DO YOU EVER WONDER why humans and other apes have no tails? A genetic parasite seems to be the culprit. Around 25 million years ago, this parasite, a small piece of repetitive DNA called an Alu element, ended up in a gene important for tail development. The single insertion altered the gene Tbxt in a way that seems to have sparked one of the defining differences between monkeys and apes: Monkeys have tails, apes don’t.
Alu elements are part of a group of genetic parasites known as transposons or jumping genes that can hop across genetic instruction books, inserting themselves into their hosts’ DNA. Sometimes, when the gene slips itself into a piece of DNA that is passed down to offspring, these insertions become permanent parts of our genetic code.
Transposons, including more than 1 million Alu elements, are found throughout our genome. Without transposons, the placenta, immune system and insulation around nerve fibers may not exist. And humans might still have tails.
To find out how apes lost their tails, researchers analyzed 140 genes involved in vertebrate tail development. The team found that in monkeys, including baboons and rhesus macaques, the Tbxt gene was missing a big portion of DNA that’s found in humans, chimpanzees and other apes.
But the missing chunk was in a part of the gene called an intron, a bit of genetic material that isn’t made into proteins. The missing bit tweaked Tbxt so that the gene makes a different form of the protein in apes than in monkeys.
Experiments in mice seemed to confirm the hypothesis. Mice, like monkeys, make normal versions of the Tbxt protein and have full-length tails. But when genetically engineered to make shortened versions of the Tbxt protein like apes do, the mice had shorter tails or none at all. Some mice also had spinal cord defects similar to spinal bifida, suggesting that there may be drawbacks to tail loss.
Mice genetically engineered to make an apelike version of the Tbxt protein had a variety of tail lengths. X-ray images show that some had no tail or were shortened compared with normal mice. Other mice still had long tails.
The genes may also be involved in tail loss. Many more Alu elements are scattered among human introns that could have yet-to-be-known effects on other aspects of human evolution.
The new findings show how apes lost their tails. But why it happened is a much harder question to answer. Research from the early 1900s linked tail loss to muscle changes that helped human bodies move upright, but shifts in posture, as well as learning to walk on two feet, didn’t happen until millions of years later. So it’s unlikely the new findings will shed light on these human traits.
A natural next step is exploring whether the genetic underpinnings of tail loss in apes also happened in other mammals with shortened tails or none at all such as koalas, capybaras and bears. By Manny Palomar, PhD (EV Mail November 11-17, 2024 issue)
