For Curtis Marean, finding a site that might yield evidence of how early modern humans survived a glacial maximum that made much of Africa uninhabitable was neither straightforward nor without incident. The work, which culminated in the discovery of the earliest known instance of modern humans incorporating shellfish into their diets, began with climbing cliffs and dodging falling rocks.

Since 1991, Marean, an archaeologist at Arizona State University in Tempe, has studied early modern humans in South Africa. On the basis of palaeoclimatic records, he says, only a handful of places on the African continent could have supported human populations during the glacial maximum that occured between 195,000 and 130,000 years ago. One potential refuge was along the southern coast of South Africa, where ocean currents would have kept temperatures warmer and provided conditions in which food would have been plentiful on land and at sea.

What Marean sought was a cave that would have been near enough to the low sea at that time for early man to have had sufficient food and warmth, but high enough that its contents would not have been washed out by the elevated sea levels that followed soon afterwards, some 123,000 years ago.

Peter Nilssen, a friend and collaborator of Marean's from the Iziko-South African Museum in Cape Town, had a few photographs of caves in cliff faces at Pinnacle Point, which overlooks the Indian Ocean, that looked promising. The two carried out reconnaissance work in 1999, climbing down some 60 metres of sheer cliff face to the rocky beach and then scrambling back up another 15 metres to the mouth of a cave dubbed PP13B. On one trip, Marean recalls, “Peter was ahead, and he dislodged a rock on the way up. It just missed my head, and it gave me quite a fright.” At the top, they both came to a realization — they may have found the perfect location for their work, but they could never run a project there, for fear of losing graduate students to the danger.

A solution came in the form of Ricky van Rensberg, a nearby ostrich farmer with a knack for building innovative structures. By 2000, the team was excavating, thanks to a 200-step wooden stairway leading from the cliff top down to the beach. In 2003, a bridge and a walkway up to the cave were added. But occupational hazards still exist — the cave opening is so narrow and the water so close that it's like being at sea. “Sitting in the cave, all you see is ocean — two students actually got seasick,” Marean says.

But the difficulties faced in getting to and working at the cave turned out to be worthwhile when excavations got underway. The team found remains of molluscs in sediments on the cave floor that dated to around 164,000 years ago. This was much earlier than previous reports of shellfish consumption by humans — the earliest documented evidence before this dated to about 125,000 years ago (see page 905).

How people first came to sample seafood, however, remains a mystery. They may simply have been curious, inspired by the sight of birds feasting on mussels and crabs exposed at low tide. Or perhaps desperation led some brave early human to take a gamble. Far from being just a tasty snack, Marean says, seafood may have contributed to the survival of our species.