Why did we lose the tail?

The loss of the tail occurred about 25 million years ago, when the lineage of humans and great apes split from that of the ancient apes. The researchers identified the insertion of a specific AluY element into the intron of the TBXT gene as a key event that could explain the loss of the tail

A new study published in the journal Science presents interesting insights into the genetic basis of tail loss during the evolution of humans and great apes. The loss of the tail occurred about 25 million years ago, when the lineage of humans and great apes split from that of the ancient apes. The researchers identified the insertion of a specific AluY element into the intron of the TBXT gene as a key event that could explain the loss of the tail.

The researchers showed that this AluY element interacts with an earlier AluSx1 element that is in the opposite orientation in the neighboring intron. This interaction results in an alternative splicing event unique to great apes that produces a missing isoform of the TBXT protein. To test the effect of this isoform, the researchers created model mice that express it together with the full isoform of the gene.

The results of the study showed that mice expressing both Tbxt isoforms develop a very short or completely absent tail, depending on the ratio between the isoforms. These findings support the hypothesis that the exon-less isoform of Tbxt is sufficient to cause the loss of the tail. Moreover, overexpression of this isoform also causes defects in the closure of the neural tube in the mice, a finding that raises the possibility that the loss of the tail in the evolution of humans involves an adaptive cost of an increased risk of birth defects. 

The study provides interesting insights into how mobile elements in the genome can influence complex traits in mammalian evolution. This genetic change may have contributed to the loss of the tail in great apes, although there may also be other genetic changes that acted in concert with it to stabilize the phenotype of the absence of an external tail.

The study examines the effect of the missing isoform on genes known to be targets of Tbxt that are important for tail and hindbody development. Analysis of the gene expression profile in murine stem cells shows that there are significant differences in the expression levels of these genes when both the short isoform and the full isoform are expressed.

The researchers hypothesize that the short isoform may create a shortened version of the transcription factor Tbxt with different functionality. However, more research is needed to better understand the exact mechanism by which this isoform works, and how it contributes to the loss of the tail. 

In addition, the study shows that the introduction of the AluY element into the genome can also cause serious birth defects such as failure to close the neural tube. It is therefore possible that the loss of the tail in human evolution involves a certain adaptive price that continues to affect human health even today. The study provides important insights into the relationship between genetic changes and the development of complex traits in mammals.

for the scientific article

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