Why humans lack tails? Junk DNA

Around 25 million years ago, the chance insertion of a transposable element occurred in a gene in the zygote of an ancient creature. The probability of the insertion occurring in that exact region was around one in a million. Yet it did, causing the creature to not develop a tail, a study has found.

One of the most striking anatomical features of apes, which sets them apart from monkeys, is the absence of a tail. All mammals have a tail at some point during their development, but apes, including humans, chimpanzees, bonobos, gorillas, orangutans, and gibbons, lose them in utero, leaving behind three to five vestigial vertebrae called the coccyx, or tailbone.

Apes started to lose their tails in this way around 25 million years ago, when the ape and monkey lineages split from a common ancestor. And until recently, nobody knew why apes started to do this.

The compact genome:

Every cell of an organism contains a full copy of that organism’s DNA, called the genome. The genome contains the information that the cell uses to make proteins, the workhorses of the cell. Each protein is coded by a specific section of the genome, called the gene.

Not all cells make all the proteins encoded in the genome. For instance, pancreatic cells make insulin, but skin cells don’t. Skin cells make other proteins, such as keratin, that the pancreas cells don’t. A cell achieves this selective protein production by first making a temporary copy of the gene, called the mRNA, that then drives protein production. So pancreas cells will first copy information in the insulin gene into insulin mRNA, and the insulin mRNA will be used to make insulin protein. Skin cells follow the same process to make keratin.

As scientists began to determine the genome sequence of organisms in the mid-1990s, they realised simple organisms like bacteria keep their genomes very compact while more complex life forms don’t. In the bacterial genome, the genes are arranged in tandem: where one gene ends, another begins. As a result, genes make up 85–90% of the bacterial genome.

‘Junk’ DNA:

But in complex organisms, genes are spaced far apart. In humans, for example, only 1.5% of the genome codes for proteins. At the time, scientists didn’t know what the rest did and called it ‘junk’ DNA.

Today we know this ‘junk’ DNA is responsible for various functions, including controlling when to make a protein and when not to. A significant fraction of the ‘junk’ also contains transposable elements. These are pieces of DNA that can shift their positions within the genome.

One such element, called Alu, is unique to primates (both apes and monkeys). It is tiny, being made up of around 300 base pairs (the human genome is approximately 3 billion base pairs). But due to its ability to copy itself and ‘jump’ within the genome, it is present in 1.4 million different locations in the human genome. Normally, in nearly all cell types, these elements copy themselves, switch to different locations, and insert themselves into the genome again with minimal consequence to health or evolution. This is because the insertion event is unique to a given cell.

For example, if it happens in an essential gene, only that cell will die; others around it will function normally. The sole exception to this rule is if the insertion happens in the zygote: the fertilized cell after fusion of the sperm and egg that develops into the offspring. Then the change to the DNA will be permanent: it will be reflected in every cell of the offspring.

The Alu accident:

Twenty-five million years ago, after the ape and monkey lineages separated, a chance insertion of an Alu element occurred in an important gene in the zygote of an ancient creature. The probability of the insertion occurring in that exact region was around one in a million. Yet it still occurred, and it caused that ancient creature to not develop a tail.

And because the insertion had happened in the zygote, it was imprinted in the DNA of every cell of that creature, and its subsequent offspring — all of them. That creature was the ancestor of all modern apes.