A team of scientists in New York claims to have identified the genetic mutation that may have resulted in the elimination of human tails. According to a new study, when scientists induced this genetic change in mice, the animals did not grow tails.
This major modification in our ancestral anatomy was first noted by Charles Darwin. But how and why it happened is still unknown.
Darwin stunned the world when he claimed that humans originated from primates with tails. While humans and apes do not have a visible tail, they do share a tiny group of vertebrae that extend beyond the pelvis - a structure known as the coccyx, commonly known as the "tailbone."
This tailbone is actual evidence that something happened along our evolutionary path that caused us to lose our tails. Humans are not alone either.
Humans are classified as great apes, and like gorillas, orangutans, chimps, and bonobos, we do not have tails. Lesser apes, such as gibbons, lack tails as well. So, what exactly happened?
"This question - where's my tail? - has been in my head since I was a kid," Bo Xia, a graduate student in stem cell biology at NYU Grossman School of Medicine, said.
Xia studied how other animals' tails form to better understand how apes and humans lost theirs. During the early stages of embryonic development, a group of master genes activates, coordinating different sections of the spine to establish separate identities, such as the neck and lumbar region. A tail bud arises at the far end of the embryo, within which a unique chain of vertebrae, muscles, and nerves develops.
Xia reasoned that human ancestors lost their tails as a result of mutations in one or more of these genes. He examined the DNA of six species of tailless apes to nine species of tailed monkeys to look for those mutations. He eventually uncovered a mutation in a gene called TBXT that is shared by apes and humans but absent in monkeys.
The mutation discovered by Xia has never been seen before. It was made up of 300 genetic letters located in the middle of the TBXT gene. This stretch of DNA was nearly identical in humans and apes, and it was placed in the exact same location in both genomes.
To test the theory that the mutation was responsible for the removal of our tail, Xia and his colleagues genetically engineered mice with the human TBXT mutation. Many of the creatures that emerged from these embryos did not have a tail. Others grew only a short one.
The genetically engineered mouse embryos developed a variety of different short tails. The human tailbone, on the other hand, appears to be nearly same among individuals, implying that other mutations may be involved in its evolution.
Xia and his colleagues hypothesize that this mutation happened by chance to an ape 20 million years ago, causing it to grow only a stump of a tail or none at all. Despite this, the tailless animal survived and thrived, passing the mutation on to its descendants. The mutant form of TBXT eventually became the norm in living apes and humans.
The new study appears in the preprint server bioRxiv.
