From a Jared Diamond review of David Reich's new book, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past:
All modern humans outside Africa, but no modern Africans, carry about 2 percent of Neanderthal genes. That means that, as modern humans expanded from their African origins into the Near East between 50,000 and 100,000 years ago, they met and occasionally had sex with Neanderthals: perhaps mainly when a lonely modern human found few other moderns with whom to mate. Their hybrid descendants then spread around the world, carrying their 2 percent of Neanderthal genes to everywhere outside Africa.Also from the review:
Native Americans may not like hearing that 10,000-year-old Native American skeletons whose repatriation and reburial they seek have no demonstrable connection to tribes living in the same geographic area today.