Just in from Atapuerca, northern Spain: mitochondrial DNA has been retrieved from the bones of Homo heidelbergensis.
The Sima de los Huesos, or pit of bones, has been a treasure trove of human remains, and has yielded a minimum number of 28 individuals dating to at least 300,000 years ago. This type of preservation and concentration of human remains is rare—most excavations are lucky to turn up a stray hominin tooth every couple of seasons.
Homo heidelbergensis cranium number five from the Sima. Photo by José-Manuel Benito Álvarez
Not to mention mitochondrial DNA typically does not have such a long shelf-life. After a successful attempt at analyzing cave bear DNA from the Sima, researchers decided to risk portions of valuable human remains for testing. The MtDNA did turn out to be viable, and the provided some seriously unexpected results.
The Heidelberg femur that yielded the mtDNA. Image from Meyer et al. 2013.
The outcome of the mtDNA analysis demonstrates that the Sima matrilineal tree shares its roots with Denisovan populations, a group of archaic humans discovered recently in Siberia. Previously it would have been expected that the Sima Heidelberg mitochondrial genome would share most affinities with Neandertals.
So what exactly does this mean for our current understanding of human evolutionary models?
The findings imply that the Sima hominins share a common ancestor with Denisovans rather than Neandertals. For a long time, the Heidelberg population of the Sima was looked at as a group of hominins en route to becoming Neandertals—now the picture isn’t as clear.
It is possible that the Sima hominins are a distinct line of humans that later contributed genetically to Denisovans. This wouldn’t however explain the morphological traits they exhibit which are consistent with Neandertals, and would mean the same traits evolved in two separate contexts.
It is also possible that the Sima population was ancestral to both Neandertals and Denisovans. This answer would require further explanation for why the genomes of Neandertals differ significantly from the recently sequenced genome, though.
Right now, there isn’t one answer. As DNA processing methodology gets better and better, the picture of what was going on will become more evident. We’ve only begun to scratch the surface of what was really going on, and it’s going to get a lot more complicated before it gets any clearer.