For nearly 500 million years, our ancestors had tails. Like fish, they used to swim across the Cambrian seas. Much later, as they evolved into primates, their tails helped them maintain balance as they ran from branch to branch through the jungles of the Eocene. But about 25 million years ago, their tails disappeared.
Charles Darwin was the first to recognize this profound change in the anatomy of our ancestors. However, how and why this happened remains a mystery.
Now, a team of scientists in New York are claiming to locate the genetic mutation that may have wiped out our tails. When scientists performed this genetic modification on mice, the animals didn’t stand in the queue, according to a new study published online last week.
This dramatic anatomical change has had a profound effect on our evolution. Our ancestors’ tail muscles evolved into a hammock-like network across the pelvis. When human ancestors stood and walked on two legs millions of years ago, that muscular swing was ready to bear the weight of the erect organs.
Although it’s impossible to prove that this mutation cut off our ancestors’ tail, said Cedric Viscott, a geneticist at Cornell University, who was not involved in the study, “it’s as close as we can hope.” .
Darwin caused scandal to his Victorian audiences by claiming that we descended from tailed primates. He noted that although humans and apes did not have a visible tail, they shared a small group of vertebrae that extend beyond the pelvis, a structure known as the coccyx.
“I have no doubt that it is a primitive tail,” he wrote.
Since then, paleoanthropologists have discovered fossils that illustrate this transformation. The oldest known primates date back about 66 million years and have fully grown tails which may have been used for balancing in trees. Today, most living primates, such as lemurs and almost all monkeys, still have tails. However, when apes appeared in the fossil record, about 20 million years ago, they did not have a tail.
“This question — where’s my tail? — has been in my head since I was a kid,” said Bo Shea, a graduate student specializing in stem cell biology at New York University’s Grossman School of Medicine.
In 2019, a bad trip with Uber, in which Shea struck a tailbone, reminded him of renewed urgency. “It took me a year to recover and it really got me thinking about my tailbone,” he said.
To understand how apes and humans lost their tails, Shea looked at how other animals’ tails formed. In the early stages of embryonic development, a set of master regulators are triggered, to coordinate different parts of the spinal cord to develop distinct traits, such as the neck or lower back. At the far end of the embryo, a tail cocoon arises, within which a special series of vertebrae, muscles and nerves develop.
Researchers have identified more than 30 genes involved in the development of the tails of different species, from the long whip of the iguana to the tail of the Manx cat. All of these genes are also active in other parts of the developing fetus. Scientists continue to learn how their unique activity at the end of the embryo leads to the appearance of the tail.
Xia concluded that our ancestors lost their tails when there were mutations that altered one or more of these genes. To look for these mutations, Chia compared the DNA of six species of tailed monkeys with that of nine species of tailed monkeys. Eventually, he discovered a mutation that monkeys and humans share – but is missing in monkeys – in a gene called TBXT.
TBXT was one of the first genes that scientists discovered more than a century ago. At the time, many researchers searched for genes by bombarding animals, plants, and microbes with X-rays, hoping that mutations would bring about visible change.
The mutation that Xia discovered had not been observed before. It consisted of 300 genetic letters in the middle of the TBXT gene. This stretch of DNA was nearly identical in humans and monkeys and was inserted at exactly the same location in their genomes.
Shea took the discovery to his supervisors, Itai Yanai and Jeff Boeke, for their opinion. “I almost fell off a chair, because it was just such a great result,” Yanai recalls.
To test the idea that the mutation was involved in the disappearance of our tail, Shea and colleagues genetically engineered mice bearing the human-borne TBXT mutation. When these embryos developed, many animals’ tails did not grow. Others only got one short.
Shea and colleagues suggest that this mutation randomly infected a monkey about 20 million years ago, causing only a trunk to grow out of the tail or nothing at all. However, the brilliant animal has survived and even evolved, to pass the mutation on to its offspring. Eventually, the mutated form of TBXT became the standard for apes and humans today.
According to scientists, the TBXT mutation isn’t the only thing that explains why we have a tailbone instead of a tail. Although the mice in the experiment produced a series of altered tails, our tailbones are nearly identical from person to person. There must have been other genes that mutated later and helped produce the uniform anatomy.
Although genetics have begun to explain how our tails disappear, scientists remain puzzled as to why this happens.
The first apes were larger than the ape, and their increased size would have facilitated falling off the branches, which would have been fatal. It is hard to explain why monkeys without tails to help them balance would not suffer much evolutionary damage.
In addition, losing the tail may also lead to other risks. Shea and colleagues found that the TBXT mutation not only shortens the tail but sometimes also causes defects in the spinal cord. However, somehow, tail loss proved to be a major evolutionary feature.
(c) The New York Times