Evrim Yazgin
Cosmos science journalist
The genetic code that saw ancient human ancestors lose their tails and walk upright has been found. And the same DNA might also have some negative side effects.
It is still not clear exactly when our ancestors first walked upright. Bipedalism – walking on two legs, instead of four – appears to have emerged as far back as 7 million years ago. It enabled hominins to navigate the African savannahs which were growing and encroaching upon their dwindling forest homes as the climate began to dry.
A new study, published in Nature, compared tail-less apes and humans to those of monkeys which have tails. The researchers scanned 140 genes linked to vertebrate tail development. The research revealed a DNA insertion in a gene called TBXT present in apes and humans, but not in monkeys.
When inserting the same gene into mice, they were born with various effects in their tails – including some without tails at all.
“Our study begins to explain how evolution removed our tails, a question that has intrigued me since I was young,” says corresponding study author Dr Bo Xia who was a student at New York University at the time of the study.
Past research linked more than 100 genes to tail growth and hypothesised that mutations to one or more of these genes caused a loss of tails. But, surprisingly, this evolutionary change in apes occurred through an insertion, not mutation.
Even more surprising is that the insertion of interest, called AluY, which is a transposon – also called “jumping genes” or “mobile elements” which move around – randomly insert themselves into the genetic code, but don’t play a role in the genes which code different proteins.
“This finding is remarkable because most human introns carry copies of repetitive, jumping DNAs without any effect on gene expression, but this particular AluY insertion did something as obvious as determine tail length,” said Dr Jef D Boeke.
Tail loss in the primates that includes humans and apes is believed to have occurred 25 million years ago.
It is not clear why this change took place, but tail-less life might be better for life on ground rather than in the trees. Whatever the exact reason, the evolutionary advantage must have been significant given the authors also found serious risks associated with the new tail suppressing DNA.
The team found a small uptick in neural tube defects in mice with the modified TBXT gene.
Among these were spina bifida, also called “split spine.” 1 in 1,000 babies are born with this defect in which the spinal cord fails to develop or close properly while in the womb.
“Future experiments will test the theory that, in an ancient evolutionary trade-off, the loss of a tail in humans contributed to the neural tube birth defects, like those involved in spinal bifida, which are seen today in one in a thousand human neonates,” says co-author Dr Itai Yanai.
