Chromosomal Evidence That Mankind Nearly Went Extinct

A leading causal candidate for the human genetic bottleneck is the volcanic supereruption that formed Lake Toba in Sumatra, Indonesia.

Sam Kean, in Slate, explains that the number of human chromosomes suggests that modern humanity emerged from a small, inbred population.

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

Let’s go back a million years, when most proto-humans had 48 chromosomes, and follow a hypothetical Guy who has 47. Again, a chromosome fused at the tips won’t affect Guy’s day-to-day health. But having an odd number of chromosomes will cripple the viability of his sperm. (If you prefer to think of a female, the same is true of her eggs.)

Say the fusion left Guy with a normal chromosome 12, a normal 13, and a 12-13 hybrid in each cell. During sperm production his body has to divide those three chromosomes into two cells, and there are only a few possible ways to divvy them. There’s {12} & {13, 12-13}, or {13} & {12, 12-13}, or {12, 13} & {12-13}. The first four sperms are either missing a chromosome or have a duplicate, practically a cyanide capsule for an embryo. The last two cases have the proper amount of DNA for a normal child. But only in the sixth case does Guy pass the fusion on. Overall, then, two-thirds of Guy’s children die in the womb, and just one-sixth inherit the fusion. And any Junior with the fusion would then face the same terrible odds trying to reproduce. Not a good recipe for spreading the fusion—and again, that’s still only 47 chromosomes, not 46.

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.

And here’s the payoff: Junior and his 46 chromosomes would likely have an easier time having children than his 47-chromosomed parents. Remember that the fusion itself doesn’t ruin you—lots of healthy people have fusions. It’s only reproduction that gets tricky, since fusions can lead to an excess or deficit of DNA in embryos. But because he has an even number of chromosomes, little Junior wouldn’t have any unbalanced sperm cells: Each would have the right amount of DNA to run a human, just packaged differently. As a result, all of his children have a good chance of being healthy. And if his children start having their own children—especially with other relatives with 46 or 47 chromosomes—the fusion could start to spread. …

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.

Read the whole thing.