Focus on the Sima de los Huesos

Probably in the next months we will have results about the nuclear DNA analysis of the hominid bones found in the Sima de los Huesos, dated to 430,000 years. I wanted to recap the background and importance of Sima, its findings, and what’s going on there at the present time.

Sima de los Huesos (‘Pit of Bones’) is one of the world’s most important sites of human fossils.  Located in Atapuerca, Spain, it is a sock-shaped pit with a 12 m deep vertical shaft, which turns into an inclined passage 10 meters in length, opening onto a 15 m2 chamber. So far, 7,000 hominid fossils have been found in this chamber…

A bit of history

In 1976 some human remains were found there, but it was not until 1984 when it was cleaned of rubbish and sediment, and dozens of human fossils started to appear. In 1992 three key specimens were discovered: ‘Agamenón’ (Cranium 4), ‘Miguelón’ (Cranium 5, among the most complete Homo skulls in the world’s fossil record), and Cranium 6.

Since then, every year only 20 cm have been excavated from a 1 m2 area, but this has provided more than half of the Homo fossils found worldwide: 7,000 human fossils from 28 individuals, dated to 430 Ka.

Among this huge set of fossilised bones: 17 skulls, the most complete pelvis in the fossil record, nicknamed ‘Elvis’, and some tiny bones of the middle ear which are really strange to find.

Some key data of the Sima population

  • All the individuals found there belong to the same biological population, which is terribly valuable to analyse their variations, sexual differences and patterns of development.
  • They are ancestor of Neandertals. For many years this population has been assigned to the species Homo heidelbergensis, but the extended study of such huge amount of fossils is suggesting the possibility of a different species. For the moment, the Atapuerca scientists prudently refer to them as pre-Neandertals.
  • No children under 3 were found among the fossils. The most represented group is between 13 and 17 year-old specimens. Only 3 individuals were over 30.
  • The sex of 18 individuals was determined: half men, half women. Males were 1.75 m tall and females 1.70 m tall.
  • They show heavily worn teeth, probably caused by the consumption of uncooked fruit or vegetables, but not a single cavity has been found. Their teeth show marks of toothpicks. They were right-handed: wear on the preserved teeth suggest that food was usually brought to the mouth with the right hand.
  • The ear canals in ‘Agamenón’ are blocked, suggesting long-term deafness.
  • ‘Miguelón’ probably died of septicaemia, after a tooth had been broken in life by a strong blow, so that the flesh had been exposed and led to an infectious process that continued until nearly the orbital bone.
  • Cranium 17 shows evidence of two fractures produced by the same object in a face-to-face interpersonal conflict.

The latest findings and the future

In the 2014 season, 200 more hominid fossils were found. This figures doubles the total number of human fossils extracted from the rest of sites worldwide. Besides, two important studies were issued on Sima findings:

1) Analysis of the 17 skulls collection of Sima (7 of them not released before), publised by Science Magazine in June. The key conclusions were:

  • About the evolution of the Neandertal skulls in Sima. Not all the skeleton parts developed simultaneously. The first neandertal features developed were the teeth, the jaw and the face, which suggest some masticatory specialization.
  • Some Neandertal traits showed by the Sima individuals are: pronounced brow ridge, not sunk over the nose; cheeks projected towards the nose, giving a triangular look to the face; engagement of the jaw to the cranium.
  • The braincases are not Neandertal-type. Their average brain size is less than modern humans and less than Neandertals. They developed the brain separately, and later.
  • The uniformity of the Sima population contrasts with the variation when comparing to other European fossils in Middle Pleistocene (780-130 kya). This can be explained by age and regional variations, also by interbreeding with archaic populations.

2) DNA of the Sima population

In Dec 2013 it was published the analysis of mitochondrial DNA of a 427 ky femur, surprisingly showing that the genoma of Sima was linked to the 700 ky Denisovan population, but no links with Neandertals.

In Sep 2014 it was announced that nuclear DNA were recovered from Sima fossils. The analysis of the nuclear DNA (to be published likely in this year 2015) will decisively enrich the information about the origins of the people from Sima, about the pre-Neandertal populations in Europe, and maybe give some light over the Homo heidelbergensis taxon.

Finally…

The following presentation describes the key fossils extracted from Sima. Most of them are exhibited at the Museo de la Evolución Humana (Museum of Human Evolution), in Burgos, a city located 14 km from the Atapuerca sites.

5 pensamientos en “Focus on the Sima de los Huesos

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