Philippe Desmazes/AFP/Getty Images
Workers excavate a section of the Sierra de Atapuerca mountains in northern Spain in June.
Workers excavate a section of the Sierra de Atapuerca mountains in northern Spain in June. Philippe Desmazes/AFP/Getty Images
Human fossils have been found from the Ethiopian highlands to the Indonesian island of Java. However, the single site with the biggest deposits is located in northern Spain.
About 150 miles north of Madrid, a jeep pulls up to a clump of trees in the Sierra de Atapuerca, a collection of hills that are rich with caves.
A man with a helmet and a miner's headlamp gets out. He looks more like a mountain guide than a scientist. He's Juan Luis Arsuaga, Spain's best-known paleontologist.
Philippe Desmazes/AFP/Getty Images
This skull, named Miguelon, is estimated to be 400,000 years old and was uncovered at the Atapuerca site.
This skull, named Miguelon, is estimated to be 400,000 years old and was uncovered at the Atapuerca site. Philippe Desmazes/AFP/Getty Images
He walks into a large cave, which is marked by a pirate flag. "This is the entrance to the site that has produced the most human fossils in history," Arsuaga says. "What better way to mark it?"
The Atapuerca hills are made of what's called karstic limestone, which means they're riddled with subterranean tunnels and caverns. In the 19th century, a British mining company discovered them when it blasted through a hill to lay down a railway.
At first, only animal bones were found. Then in 1976, a paleontology student found the first human remains. Since then, an abundance of human fossils and stone tools have been found.
Inside the cave, a group of paleontologists prepares to go even deeper underground. One of them is Rolf Quam, a paleoanthropologist from Binghamton University in New York.
"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."
Patrick Kovarick/AFP/Getty Images
An archaeologist sets up pieces of an 800,000-year-old human skull found in the Atapuerca hills for a museum exhibition in Paris.
An archaeologist sets up pieces of an 800,000-year-old human skull found in the Atapuerca hills for a museum exhibition in Paris. Patrick Kovarick/AFP/Getty Images
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.
The American member of the team says scientists are trying to solve another riddle.
"Why did humans live here continually for a million years? That's one of the big questions we're trying to answer," Quam asks. "What is it about the Sierra that makes it so inviting for human occupation?"
One theory is that the fauna was particularly rich in the Sierra de Atapuerca hills, which lie at the confluence of several geological systems.
Others believe the abundance of caves made it a desirable place to live.
But as the recent find of two more skull fragments demonstrates, there's still a lot to be learned from what lies buried in the hills.