Today it looks like humans will have to launch themselves into space to get enough room for our collective population, but we weren’t always so robust a species. There are three times in history during which humans nearly went extinct. Here’s what threatened us, and how we survived.
In 2009 scientists spent some time analyzing two completely sequenced human genomes. They were looking for the time to most recent common ancestor, or TMRCA, and they knew what to look for in order to find it.
Mobile genetic elements are little sequences of DNA that don’t stay put. They move around, and multiply, within the genome. It’s much easier to copy and paste these elements than delete them, so once they’re in a gene, they tend to stay. As such, they’re genetic mile markers in individuals—signs that they have traveled a certain distance together. One of the most common of these elements is the Alu element. Scientists were looking for it because it separated “old DNA” from “new DNA.” According to the study, “the genealogy of a region that contains a mobile element is on average older than that of the rest of the genome. Because genealogies that contain polymorphic mobile elements are old, they are shaped largely by the forces of ancient population history and are insensitive to recent demographic events, such as bottlenecks and expansions.”
They read humanity’s history on its genes, and it seems that 1.2 million years ago, things weren’t looking good. Homo sapiens, Homo ergaster, and Homo erectus had, worldwide, a breeding population of about 18,000 people—no more than 26,000 people. This means that all over the world, every human-like species that could possibly contribute genes to a current human added up to a smaller population than that of gorillas today. Considering gorillas are only on one continent, and humans occupied Africa and Eurasia, that’s a very small population indeed.
This result came as a surprise because other evidence indicated that humans were doing very well. They occupied a great deal of territory. Scientists found stone tools in Turkey that date back 1.2 million years. In 2008 archaeologists found the jaw of a thirty-something human who lived in Atapuerca, Spain about 1.2 million years ago. We were all over the place. Why were we so very near extinction?
According to the Chad Huff, a co-author of the study, this might not have been a population anomaly. It might have been humanity’s normal state. He told one newspaper, “either the population got large and collapsed or the ancestors of modern humans were always a very small population for millions of years.” Human might just have been what we would consider now an “endangered species” for most of their history. The small world-wide population of humans suggests that the expansion of humans past the borders of Africa might be, if not an incorrect observation, then perhaps not genetically significant. It’s possible that those far-flung tool-makers died off, and only a core population which stayed close to home contributed to our genes.
If that’s the case, then the survival lesson is clear: stay put.
Around 195,000 years ago, the world changed. The temperature dropped in the winter, and then dropped in the summer as well. Glaciers expanded. Entire habitats were destroyed. The era is officially called Marine Isotope Stage 6 (because we know of its existence in part by analyzing oxygen isotopes from deep sea sediment samples), and informally called a “glacial stage,” but it was likely more of an “ice desert” stage. Deserts expanded as well as glaciers, and much of the world was cold and dry.
Humans in Africa split up. Possibly this happened intentionally as groups had to leave or were driven out of shrinking habitable areas, or possibly it happened by chance as what had been a continuous habitable range of territory shrank to little pockets of land. However it happened, it took a toll on the human population. Some believe that the human breeding population shrank down to only 600 people. Some of those pockets didn’t make it.
The ones that did survive got both lucky and smart. They were lucky because many of them happened to settle beside the sea in what’s now South Africa. Their particular spot of ground happened to be rich in plants that stored their energy in starchy tubers below the surface the soil. The water beside their area was warmish, and nourished a supply of shellfish. Between the two, humans managed to gather enough food to get by. They helped themselves along, though. A cave called PP13B, near Pinnacle Point in South Africa, shows evidence that the people there used the shellfish shells as tools. It’s also possible that they heated and tempered their stone tools, making them more behaviorally modern than people 150,000 years ago generally get credit for being.
The survival lesson of this one is two-fold: it pays to be smart, but also location, location, location.
About 70,000 years ago, Sumatra blew apart. The explosion is now known as the Toba super-eruption. Super-eruptions differ from regular volcanic eruptions and other environmental catastrophes like earthquakes because, according to The Geological Society, “their environmental effects threaten civilization.” The Toba eruption, locally, created a crater that eventually became Lake Toba. Globally, it threw enough ash and debris into the air to, by some estimates, dim the sun for six years.
This happened at a time when humans didn’t exactly have it easy. They were already suffering from the cool, dry climate. Now they had to contend with multiple years of winter and an atmosphere full of noxious gases. Some experts believe the entire population could have been whittled down to between 1,000 and 10,000 people.
If Toba had as much of an impact as some people believe it did, from one year to the next, people were living in a different world. Analysis on the pollen before and after the Toba explosion, as taken from a core sample from the Bay of Bengal, show that tree cover died back and grassland grew in. Whole ecosystems changed.
According to the earliest theories, humans didn’t survive this so much as diverge and thrive. Steven Ambrose, a proponent of the idea of the Toba eruption as a cataclysm states, “One consequence of volcanic winter may have been rapid differentiation of small, isolated African immigrant populations into modern human races. The bottleneck resolves the paradox of the Out of Africa Replacement model: why do human populations look so different yet have such a recent African origin? When the modern African human diaspora passed through the prism of Toba’s volcanic winter, a rainbow of differences appeared.”
Or possibly not. The Toba idea is hotly contested. While some believe that it created a series of bottlenecks that turned humans into the weird and wild bunch we are today, others think it meant a few crappy years that left humanity essentially unchanged. The Bay of Bengal might have been worked over by the explosion, but analysis of core samples from Lake Malawi indicate the effect on the climate and environment was minimal. People changed and diverged because of a host of climatic, behavioral, and environmental reasons—no volcano needed.
So what’s the survival lesson in this one? Perhaps the lesson is to keep our eye on volcanos, just in case.