When the Sea Saved Humanity

Shortly after Homo sapiens arose, harsh climate conditions nearly extinguished our species. Recent
finds suggest that the small population that gave rise to all humans alive today survived by exploiting
a unique combination of resources along the southern coast of Africa


With the global population of humans currently approaching seven billion, it is difficult to imagine that Homo sapiens was once an endangered species. Yet studies of the DNA of modern-day people
indicate that, once upon a time, our ancestors did in fact undergo a dramatic population decline.
Although scientists lack a precise timeline for the origin and near extinction of our species, we can
surmise from the fossil record that our forebears arose throughout Africa shortly before 195,000
years ago. Back then the climate was mild and food was plentiful; life was good. But around
195,000 years ago, conditions began to deteriorate.

The planet entered a long glacial stage known as Marine Isotope Stage 6 (MIS6) that lasted until roughly 123,000 years ago. A detailed record of Africa’s environmental conditions during glacial stage 6 does not exist, but based on more recent, better-known glacial stages, climatologists surmise that it was almost
certainly cool and arid and that its deserts were probably significantly expanded relative to their
modern extents. Much of the landmass would have been uninhabitable. While the planet was
in the grip of this icy regime, the number of people plummeted perilously—from more than 10,000 breeding individuals to just hundreds. Estimates of exactly when this bottleneck occurred and how small the population became vary among genetic studies, but all of them indicate that everyone alive today is descended from a
small population that lived in one region of Africa sometime during this global cooling phase.
I began my career as an archaeologist working in East Africa and studying the origin of modern humans. But my interests began to shift when I learned of the population bottleneck that geneticists had started talking about in the early 1990s. Humans today exhibit very low genetic diversity relative to many other species with much smaller population sizes and geographic ranges—a phenomenon best explained by the occurrence
of a population crash in early H. sapiens. Where, I wondered, did our ancestors manage to survive during the climate catastrophe?

Only a handful of regions could have had the natural resources to support hunter-gatherers. Paleoanthropologists argue vociferously over which of these areas was the ideal spot. The southern
coast of Africa, rich in shellfish and edible plants year-round, seemed to me as if it would have
been a particularly good refuge in tough times. So, in 1991, I decided I would go there and look
for sites with remains dating to glacial stage 6.