Irish cannibals and Neanderthal sex: Just how big is our species?

When the first fossils of Neanderthals came to light 155 years ago, they raised a tough question: did they come from a member of our own species, or a separate one? For all the progress scientists have made in studying Neanderthals since then, the answer remains tough--in part because it's not that easy to define a species. NOVA asked me to write about this enduring question. You can read my answer here.