Where was Eden? Perhaps in a sun-baked salt plain in Botswana

NORTHERN BOTSWANA is a land of strange contrast. Drive west from Francistown, the country’s second city, and you skirt Makgadikgadi, a white, salt-encrusted plain that is bone dry for most of the year, but which blossoms into sudden, abundant life during the wet season. Follow the road farther and you arrive at Maun, on the edge of the lush inland delta of the Okavango river, the fourth-longest in southern Africa. Two hundred thousand years ago, though, Makgadikgadi was also lush. Both it and the delta were part of a lake, then the largest in Africa, surrounded by wetlands. For wildlife, the result was a veritable paradise—and also for people, for, if the latest research is correct in its claims, an intriguing episode in humanity’s origins was played out there.

That Homo sapiens began as an African species was pretty-much proved in the 1980s by Allan Wilson of the University of California, Berkeley. He developed what has come to be known as the Mitochondrial Eve hypothesis by looking at a special type of DNA which is passed, unmixed by sexual reproduction, from a mother to her children. This so-called mitogenome is independent of a cell’s nucleus, where the rest of the genes are found. It resides in structures called mitochondria that are the descendants of once-free-living bacteria and...