iStockphoto
It only took us around 25 million years to figure out why humans don’t have tails, but now a group scientists believe they have cracked the case.
A new study published on Wednesday in the journal Nature identifies a unique DNA mutation located in the gene TBXT. TBXT is a gene which is known to control tail development in animals.
According to a statement published by The Dark Matter Project at The Center for Synthetic Regulatory Genomics at NYU Langone Health, “It has long been speculated that tail loss in hominoids has contributed to bipedal locomotion, whose evolutionary occurrence coincided with the loss of tail. Yet, the precise genetic mechanism that facilitated tail-loss evolution in hominoids remains unknown.”
“The recent primate genome sequencing projects have made possible the identification of causal links between genotypic and phenotypic changes, and enable the search for hominoid-specific genetic elements controlling tail development.
“In this project, we found evidence that tail-loss evolution was mediated by the insertion of an individual Alu element into the genome of the hominoid ancestor.”
The statement did add, however, that “the exact evolutionary advantage that drove the selection for the loss of the tail along the hominoid lineage is unclear, we propose that such selection was associated with an adaptive cost of potential neural tube defects and that this ancient evolutionary trade-off may thus continue to affect human health today.”
Jennifer Zieba of Live Science explains, “So-called Alu elements are repetitive DNA sequences that can generate bits of RNA, a molecular cousin of DNA, that can convert back to DNA and then insert themselves randomly into the genome. These ‘transposable elements,’ or jumping genes, can disrupt or enhance a gene’s function upon insertion. This specific type of jumping gene exists only in primates and has been driving genetic diversity for millions of years.”
“We did a lot of other analyses of other genes implicated in tail length or morphology. And, of course, there are differences, but this was like a lightning bolt,” Jef Boeke, director of the Institute for Systems Genetics at NYU Langone Health and a senior author of the study told Live Science. “And it was noncoding DNA [introns] that was 100% conserved in all the apes and 100% absent in all the monkeys.”
Interestingly, the researchers found inserting these same jumping genes into mice caused them to lose their tails. They also learned that the mice that lost their tails had a higher risk of having the birth defect spina bifida, which affects approximately 1 in 1,000 humans.
“It may be a sort of unintended consequence that TBXT deficiency gives you a short tail … but it makes it more likely that you don’t get that complete neural closure,” Boeke said.
Itai Yanai, scientific director of the Applied Bioinformatics Laboratories at NYU Langone Health and a senior author of the study, added, “No one ever thought that, by just following our curiosity, we would make a mouse lose their tail by putting in the same mutation … and then we see the mouse also has a neural tube defect.”