Thursday, July 19, 2012

When there were only 40 adult humans in the world

Humans have 46 chromosomes. Our closest primate relatives have 48. So where did those extra two disappear to? ...

Around a million years ago, in some fateful man or woman in Africa, what were the 12th and 13th human chromosomes (and still are the 12th and 13th chromosomes in many primates) got entangled at their tips. Instead of separating cleanly, 12 and 13 fused together, like one belt buckled onto another. This amalgam eventually became human chromosome 2.

Fusions like this are not uncommon—one in every 1,000 babies has some sort of chromosomal fusion—and most go unnoticed because they don’t upset anyone’s health. (The ends of chromosomes contain few genes, so often nothing gets disrupted.) However, a fusion by itself can’t explain the drop from 48 to 46. A fusion leaves a person with 47 chromosomes, not 46, and the odds of two identical fusions in the same cell are remote. And even after the drop to 47, the person still has to reproduce to pass on the trait, a serious barrier. ...

What Guy needs is a Doll with the same two fused chromosomes. Now, the odds of two people with the same fusion meeting might seem infinitesimal. And they would be—except in inbred families, where the chances of finding a cousin or half-sibling with the same fusion don’t round down to zero so easily. What’s more, while the odds of Guy and Doll having a healthy child remain low, every 36th spin of the genetic roulette wheel (because 1/6 x 1/6 = 1/36), the child would inherit both fused chromosomes—giving him 46 total. ...

How did having 46 chromosomes then spread worldwide? It’s possible that having two fewer chromosomes than everyone else gave Guy and Doll’s family a whopping evolutionary advantage, allowing them to out-compete the 48-chromosome sluggards. But probably not. More likely, they happened to be living at a point when the human race nearly got wiped out. ...

But humans have far less genetic diversity than most other species, and the most reasonable explanation for this is a genetic bottleneck: a severe reduction in the population of humans in the past, perhaps multiple times. One study suggested that our population, worldwide, might have dropped as low as 40 adults.
--Sam Kean, Slate, on our genesis