Why Humans Don’t Have A Tail? A Single Genetic Tweak Lead Human Ancestors To Lose Their Tails 25 Million Years Ago

A distinct DNA mutation in the TBXT gene has been found via research. The study represents a substantial advance in our knowledge of human evolution and was published in the Nature Journal. It is thought that 25 million years ago, the mutation led to the loss of our ancestors’ tails. The genetic tweak did come with a price, though, still leading to the rare birth condition spina bifida. Bo Xia, a major investigator at the Broad Institute and a graduate student at New York University; after breaking his tailbone, he found the source of the structure.

The Role Of Alu In The Genetic Tweak

Researchers at NYU Langone Health have identified a short segment of genetic code known as Alu. The genetic tweak caused early humans and non-tailed apes to lose their tails. According to the study, humans and tailless apes have a DNA characteristic that sets them apart from monkeys. The introduction of the genetic code fragment altered the story of our evolutionary path. Combining this new gene with another called TBXT. Thus resulting in two different kinds of ribonucleic acid essential to cellular structure. Which explains how the tail loss was genetically programmed in both humans and apes. The study also suggests that the genetic tweak caused the tail loss in humans, gorillas, and chimpanzees. And it may have influenced the evolution of the tailbone. But the unanswered question is why the tail loss was the best life course, as dictated by ancient evolutionary processes.

A Deeper Look

Evolution is facilitated by changes in an animal’s DNA over many years, with some modifications being more advanced than others. Repeated DNA sequences known as alu elements produce RNA fragments. These randomly insert themselves into the genome and transform back into DNA. The jumping genes, or transposable elements, have been the primary source of genetic variety in primates for millions of years. Two Alu elements, absent in monkeys but present in great apes, were discovered to be part of the TBXT gene. These components are present in DNA segments called introns, which surround exons and were previously thought to be non-functional.  The repeated pattern of the Alu sequences leads them to join together when cells create RNA with the TBXT gene. This cuts off the bigger RNA molecule but takes a whole exon with it, altering the final coding and structure of the resultant protein.

Noncoding DNA introns are 100% missing in monkeys and 100% conserved in all apes. The TBXT gene in human cells has identical Alu sequences, which lead to the exon‘s loss. This process is also known as alternative splicing and allows the linked RNA molecule to be cut in different ways. Hence producing different proteins from the same gene. 

The Downside Of The Genetic Tweak: Spina Bifida

The investigation also revealed a dark side effect of the mutation. Mice with TBXT genes that have Alu sequences added to them have abnormally high levels of neural tube defects. This affects the development of the nerves in the brain and spine, in addition to having their tails cut off. Spina bifida is a condition known to result from this abnormality and can cause incontinence or paralysis.

According to some experts, a tail would have interfered with the delicate balance that keeps us upright. Therefore, the genetic tweak was a necessary sacrifice for us to be able to stand. Further research could explore how the loss of tails in humans may have contributed to neural tube birth abnormalities.

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