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.
Originally published September 21, 2012. Copyright 2012 Carl Zimmer.