Social polarization wasn't invented yesterday. Ask the scientists studying the bones of prehistoric Europeans. Hundreds of skeletal remains, many from a newly discovered cave in Germany, have produced a startling reminder of the power of social boundaries.

When farmers showed up from the Near East about 7,500 years ago, eager to grow their grains in the soil of Central Europe, they were met by indigenous hunters and gatherers. The locals, apparently, did not welcome them with open arms.

Two new scientific techniques, ingeniously paired, suggest that for some 2,000 years, these distinct groups rarely crossed their cultural boundaries to find a mate.