Saturday, July 21, 2012

chromosomal evidence of your species near demise...,

slate | 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.

Take your pick for the cause of our near-extinction—ice ages, plagues, Indonesian gigavolcanoes. 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. (The world record for fitting people in a phone booth is 25.) That’s an outlandishly pessimistic guess even among disaster scientists, but it’s common to find estimates of a few thousand adults, below what some minor league baseball teams draw. Consider that these humans might not have been united in one place, but scattered into small, isolated pockets around Africa, and things look even shakier for our future. Had the Endangered Species Act existed way back when, human beings might have been the equivalent of pandas and condors.

But however alarming, bottlenecks and near-extinctions aren’t necessarily all bad. With less competition around, a beneficial brain-boosting gene, say, could have an easier time spreading. And those who slip through the bottleneck become big Darwinian winners, because whatever genes those dumb-lucky survivors have can spread far and wide. Guy and Doll were likely two of those survivors, and they bequeathed to the rest of us our unique arrangement of 46 chromosomes.