Schraiber is pretty open about the limitations of his own findings, and even cautions against outright buying into the new paper's model. There's significant room for error in accurately identifying DNA bases. And he says matings between the two species must have involved very closely related Neanderthals. When looking at fragments from another early human group, the Denisovans (Neanderthals weren't our only special friends), researchers can clearly identify that the DNA came from different, distinct populations on different occasions. "However, the fragments we see from Neanderthals are consistent with coming from a single Neanderthal population," says Schraiber. "This perhaps isn't too surprising, because Neanderthals had very low genetic diversity. But it is definitely worth following up on."