An archeological site known as the Sima de los Huesos or Pit of Bones was made famous in 1993 when three fossilized human skulls were found there.

Since that time, researchers have been slowly excavating the site, removing bone fragments from 30 other individuals that were broken and jumbled together under layers of sediment. Individual bone fragments have been matched and the remains of individuals reassembled like human jigsaw puzzles.

Work has also been done on the geology and dating of the site. Researchers hope that this will provide clues as to how and why so many human remains were gathered in one spot. The Pit of Bones contains the largest known deposit of bones from an extinct human species.

Multiple independent dating methods show the site to be approximately 430,000 years old, which would place it in the Middle Pleistocene era.

The Atapuerca research team maintains that the Sima population is closely related to Neanderthals. The remains show clear Neanderthal facial features and demonstrate, say researchers, that the evolutionary pattern of Neanderthals is “mosaic” in nature.

Mosaic evolution is the idea that change takes place in some parts of the body without simultaneous changes in other parts of the body. In the case of the Spanish site, this is opposed to the idea that change takes place gradually over the entirety of the skull.

The remains at Sima de los Huesos indicate that the changes that are associated with Neanderthal skulls occurred first in the teeth, jaw and face. This would indicate that these adaptations represent a single complex change to the chewing apparatus. Other areas of the skull underwent changes at a later date.

Because all of the remains recovered from the site represent a single population it is also possible for researchers to study variation across gender and age groups.

“This combination of mosaic evolution and anatomical homogeneity led the authors to favor a branching pattern of evolution, known as cladogenesis in evolutionary studies, in the European Middle Pleistocene,” according to a statement.

While the samples recovered demonstrate some Neanderthal characteristics, they are not purely Neanderthal. Researchers have not assigned a particular species to the fossils recovered as they do not fully match any human remains found to date. However, the research which began 40 years ago at the “Pit of Bones” is still ongoing, with no end date in sight.

The latest findings can be found in the journal Science.