Determining the age of fossils found in conditions like those at Sima de los Huesos is notoriously difficult. At Gran Dolina, clues from the distinct archaeological layers helped pinpoint the age of the fossils found there. But the Pit of the Bones is filled with muddy sediments with no distinct geologic layers that can be easily dated.

Pelvis Homo heidelbergensis or related species 400,000 years old photo M. Carlough/AMNH

To establish the age of this pelvis and other Sima fossils, the Atapuerca team had to rely on the secondary limestone deposits that periodically formed across older sediments. Such deposits (similar to the stalagmites and stalactites on the floors and roofs of caves) can often be chemically analyzed to determine the age of the sediments below. Using the technique known as uranium-series dating, which measures the amount of radioactive-isotope decay over time, investigators concluded that the early human fossils from Sima de los Huesos are between 350,000 and 500,000 years old.

Although this pelvis is essentially fully modern (it is configured for upright walking in the modern style, for example), it has some archaic characteristics. Wider and more robust than a modern human pelvis, this fossil reveals that females of this species had a larger birth canal than modern females (Homo sapiens). A modern human newborn could easily fit through the central opening of this pelvis, which is thought to be from a male. The opening in a female pelvis would have been even larger.