A Powerful Thirst

Paleoanthropologists, and some evolutionary ecologists, study the prehistory of our human past, take the stuff of excavations—ancient skulls, fossil bones, stone tools—and create a narrative, or theory, to fit it. This is a long-standing practice. And now, there is a new theory that is sure to draw both criticism and acclaim. It comes from Clive Finlayson, the co-director of the Gibraltar Caves Project and 2003 member of the Order of the British Empire (for archaeological and museum services in Gibraltar), as presented in his new book, The Improbable Primate: How Water Shaped Human Evolution.

The title leaves little doubt what Finlayson has in store for the reader with this double-barrelled work of popular science. The first barrel, the improbable primate part, refers to Finlayson’s hypothesis—that there is, and only ever was, one Homo species, an improbable one from...