Scientists said yesterday that they have discovered a tiny genetic mutation that largely explains the first appearance of white skin in humans tens of thousands of years ago, a finding that helps solve one of biology's most enduring mysteries and illuminates one of humanity's greatest sources of strife. The work suggests that the skin-whitening mutation occurred by chance in a single individual after the first human exodus from Africa, when all people were brown-skinned. That person's offspring apparently thrived as humans moved northward into what is now Europe, helping to give rise to the lightest of the world's races. Leaders...