Cooties, of course, are a colloquialism for lice. And our cooties, so says Smithsonian, have a lot to tell us.
For instance, a 2008 study of ancient Peruvian mummies found that lice were already making themselves at home in the Americas, long before the arrival of Europeans. And the DNA of those lice is identical to that of lice we know originated in Africa. If the lice moved to the New World from Africa, it's likely the people they lived on did, too.
Even more intriguing—there's actually a type of head louse that only lives in the Americas. And scientists think it's a remnant of interaction between Homo sapiens and our Asian cousins, Homo erectus.
They found two genetically distinct types of head louse, one found worldwide and another exclusive to the Americas. Strangely enough, this would be possible if the two groups of louse had been living on the heads of two different species on different continents, the scientists say. Reed argued that both modern and archaic humans had their own types of lice. As modern humans–Homo sapiens–began to move out of Africa, they would have intermingled with Homo erectus–Homo sapiens' evolutionary predecessors that were living in Asia and East Africa–picking up their archaic parasites along the way to the New World.
Read more, and get links to the original research papers, at Smithsonian