Skulls reveal Neandertal’s hodge-podge genealogy

The Neandertal branch of the hominid family tree just got a lot more shrublike. Ancient skulls from a desolate Spanish cave have a hodge-podge of Neandertal and non-Neandertal features, suggesting the species underwent a long period of evolutionary fits and starts before emerging as full-fledged Neandertals some 200,000 years ago.

A battery of dating techniques indicates that the 17 skulls, seven of which were analyzed for the first time, are roughly 430,000 years old. The age means the fossils are the oldest reliable evidence of recognizable Neandertal features, says paleontologist Juan Luis Arsuaga of Complutense University of Madrid, who led the new analysis. The ancient age also suggests that the Neandertals’ evolutionary roots reach much farther back in time than that of humans, whose characteristic features don’t appear in the fossil record until some 200,000 years ago in Africa.

The skulls’ teeth and jaws are Neandertal-esque, with protruding teeth and no chin, hinting that chewing or using the mouth in a grasping, tool-like manner was important in becoming a Neandertal, says Arsuaga. He and an international team of colleagues report the findings in the June 20 Science.

But random evolutionary processes might also have led to the suite of traits, says Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Europe underwent major environmental fluctuations from about 780,000 to 125,000 years ago, during the Middle Pleistocene. “We have dramatic swings between periods of hominids swimming where London is and of reindeer living in Spain,” says Hublin, who was not involved with the study.

Small, isolated populations of the proto-Neandertals probably crashed and burned repeatedly during this time. Limited genetic diversity in these small groups could have randomly led to the odd mix of facial traits seen in the remains.

None of the skulls has a hefty brain case like that of a proper Neandertal, which means that the big brains of Neandertals and the big brains of humans evolved independently, says Hublin. The origin of larger brain sizes in both hominid species probably resulted from intense selection, he says. And differences between those big brains very likely led to Homo sapiens’ success and the demise of Neandertals.

How these “incipient” Neandertals ended up more than 13 meters deep in a cave shaft in Spain remains a mystery, Arsuaga says. The site known as Sima de los Huesos, the pit of bones, was known to cave explorers from the middle of the 19th century, he says. When paleontologists first examined the hominid fossil assemblage, found beneath a mess of carnivore bones including bears, foxes and a jaguar, they were stunned to discover that the skeletons were not from modern humans, but humans’ ancient relatives.

“Someone, some agent had to put them there,” Arsuaga says. “It’s a problem that is very difficult to solve.” Since many of the 28-odd skeletons are complete, the bodies are thought to have been dumped in the shaft as corpses, probably from a more accessible cave entrance that no longer exists. There are no known funeral practices from more than 400,000 years ago. If intentional burials did occurr at Sima de los Huesos, it would be the earliest evidence of such symbolic ritualized behavior, Arsuaga says.