http://www.sciencemag.org/news/2020/02/mysterious-ghost-populations-had-multiple-trysts-human-ancestors

Human ancestors mated with “super-archaics,” perhaps like this 700,000-year-old Homo erectus from Java.

The story of human evolution is full of ancient trysts. Genes from fossils have shown that the ancestors of many living people mated with Neanderthals and with Denisovans, a mysterious group of extinct humans who lived in Asia. Now, a flurry of papers suggests the ancestors of all three groups mixed at least twice with even older “ghost” lineages of unknown extinct hominins. One candidate partner: Homo erectus, an early human who left Africa by 1.8 million years ago, spread around the world, and could have mated with later waves of human ancestors.