Can Snail Shells Solve a Mystery?

Things have changed dramatically in the Sahara, according to Washington University Assistant Professor of Earth Sciences Jennifer Smith, who has studied ancient snail shells taken from some of the Sahara's vanished lakes. Specifically, she dug shells of the freshwater gastropod Melanoides out of silts laid down in the bottom of a small lake in the Kharga oasis of western Egypt. She concluded that 130,000 years ago tool-using human beings lived along the shores of the lake, along with hippos, giraffes, and other wildlife.

As it happens, this was the very period when the genetic and archaeological evidence suggests human beings were migrating through this lake-graced savannah and then fanning out across Europe, Asia, and North America. DNA analysis has already shown that all modern human beings alive today have a common ancestor who lived in southern Africa between 70,000 and 180,000 years ago. Although other upright walking hominids lived on other continents, they all apparently vanished as modern homo sapiens spread out across the planet. Therefore, the state of the Sahara grassland 130,000 years ago has a direct bearing on all of human evolution.

The Washington University researchers dug through 15-foot-thick (5 m) silt layers on the now dry lake bed, most of it laid down during a wet phase that lasted for thousands of years. Today, the area gets only a trace of rain. But 130,000 years ago, the rainfall averaged 23 inches (600 mm) per year. The researchers analyzed the shells of 20 freshwater snails, because the chemical composition of the shells gives a clue to past climate.

The snails incorporate into their hard shells certain isotopes that indicate whether the lake in which they were living was stable or subject to strong evaporation. Lakes that are evaporating have higher concentrations of salt and other minerals in the water, like today's Salton Sea or Dead Sea. The researchers measured the calcium carbonate levels in the silts of the long-vanished lake and the isotopes the snails had incorporated into their shells. The scientists paid special attention to elements sensitive to evaporation rates. The study demonstrated that 130,000 years ago the lake had a stable water level with only modest net evaporation for long periods of time.