New genetic analysis of prehistoric fossils has shown that most modern humans outside Africa carry the genes of Neanderthals and other extinct human species — DNA evidence reveals our ancestors interbred with Neanderthals, the newly discovered Denisovans and possibly other unknown species of humans.

These discoveries change our understanding of human origins and what it means to be human, Toronto-based science journalist and author Alanna Mitchell reports on Quirks & Quarks.

Mitchell explored the science of inter-species human breeding and its implications, speaking to:

• John Hawks, an anthropologist at the University of Wisconsin, who summarized what is known about Denisovans, whose remains were found in a particular cave in Russia where the remains of modern humans and Neanderthals were also found; and other archaic human species that may once have co-existed with our own.
• Chris Stringer, a paleontologist at London's Natural History Museum, who explained why modern humans are considered separate species from the others, despite evidence that we could interbreed.
• Peter Parham, who studies the human immune system at the Stanford School of Medicine in California. He explained how the DNA of other species may have affected our immune system and continues to do so today.
• David Reich, a geneticist at Harvard Medical School in Cambridge, Mass., who pointed out that this is the first time in four million years of human history that we've been the only humans on the planet.