Research suggests that prehistoric humans may have interbred with two separate "superarchaic" species, with some studies indicating that this interbreeding occurred as far back as 700,000 years ago. The discovery has implications for our understanding of human evolution, which may be more complex than previously thought, with some describing it as a "braided stream" rather than a simple family tree. The existence of "ghost lineages" in human dna also implies that there may have been interactions with other, unknown species.