The Sima de los Huesos (pit of bones) is a cave in northern Spain from which 6500 human fossils from at least 28 individuals have been recovered to date. Analysis of skulls from the earliest humans with Neandertal-like features reassigned the age of the fossils to about 430 000 years ago.
Dr Lee Arnold, from the Environment Institute at Adelaide University, was one of the lead authors on the research paper published in the journal Science. The research addresses controversy associated with human evolution during the Middle Pleistocene period, in particular the origin of Neandertals and modern humans.
Previous studies of fossils in the cave in the Atapuerca Mountains reported the age of the skulls at more than 530 000 years old. Dr Lee Arnold is a geochronologist and led the dating part of the study. He conceded that “This age range is one of the most difficult to date”. However, Arnold says that to arrive at the new date range: “rather than relying on a single dating technique, we used six different techniques to produce a robust chronological study which would not have been possible a few years ago”.
The result is more compatible with morphological and genetic evidence for human evolution of the time. “We’ve resolved the age of the fossils at 100,000 years younger than previously reported, which makes them the oldest reliably-dated humans to show clear Neandertal morphology.” Dr Arnold and Dr Martina Demuro, geochronologists from Adelaide University’s Environmental Luminescence group, conducted dating of the site while at Spain’s National Research Centre for Human Evolution (CENIEH).
Along with a more accurate age of the fossils, studies of the specimens cranial, facial, and dental features of the Atapuerca hominins allows more precise evolutionary positioning of these Neandertal ancestors. The analysis has allowed for testing of the “accretion” model which proposes that Neandertal features appeared separately rather than at the same time. For example the facial features evolve at a different time to the neocranium.
The skulls from this population show jaws and teeth which are more typically Neandertal and upper cranial features more like Homo heidelbergensis, suggesting the fossils may belong to a new species or sub-species. “A picture is emerging of human evolution which is way more complex than has been considered over the past couple of decades,” says Dr Arnold.
More skulls of extinct human species have been found at the Sima archaeological site than anywhere else in the world. “This collection of bones, which is expected to continue growing in the coming years, is becoming increasingly important for the study of human evolution.” says Professor Juan Luis Arsuaga, from Madrid’s Complutense University and the ISCIII Joint Centre for Evolution and Human Behaviour in Spain.
Obtaining the fossils is difficult, with access limited to a 500 metre crawl through underground caves and a 13 metre abseil down a deep vertical shaft. A career as a research scientist really can take you to some amazing places!