpointed a genetic mutation that may have wiped out our tails. According to a new study, when scientists made this genetic change in mice, the animals’ tails did not grow.
This dramatic physical change had a profound effect on our development. The tail muscles of our ancestors evolved into a hammock-like mesh throughout the pelvis. When the ancestors of humans stood up and walked on two legs a few million years ago, that muscular hammock was designed to support the weight of upright limbs.
While it’s impossible to definitively prove that this mutation truncated the tail of our ancestors, “it’s as close to a smoking gun as one might expect”, says Cedric Fashote, a geneticist at Cornell University who studied were not involved.
Darwin shocked his Victorian audience by claiming that we descended from the tailed primate. He notes that while humans and apes lack a visible tail, they share a small set of vertebrae that extend beyond the pelvis—a structure known as the coccyx.
“I cannot doubt that this is a rudimentary tail,” he wrote.
Since then, paleontologists have discovered fossils that shed some light on this change. The oldest known primates, which were 66 million years old, had full tails, which they probably used to keep their balance in trees. Most living primates today, such as lemurs and almost all monkeys, still have tails. But when apes appeared in the fossil record about 20 million years ago, they had no tail.
“This question – where is my tail? – has been on my mind since I was a kid,” says Bo Xia, a graduate student in stem cell biology at the NYU Grossman School of Medicine.
Uber’s poor ride in 2019, in which Xia injured her coccyx, brought her back to her mind with a fresh mind. “It took me a year to recover, and it really got me thinking about the tailbone,” he says.
To understand how apes and humans lost their tails, Xia looked at how tails form in other animals. In the early stages of embryonic development, a set of master genes are turned on, which modulate different parts of the spine to develop specific identities, such as the neck and lumbar region. At the distal end of the embryo, a tail bud emerges, inside which a specialized series of vertebrae, muscles and nerves develop.
Researchers have identified more than 30 genes involved in tail development in a variety of species, from the long whips of an iguana to the stubble on a Manx cat. All these genes are also active in other parts of the developing embryo. Scientists are still learning how their unique activity gives rise to a tail at the end of an embryo.
Xia argued that our ancestors lost their tails when mutations changed one or more of these genes. To discover those mutations, they compared the DNA of six species of tailless apes with nine species of tailless monkeys. Eventually, they discovered a mutation shared by apes and humans — but missing in monkeys — in a gene called TBXT.
TBXT was one of the first genes discovered by scientists more than a century ago. At the time, many researchers discovered genes by zapping animals, plants or microbes with X-rays, hoping the mutation would produce a visible change.
In 1923, Russian geneticist Nadezhda Dobrovolskaya-Zavadskaya X-rayed male mice and then allowed them to breed. He found that some of them acquired a mutation that caused some of their descendants to develop kinked or short tails. Subsequent experiments showed that the mutation was on the TBXT gene.
The mutation that Xia discovered had not been seen before. It contained 300 genetic letters in the middle of the TBXT gene. This stretch of DNA was almost identical in humans and apes, and was inserted at the exact same location in their genomes.
Xia brought in her supervisors, Itai Yanai and Jeff Boecke, to see what they thought. “I almost fell out of my chair because it’s just a surprising result,” Yanai recalls.
To test the idea that the mutation was involved in the disappearance of our tails, Xia and his colleagues genetically engineered mice with the TBXT mutation that humans carry. When these embryos developed, many animals failed to develop tails. Others only grew a little.
Xia and his colleagues propose that this mutation randomly killed an ape about 20 million years ago, making it grow just a stump of a tail, or not grow at all. Nevertheless, the tailless animal survived and even thrived, passing the mutation on to its offspring. Eventually, a mutant form of TBXT became the norm in living apes and humans.
Scientists say the TBXT mutation isn’t the only reason we develop a coccyx instead of a tail. While rats produced many altered tails in their experiments, our coccyx is almost always the same from person to person. There must be other genes that are subsequently mutated, helping to produce a similar anatomy.
Even as geneticists are beginning to explain why our tails disappeared, the question of why still puzzles scientists.
The first apes were larger than monkeys, and their increased size made it easier for them to fall from branches, and were more likely to fall. It is difficult to explain why tailless apes would not have suffered a significant evolutionary disadvantage to help them balance.
And losing a tail could bring other dangers as well. Xia and his colleagues found that the TBXT mutation not only shortens the tail, but sometimes also causes defects in the spinal cord. And yet, somehow, losing a tail proved to be a major evolutionary advantage.
“It’s very confusing why they lost their tails,” says Gabriele Russo, an evolutionary morphologist at Stony Brook University in New York who was not involved in the study. “That’s the next excellent question: What on earth will it gain?”
This article originally appeared in the new York Times
Credit: www.independent.co.uk /