Humanity’s ancestry has grown far clearer thanks to our ability to obtain ancient DNA. We now know that, as humans left Africa, they interbred with the groups they met there, Neanderthals and Denisovans. Evidence from the Denisovan genome also suggests that this was nothing new; the Denisovans had apparently interbred with an even earlier group. But the identity of that group remained a bit of a mystery. Now, some evidence from ancient proteins suggests that the mystery group was Homo erectus , a species that left Africa over a million years ago and spread throughout Eurasia. And, thanks to the Denisovans, it appears that modern humans inherited some of that Homo erectus DNA. In the teeth Without access to all the repair enzymes made by living cells, DNA rapidly degrades. The double helix fragments, and bases change identity or fall off entirely. While cooler, drier environments slow this process, it sets a hard limit on how far back in time we can obtain DNA sequences.…