José Mª Bermúdez de Castro, coordinator of the Paleobiology Program at the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), has led a paper just published in the journal Anatomical Record on the population at the Sima de los Huesos site, in the Sierra de Atapuerca (Burgos, Spain), which confirms that this Middle Pleistocene human group was comprised of 29 individuals, instead of the 28 estimated in earlier studies.

During the 2019 excavation campaign, the impressive total of 7600 human fossils from the Sima de los Huesos was reached, forming a collection unique in the world for studying the variability of a past species. In 2003, when the number of fossils stood at 4000, it had been calculated that 28 different individuals were represented.

During recent excavation campaigns, very complete bone remains have been encountered, as well as others that have allowed craniums, jaws and postcranial skeleton bones to be reconstructed. For this reason, it appeared necessary to carry out another demographic study of the Sima de los Huesos, and this took place in January 2020, starting with the hypothesis that the number of individuals in the collection could have increased significantly. Nevertheless, and surprisingly, the estimated number of individuals has only risen from 28 to 29.

“We have been able to verify that some individuals have been becoming more complete, while others are still represented by the same teeth and jaw fragments recovered during the first years of excavation”, says Bermúdez de Castro. “It appears to be clear that the damage to the site before the 1976 discovery of the first human fossils was major and a lot of information about some individuals has been lost”, adds the co-director of Atapuerca.

Sex and age

In addition, the sex of the individuals from the Sima de los Huesos was reviewed in this work. It is estimated that nine of them could be male and fifteen female. Despite this imbalance, the statistical analyses concluded that the 8:15 ratio could have come from a population in which both sexes were equally represented.

Moreover, there is an excess of teenagers and very young adults (82.8%). Only one child individual has been recorded, while four adults had reached maturity, possibly in the age range 40 to 45.

“With such high mortality at such young ages, the reproductive viability of a Pleistocene hunter-gatherer group could not be sustained. Given that the overall mortality at the Sima de los Huesos matches a catastrophic profile better, it is possible that their lives were ended by a single event of unknown type”, says Bermúdez de Castro.