Why Don’t Humans Have Tails? Scientists Found the Answer From a Single Genetic Change That Occurred 25 Million Years Ago

Around 25 million years ago, an ancestor of humans and apes genetically diverged from the family of monkeys and lost its tail. So, if the ancient animal ancestors of modern humans had tails, why don't we?

(Photo: Wikimedia Commons/ Tom Junek)

For many years, experts have debated the reasons for this tail loss variation, trying to decode the reasons behind this difference. In a recent study, scientists have finally identified the genetic mechanism responsible for our tailless condition.

Jumping Genes

Animals evolved from millions of years of having changes in their DNA. Some of these changes involve only a single ladder in the DNA's spiral ladder, while others are more complex.

In primates' genetic materials, repetitive DNA sequences called Alu elements exist. Also known as 'jumping genes', these elements can generate bits of RNA which do not only convert back to DNA, but also insert themselves randomly into the genome.

As mobile DNA sequences, these transposable elements can either disrupt or enhance the function of a gene upon insertion. This specific type of gene is not observed in animals outside the Order Primata, and it has been driving diversity for millions of years.

Evolutionary Loss of Tail

In a study entitled "On the genetic basis of tail-loss evolution in humans and apes," the researchers discovered a unique DNA mutation that drove the loss of our ancestor's tails. This is located in the gene TBXT, a sequence known for its involvement in tail length in tailed animals.

Bo Xia led the researchers to discover two Alu elements in the gene TBXT, which are present in great apes but not in monkeys. These are not found in the part of the gene that codes for proteins called exons but instead in introns.

Introns are regions within a gene that are known to flank exons. They are known as "dark matter" of the genome because they were historically assumed to have no important function. They get removed from the sequence before an RNA molecule is converted into protein.

The repetitive nature of the Alu sequences causes cells to bind together when they use the TBXT gene in generating RNA. The complex structure still gets spliced out of the larger RNA molecule but carries an entire exon with it, changing the resulting protein's final code for and structure.

In human cells, the same Alu sequences appear in the TBXT gene, causing the removal of the same exon. The scientists also found that generating multiple proteins from the same gene can happen since the related RNA molecule can be cut differently.

In the study, scientists inserted the same jumping gene into mice and discovered that they lost their tails. It was found that mice make only one version of the protein, so having both versions seems to hinder the formation of tails. Creating various proteins from the same gene is known as "alternative splicing." It is one of the main reasons why human physiology is very complex.