Neanderthal-like skulls collected from northern Spain are revealing new information about the prehistoric hominins. A total of 17 skulls of the long-extinct species were unearthed in the same location, which also offered up three similar artifacts 21 years ago.

Sima de los Huesos (Pit of the Bones) in the Atapuerca hill, located in northern Spain was the site of the discoveries. This area is extremely difficult to access, and researchers spent two decades searching for fossils. They would then bring fragments back to their laboratory, were the scientists painstakingly re-assembled the skulls.

As many as 30 individual Neanderthal relatives are believed to be among the dead found at the site. The bones are largely fractured, and the remains are all hap-hazardously jumbled together in a large pile.

Each of the individuals found at the site are from one genetic stock. This genetic homogeneity allows scientists to better study individual differences in the remains, including those between the genders.

The combination of mosaic evolution and genetic similarities suggest to researchers that those hominids could have been experiencing a wide range of genetic changes. This would have produced a wide range of evolutionary changes at the same time, in a process called cladogenesis. A similar process took place on the Hawaiian Islands, after species arrived there from across the Pacific Ocean.

Several dating techniques suggest the remains are roughly 430,000 years old, during the height of the last period of global glaciation. Researchers are attempting to better-understand the age of the site in order to determine how all the bodies came to be in that area.

Neanderthal-like remains from the Sima site are the oldest such fossils to show clear facial structures similar to those of the hominids. Researchers believe evolutionary changes occurred in a "mosaic" fashion, with development of the jaw and lower face happening first, possibly driven by change in diet. One of these modifications could have been front teeth that could act like an extra hand.

Changes in the upper skull are thought to have formed later in Neanderthal evolution. Genetic changes recently measured in mitochondrial DNA from Sima remains show they were not likely direct ancestors to the better-known hominids.

Archaic humans are believed to have evolved in Africa roughly two million years ago. Prehistoric humans arrived in Europe and Asia roughly a million-and-a-half years later. There, they would slowly develop in Neanderthals, which disappeared around 40,000 years in the past. Modern humans came into being in Africa between 100,000 and 200,000 years ago, and the two species were genetically similar enough to breed. Between 1.5 and 2.1 percent of modern human DNA is left over from Neanderthals.

"With excavations continuing and new fossils being discovered each field season, there is certainly reason to believe that the Sima de los Huesos will yield more surprising findings in the future," investigators stated in a press release announcing their findings.

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