Humans may be paying a price for the absence of a tail.

Scientists have found that tail-loss evolution in apes and humans has increased their risk of developing neural tube defects, or birth defects that affect the spine or central nervous system.

A study by US researchers published on Wednesday has found that a specific genetic change in ancestral species may partly explain why apes and humans don’t have tails like monkeys.

The scientists who compared genome sequences of apes and humans with those of tailed monkeys have identified a genetic sequence called an “insertion” shared by apes and humans in a key gene called TBXT involved in tail development in tailed animals. The insertion is missing in monkeys.

When the scientists genetically engineered mice to introduce the insertion, they found it affected their tail development and some mice were born without tails.

“Our study begins to explain how evolution removed our tails,” said Bo Xia, a genome scientist and the study’s first author at the Broad Institute of the Massachusetts Institute of Technology and Harvard, in a media release from the New York University Grossman School of Medicine.

Scientists believe the tail loss in gorillas, chimpanzees and humans occurred about 25 million years ago when the group evolved away from monkeys. After the tail loss, apes and humans evolved with fewer tail vertebrae, giving rise to the coccyx or the tailbone.

The study has also suggested that tail loss may have come with a cost. Genes can influence multiple biological mechanisms in the body and, the scientists say, an advantage in relation to one mechanism might have adverse effects elsewhere.

The researchers observed a slight rise in neural tube defects in mice that had the insertion in the TBXT gene. Neural tube defects such as a condition called spina bifida occur in about 1 in every 1,000 newborns in humans.