"In the field of human evolution which is what I'm in, Atapuerca is a world reference site," Quam says. "This is the richest fossil bearing deposits in the world. And every single site in Atapuerca that has been excavated has yielded human remains, which is something that is very unusual."
Last year, the team uncovered a 1.2 million-year-old jawbone fragment from a species known as homo antecessor. It's the oldest hominid fossil ever found in western Europe.
Near the railway trench, another site yielded human remains of 28 individuals, dating back at least half a million years. The Spanish paleontologist believes it's a mass grave.
"This was a collective act, something a group did with its dead," Arsuaga says.
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--from NPR.org Morning Edition 3 Aug. 2009