Sam Kean. 19 Jul 2012. Blogging the Human Genome Entry 10: The chromosomal evidence that mankind nearly went extinct. Slate.com
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.
Read more about the human bottleneck here: Wikipedia. Population Bottleneck.
Analysis of our genome has revealed we probably went through a time when we were reduced to a population between 40 and 15,000. If we reach such low numbers again from all of the factors in Collapse and some or all of the 9 boundaries being crossed, and other potential extinction events and damage to the ecosystem, it could be harder to survive this time around.
However bleak the world was during the last bottleneck, it wasn’t too hot, poisoned by chemicals, and didn’t have acidic oceans killing off life on the bottom of the food chain. There’s some speculation that the survivors lived along the ocean and were able to survive by eating shellfish and seafood. That won’t work this time around because shellfish are the first to go in acidic oceans (they can’t make shells), and we’ve consumed a lot of the world’s fish to the point that we’re eating the sardines and anchovies that fish depend on to survive, further weakening their wild populations.