( e and rats only have around 600 numts. ...)Completely Neutral Evolutionary Changes Helped Shape Our Genome,Health

e and rats only have around 600 numts. Since they increase in frequency as species advance, it suggested there was some evolutionary purpose to keeping them around.

Strikingly, however, none of these numts contained the blueprint (an actual gene) to make a protein that does anything, nor did they seem to control the function of any nearby genes.

"At best, it seems numts are a neutral part of our genome. If anything, they may be mildly negative since long repeat sequences can be unstable or get inserted inside genes and disrupt them," said Katsanis.

The researchers believe they have uncovered a possible reason why these potentially damaging but mostly neutral bits of DNA accumulate over time by comparing the sequences of human numts with those in different animals. How closely the different species' sequences match can provide an estimate of when that particular sequence got inserted into the ancestor of the human genome.

Their calculations revealed that most numts became embedded in our genome over a 10-million-year period centered roughly 54 million years ago - right around the time when the first primates emerged.

"When new species emerge, their numbers and therefore their genetic differences are very small. This creates a genetic bottleneck during which any changes in the genome will either get eliminated quickly or spread to the whole population quickly," Katsanis notes.

Katsanis proposes that numts, being "neutral," were generally at low levels in ancient mammals, but during the primate emergence 54 million years ago, they accumulated and spread through the small early primate populations precisely because they were not detrimental enough to be eliminated. Then, as these populations expanded, numts reached stable but higher frequencies.

The study is published online in PLoS Genetics.


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