Lake Turkana is the largest permanent desert lake on Earth, 250 kilometres of jade-green water lying in the lava badlands of northern Kenya, and its shores hold the richest record of human origins found anywhere. On the eastern side, the eroding badlands of Koobi Fora and Sibiloi have given up more early-human fossils than almost any ground on the planet. A ranger can walk you past the exposed sediment layers to a small museum where casts of the finds, some of them millions of years old, are laid out. The most complete of all came off the other shore. In 1984 the fossil hunter Kamoya Kimeu spotted a scrap of skull on a dry riverbed at Nariokotome — the first trace of a Homo erectus youth of about eight or nine who died some 1.6 million years ago. He stood over 1.6 metres already, tall and long-limbed, which is how we know our ancestors were striders by then.
Two active volcanic cones rise out of the water, and reaching them means an open crossing that can turn choppy fast, then a scramble up bare black rock. Central Island holds three crater lakes inside its rim, named Crocodile, Flamingo and Tilapia, and you do not see them until you are standing on the rim. Their warm shallows are one of the great breeding grounds of the Nile crocodile, which is why locals simply call the place Crocodile Island. On South Island a cluster of smaller pools holds water in vivid emerald, ringed by cooled black lava. On the southeastern shore live the El Molo, among the smallest of Kenya's peoples: about a thousand fishers who worked these waters rather than herding like their neighbours, and whose own language has all but vanished into Samburu. The lake sets your timetable too. A fierce afternoon wind whips it to whitecaps, so the crews mend nets in whatever shade they can find and wait it out. Then it falls away, the surface settles flat and burning orange, and the boats go down the beach and out for the night's fishing.