Last week the spread of information around Homo naledi was huge. But there was another hugely important publication in the human evolution field: the results of the partial sequencing of nuclear DNA from the Sima de los Huesos site in Atapuerca, Spain. The samples were taken from two fossils, a femur and a tooth. This project is carried out by the Max Planck Institute for Evolutionary Anthropology.
The Homo naledi’s announcement was just the day before Sima’s, so the spread of the Sima news is actually happening mostly in the current week. Unfortunately I have read a few interpretations which were exagerated or inaccurate, so I wanted to summarise below a list of key facts from the Sima DNA findings:
1) The analysis is still partial. They are only some initial results. The human nuclear DNA is c. 3 billion base pairs (nucleotides) and they are reconstructing small fragments of c. 20-30 pairs each. They have been able to sequence 1 to 2 million pair bases so far (0.1%). The amount of work is huge…! The overall target is to reach an assemblage fraction of c. 0.5% to 1% of total nuclear DNA. That will be meaningful to determine the evolutionary relationships.
2) The initial results show that Sima fossils share a close affinity with neandertals (as their morphology had already indicated), and suggests two scenarios:
- A) They are early neandertals (‘pre-neandertals’) or related to early neandertals: They gave rise to the ‘classic neandertals’. A formal classification would then be required for them – maybe even a new species.
- B) They are in fact the earliest known neandertals. This option is widely discarded by scientists, who are aligned more around the first option ‘A’: the partial sequence of the nuclear DNA is consistent with Sima people being pre-neandertals.
3) The full analysis, when finished, will not be enough to fully characterize the population form. For this, they would need a much higher fraction of at least 10%, which is almost impossible considering that this is the oldest human DNA recovered (430K years).
4) The results differ from the mtDNA analysis made in 2013, which showed a strong relation of the Sima hominids with the denisovans, and no so much with the neandertals. The researcher Matthias Meyer thinks this is due to some introgression of other mtDNA lineages. That suggested two scenarios:
- Eventual interbreeding between the denisovans and the Sima populations.
- A common ancestor of neandertals, denisovans and the Sima populations.
5) If confirmed, the results will push back the H. sapiens-H. neanderthalensis ancestor beyond 400 KYA. Meyer suggested that the ancestors of H. sapiens could have diverged from the branch leading to neandertals and denisovans 550 K to 765 K years ago. It may be possible that H. sapiens evolved in western Eurasia and later migrated back into Africa. The fossils from Europe, Asia and Africa in the 400 Ka. period are physically very diverse and may represent multiple species, only one of which could be the ancestor of today’s humans. For example, Chris Stringer thinks it may be needed to look at Homo antecessor and not Homo heidelbergensis as our last common ancestor with neandertals.
(L) Skull 5 of Sima de los Huesos. (R) Gibraltar 1 neandertal skull from Forbes’ Quarry. Photo: Roberto Sáez
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