.. and Africa.
by Doctor Science
Like many people in the mid-Atlantic US, I've been noticing a lot of stink bugs this summer. I figured there was some kind of weather-related stink bug boom until a story about them popped up on the NY Times. It turns out that these are actually new and different stink bugs, a recent accidental arrival from China. Most species that are introduced to a new continent don't make it, but when they do they can run wild, run free! and be extremely destructive away from their home-grown predators and parasites. Researchers are looking into introducing stink-bug-specialist parasitic wasps to control them, but that will take several years -- and it looks as though the wasps are killed by agricultural insecticides to which stink bugs are naturally resistant. Oops.
But what does this have to do with Africa?
Last year, I read Africa: A Biography of the Continent by native South African John Reader. In his prologue, he states his central question of (sub-Saharan) African* history:
While the out-of-Africa population grew from just hundreds to 200 million in 100,000 years, and rose to just over 300 million by AD 1500, the African population increased from 1 million to no more than 20 million in 100,000 years, and rose to only 47 million by AD 1500. And the disparity persists to the present day, though both groups were descended from the same evolutionary stock. Both groups inherited the talents and physiological attributes that evolution had bestowed during the preceding 4 million years in Africa.
Why did the migrant population grow so much faster? Or, to approach the disparity from another direction, what prevented the African population from achieving similar levels of growth? Since the ancestral genetic stock was identical, the divergent history of the two groups implies that Africa itself was in some way responsible.
My answer to Reader's question should be obvious. Once you think of Homo sapiens as an invasive species -- like stink bugs, or kudzu, or cane toads -- the answer is blindingly obvious.
The only part of the world to which human beings are truly native is sub-Saharan Africa. Ecologically, there are no "native peoples" anywhere else in the world, because outside of Africa Homo sapiens is always an invasive alien species. You'd think that the fact that we're adapted to Africa in a way we aren't adapted to anywhere else would be an advantage, but it turns out not to work that way. The overwhelming factor, for H. sapiens as well as stink bugs, is that our native range is adapted to us -- humans or bugs become dangerously invasive when we can escape not just the limited space of our native range, but the constraints on our population that come from other co-native species: predators or parasites (including diseases).
When scientists try to find a way to control an invasive species, they generally focus on one or a very few parasites, predators, or diseases from the species' native range, hoping that will be enough. But once you think of humans as an invasive species, it looks like a less workable strategy: it's quite clear that there is no single disease, much less a predator, that historically controlled human populations in Africa. There are a whole swarm of diseases that are prevalent in Africa -- malaria, schistosomiasis, hepatitis, yellow fever -- none of which has had a population-controlling effect outside of it.
My suspicion is that the African population is more like Gulliver on the Lilliput beach: held down by many, many ropes, no one of which is very strong. Most of those ropes are probably very small parasites or microbes; many may be part of the microbiome -- the web of bacteria, viruses, and single-celled organisms that surrounds and penetrates every human body.
* throughout this post, when I say "Africa" I mean "Africa south of the Sahara" as a biogeographic region.