When Neandertals mated with modern humans, they shared more than an intimate moment and their own DNA. They also gave back thousands of ancient African gene variants that Eurasians had lost when their ancestors swept out of Africa in small bands perhaps 60,000 to 80,000 years ago. Restored to their lineage, the diversity may have been a genetic gift to Eurasian ancestors as they spread around the world and adapted to new environments. Today, however, some of these African variants are a burden: They seem to boost the risk of becoming addicted to nicotine and having wider waistlines.
