We may have finally figured out why we begin developing tails in the womb — and why we lose them at around eight weeks of gestation. A single genetic tweak that occurred among our ancestors 25 million years ago means humans today are unable to grow a tail, according to a new study. This tweak to our genetic makeup may have given us evolutionary advantages, such as being able to walk upright — but it came at a cost.
Researchers believe it could be behind a birth defect called spina bifida that still affects babies. Humans' ancestors started off with a tail, but about 25 million years ago, they dropped the appendage, leaving it to other tree-dwelling primates like monkeys. Human fetuses start growing a tail in the womb, but after about eight weeks they lose it, leaving behind only the coccyx, a nub at the end of the spine where the tail used to be.
Scientists knew that a gene called TBXT was involved in the snipping of the tail. Mutations to this gene are linked to shortened tail phenotypes in tailed animals. It's what gave us the Manx cat, a breed of cat without a tail, for instance. A team of scientists at the NYU Langone and Grossman School of Medicine say they have finally cracked it.
The solution, they found, was in a type of 'jumping gene' called an Alu element. These types of DNA sequences are known for moving around the genome. Scientists found two Alu elements around a part of the TBXT gene, called Exon 6. The insertion of these DNA sequences seemed to result in the loss of a tail.
Scientists tested their findings by inserting Alu sequences in mice. They found they sometimes had shortened tails or no tails at all. While the work provides an unprecedented look into why humans are the way they are, it also exposes a sinister shadow of the mutation.
Scientists found that adding the Alu sequences to the TBXT gene in mice didn't only truncate their tails, it also gave them an unusually high level of a defect in the neural tube, a structure which later turns into the nerves in the spine and the brain. In humans, that defect is known for producing spina bifida, which is when the baby's spinal cord doesn't completely close, leaving a bit of the spine and some nerves exposed on the back.
Some scientists argue that it was a necessary sacrifice in our quest to stand up. They say, for instance, that a tail would have hindered the delicate balance that keeps us on our feet. Others, however, aren't sure it's that simple. Early research now suggests we may have lost our tails while we were still in trees, and that a tail could have helped, rather than hindered, our two-legged lifestyle.
It's possible that the mutation arose when humans' and apes' ancestors were isolated from other groups. They claim the loss of the tail was less of an evolutionary boon as it was a bizarre trait our ancestors kept because they didn't mix with primates with tails at the time. 'We apparently paid a cost for the loss of the tail, and we still feel the echoes,' said a study author and graduate researcher at NYU Grossman School of Medicine.
