It turns out humanity has been almost wiped out a few times in our distant past. How did it happen, and what does it mean for the future of human evolution?

Scientists can spot near-extinctions by correlating several different pieces of evidence. First, by studying DNA samples, counting mutations and measuring genetic diversity (Alu sequences and mitochondrial DNA being especially useful in this regard), they can find population bottlenecks. A bottleneck indicates that for some period of time a given species had very low numbers. This reduces genetic diversity, since every member of the species from then on is descended from this limited number of progenitors.


The next step is to find something that might cause widespread deaths – usually a catastrophe like the eruption of a supervolcano or an asteroid impact. The genetic studies can narrow down the time frame of the population bottleneck somewhat, and geologists can pinpoint the dates of massive eruptions and impacts fairly accurately. If the dates coincide, you might be onto something.

The third major clue is evidence that the catastrophe did things that would lead to mass die-offs. A huge eruption can cause a prolonged cooling effect, aka "volcanic winter," aka the name of my next band. Alternately, the injection of massive amounts of CO2 into the atmosphere can lead to a runaway greenhouse effect, not to mention the settling of toxic compounds over a wide area. All of these effects are geologically preserved, and can be found, measured and dated. If the whole thing lines up with your bottleneck and your catastrophe, you've got a pretty solid case for an extinction event.

In fact, a recent study found just such evidence for the end-Triassic extinction, caused by a series of massive eruptions in north-central Pangaea, and recently completed studies into the Chicxulub impact all but confirm that a massive impactor was indeed responsible for the extinction of the dinosaurs.


But what about humans? There is one near-extinction event that is fairly well-known, although it remains controversial. Roughly 70,000 years ago, give or take a few thousand years, an enormous eruption occurred in what is now Sumatra, leaving behind Lake Toba (the crater lake pictured above). The eruption coincides with a population bottleneck that is often cited as the reason for the relatively low genetic diversity across Homo sapiens sapiens. Research suggests as few as 2,000 humans were left alive by the eruption and its aftereffects.

A recent paper in the Proceedings of the National Academy of Sciences found another population bottleneck much farther back in human history. Genetic studies found that 1.2 million years ago there were as few as 55,000 members of genus Homo, including pre-human hominids like Homo erectus and Homo ergaster. This one is interesting because we don't have solid evidence of a catastrophic event during that period, so we're not sure what might have caused the population crash or where to look for more evidence.


The really interesting thing about a population bottleneck is the effect it has on evolution. With a small population, mutations get passed through a very large percentage of the species' members. Detrimental mutations could be devastating and lead to outright extinction. Beneficial mutations, however, could cause fairly fast shifts in the population. And if you imagine some kind of tribal arrangement in which a few dominant males were responsible for a lot of the procreation going on, this situation becomes even more pronounced. An entirely new species might be created within a few generations. Anthropologists have proposed that such bottlenecks were responsible for the rapid development of hominids.


A catastrophe induced bottleneck has another factor affecting evolution. It isn't just a bottleneck, it's a bottleneck under pressure. The kinds of dire circumstances that you can imagine would follow a supervolcanic eruption take "survival of the fittest" to a much higher level. Now, that beneficial mutation (say a larger brain that makes it easier to hunt sparse game and build crude shelters) still spreads through a large percentage of the species, but in addition, every genetic line without that mutation dies off (or moves away to somewhere they can hack it). The result: rapid speciation.

When you're in the world building phase of your SF magnum opus, keep speciation in mind. If a planet is not geologically active, it might have homogeneous flora and fauna, or else some other reason why there are diverse species present. After all, giant volcanoes aren't the only reason speciation happens (anything that puts a species under pressure, including other species, works too). On the other hand, a planet prone to sudden, rapid ecological changes might be a good place for sentience to emerge. Any environment that puts species under constant survival pressure and places a premium on adaptability might tend to create intelligent creatures. Sort of like ours did.


"Compound-specific carbon isotopes from Earth's largest flood basalt eruptions directly linked to the end-Triassic mass extinction." PNAS.

"When humans faced extinction." BBC News.

"The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary." Science.


"Humans Might Have Faced Extinction." Scientific American.

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