Crab lice and our species-liberal ancestors.
If, like me, you have a rather nerdy sense of humor, you need to read this paper published yesterday in BMC Biology.
In a nutshell, many related parasites in related species show very similar species divergence times indicating that parasites tend to follow their host populations instead of travelling. This is not always the case (for example, big cats get the bacteria that causes ulcers from eating a prehistoric human), but this is usually the case for parasites that require intimate contact to spread.
One group of parasites fitting this pattern is lice. Lice don't travel much and if they leave the host, die rather quickly. They tend to spread in circumstances resulting in very close contact. Head lice pass between snot-nosed kids at school, for example, and pubic lice (or "crab" lice) generally spread through close contact by a somewhat older crowd. The paper mentioned, calculates divergence times of head and pubic lice from those present on other primates. The head louse is closely related to a species present on chimpanzees and at six million years, the divergence time closely matches current estimates of the human-chimpanzee split. The pubic lice are closely related to a species currently present on gorillas, but while humans and gorillas diverged at around 7 million years ago, the lice diverged between 3 and 4 million years ago. That means that the lice had to spread well after the human-gorilla divergence and from the quality of intimate contact that causes lice to spread today.
The paper doesn't speculate on how this "transfer" happened, but I have a two-part hypothesis about what happened:
- Beer was first invented between 3 and 4 million years ago.
- Gorilla chicks are whores.