Tails are common in the animal kingdom, among fish, amphibians, reptiles, birds and mammals. Each species that has one uses it in different ways to help it survive. Fish use their tails for swimming. Among land animals, a tail is often used for balance in locomotion. The tails of birds are crucial to their being able to fly.
While almost all other primates have tails, Neither humans nor our closest evolutionary relatives, the great apes. Why and how did this happen? This question occurred to a graduate student, Bo Xia, in New York University’s stem cell biology program, and he sought to find answers. (Sources for this story are articles in Science and the New York Times.)
About 30 genes in mammals are involved in tail development. Xia compared this set of genes between monkey with tails and great apes without them to see if any had a mutation, and he found one, called TXBT. Indeed, the mutated TXBT gene is virtually identical in both humans and apes. The mutation involves the insertion of an “Alu” or “jumping gene” into the sequence, which in other species results in a shortened tail. However, in humans and apes, there is a second Alu nearby in the gene that, through interaction with the first one, disables the TXBT gene entirely, resulting in no tail. Inserting the mutated TXBT gene into the mouse genome resulted in mice with a variety of tail lengths, including none at all. It also resulted in an incidence of birth defects associated with lack of spinal development. These defects occur humans as well: spina bifida and anencephaly are examples.
Insight into what genes might be causing these defects in humans was an unexpected insight gained from this research.
There is an additional question that still has not been answered. The mutation resulting in apes without tails was, apparently, wildly successful, since there are no apes with tails. The question is, why is this so? Apes without tails would have a harder time balancing in trees and therefore be more vulnerable to falling to their deaths, so one might think the mutation would be disadvantageous on its face. It is not clear what makes having no tail an advantage for apes.
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