Great apes differ from other primate species in several ways: they are larger, have broader chests, and, among other traits, lack tails. It has long been known when our primate ancestors lost their tails — around 25 million years ago — but until recently, how this happened remained a mystery.
For a long time, the prevailing theory in the scientific community was that anthropoid primates closely related to humans lost their tails through a gradual shortening process that unfolded over millions of years.
A team of geneticists from the NYU Langone Health academic medical center in New York, however, has a different story. They argue in a study published in the biology journal BioRxiv that the explanation lies in a genetic mutation that occurred abruptly.
“The tail was lost in a single blow,” said Itai Yanai, director of the Institute for Computational Medicine, who led the research at NYU Langone Health at the initiative of his student, Bo Xia.
Lost instructions
The researchers set out to investigate the genetic basis of tail loss. Any relevant mutation, they reasoned, should be present in great apes but absent in other monkeys. To test this, they compared versions of 31 genes involved in tail development. They found no differences in the protein-coding regions of these genes, but they did identify changes in other DNA segments located within the genes.
“Imagine proteins as flat-pack furniture, and the instruction booklets come with many pages of nonsense that need to be removed before the instructions can actually work. These extra bits — called introns — are cut out of the mRNA copies of genes before protein production begins,” Bo Xia explained, as quoted by New Scientist.
In the ancestors of great apes, it turns out that an Alu element inserted itself into the middle of an intron in the TBXT gene, which is responsible for tail development. Alu elements are genetic parasites that copy and replicate themselves throughout the genome.
“We have about one million Alu elements scattered across the genome,” Itai Yanai noted.
Normally, an Alu element located within an intron has no effect and is eventually removed along with the intron. However, if another Alu element is nearby in the reverse orientation, the two complementary elements can bind to each other, forming a loop in the mRNA.
This loop effectively sticks together pages of the instruction booklet, rendering part of the instructions inaccessible. As a result, the assembled “furniture” — in this case, the TBXT protein — is produced without a crucial component.
The two researchers were able to demonstrate this mechanism in experiments with mice. When carrying such a mutation, mice are born with either a complete or a defective TBXT protein, and subsequent generations are typically born without tails.
The unresolved question of why we lost our tails
In conclusion, once a lineage of mammals loses a trait abruptly — rather than through the accumulation of small changes over millions of years — the fate of that species is effectively sealed. Moreover, there is no fossil evidence showing a gradual reduction in tail length among ancient primates discovered by archaeologists.
Despite the success of their experiments, Yanai and Xia were unable to answer another key question: why the ancestors of modern primates lost their tails in the first place — that is, why evolution selected this particular mutation for long-term persistence. Most theories suggest that tails became a disadvantage for ape species that began to move differently, such as walking upright along branches, even though they still relied on all four limbs.
On the downside, Yanai and Xia also pointed out that the mutation responsible for tail loss caused abnormalities resembling spina bifida in mice. The relatively high prevalence of this condition in humans could be a consequence of our ancestors losing their tails tens of millions of years ago, the two researchers speculated — while refraining from answering the fundamental question of what initially triggered the mutation that led to tail loss.