Human’s Loss of Tails

[Photo Credit: New Scientist]

The ape family, from which humans developed, has gone through a multitude of evolutionary changes since the time they first came to be. One of which is the loss of a tail―a change that occurred approximately 25 million years ago (New Scientist).

Geneticists recently discovered the very mutation present in apes that caused them to lose their tails. Instead of a gradual shortening of this body part, it was instead predicted to have been a very sudden and complete loss.

Bo Xia, a geneticist at NYU Langone Health, conducted research on this topic alongside his colleague Itai Yanai by comparing ape and monkey versions of 31 genes involved in tail development. According to their study, the protein-coding regions of DNA contained no insight into this theory that they were working to test. They then had to look at the introns, which are the bits of “junk DNA” that are cut from mRNA copies of genes before proteins begin to get synthesized. Found all over the genome of these introns were tail genes called TBXT, which are Alu elements. Alu elements are genetic parasites that copy and paste themselves throughout a genome, though they usually get edited out of introns. However, this specific example contained another Alu element nearby in inverse order, meaning that they were complementary and can bind together. This forms a loop in mRNA. This can cause some of the instructions to be lost, causing the TXBT protein to have a key piece missing.

They used mice in experiments to discover more about this. Mice were given mutations to produce both full-length and missing-bit TBXT proteins―all of which demonstrated complete tail loss. This is the reasoning as to why apes lost their tails suddenly, rather than through millions of years of successive tiny changes.

Natural selection provides reasoning for why exactly these apes lost their tails. To early apes, tails were disadvantages when it came to walking. Since apes (hominids in particular) were the first species to walk upright on branches, locomotion proved to be much easier when a tail was not present.

However, according to the mice trial, there were certain disadvantages to the mutation―some mice developed spina bifida. Spina bifida is a birth defect that causes a baby’s spinal cord to develop improperly. This results in a protrusion of the spinal cord, often seen on the skin above the defect. It is speculated that the presence of spina bifida in babies is a relic that comes from the loss of tails in apes millions of years ago.

References

[1] Gonzaga, Shireen. "When Our Human Ancestors First Walked Tall." EarthSky, 3 Sept. 2017, earthsky.org/earth/when-our-human-ancestors-first-walked-tall/.

[2] Lee, Yu C., et al. "Confirmation of Microevolutionary Increase in Spina Bifida Occulta Among Swiss Birth Cohorts." PubMed Central (PMC), 15 July 2010, www.ncbi.nlm.nih.gov/pmc/articles/PMC3082669/.

[3] Page, Michael L. "How Our Ape Ancestors Suddenly Lost Their Tails 25 Million Years Ago." New Scientist, 24 Sept. 2021, www.newscientist.com/article/2291130-how-our-ape-ancestors-suddenly-lost-their-tails-25-million-years-ago/.