"This site has been excavated continuously since 1984," added co-author Ignacio Martínez. "After 30 years, we have recovered nearly 7,000 human fossils corresponding to all skeletal regions of at least 28 individuals. This extraordinary collection includes 17 fragmentary skulls, many of which are very complete."
Using multiple, independent dating techniques, the researchers determined that the common ancestor of modern humans and Neanderthals lived more than 430,000 years ago. This alone is significant, the authors write: "With this new age, the SH hominins are now the oldest reliably dated hominins to show clear Neandertal (features)."
Other studies support that between 400,000 to 500,000 years ago, ancient humans lived in Africa and East Asia. Some split off, ultimately settling in Europe and Asia. Others remained in Africa.
PHOTOS: Faces of Our Ancestors
The separation between the groups resulted in key physical differences, which the researchers believe evolved separately, and at different times - as opposed to all at once, as some other scientists have proposed. Neanderthals found at other locations, for example, have a shorter and stockier build, angled cheekbones, prominent brow ridges and wide noses.