May 03, 2011

Homo heidelbergensis and the Ceprano calvarium

The type specimen of Homo heidelbergensis is the Mauer mandible. There is considerable controversy about the validity of this taxon, with some "stretching" it to include many different fossils from Europe, Asia, and Africa, while others limiting it to the pre-Neandertal population of Europe. There are also those who want to get rid of H. h altogether.

What constitutes the hypodigm Homo heidelbergensis? The Mauer mandible is insufficient for a definitive description of the species, as it is, well, just a mandible. A new paper suggests that the Ceprano calvarium from Italy is the "counterpart" of the Mauer mandible and can be used to complete the description of the taxon.

Personally, I'm not that invested in taxonomy. The sequencing of the Neandertal and Denisova Cave genomes suggests that modern humans were interfertile with archaic specimens that have been assigned to a different species on morphological grounds. So, if one adopts an evolutionary species perspective, it's hard to argue against those who've maintained that a single species within the genus Homo has existed long before the emergence of so-called "anatomical modernity" and Homo sapiens in the anatomical sense.

Also, the fact that interfertility of moderns with "archaics" has been proven true twice in two DNA-tested fossils has led me to recognize that all fossil hominids are relevant to the story of human origins. It may very well be true that modern humans share synapomorphies not shared by the "archaics" (and the current paper reinforces that idea), but it is not clear how they came to share these synapomorphies: by dispersal of an originally geographical circumscribed tribe (as Out-of-Africa holds) or by homogenizing gene flow between distant populations of a single Homo sapiens species.

I will sidestep all issues of taxonomy, and go right to the core of the paper which is quite interesting in itself. The auhors used a combination of geometric morphometrics and scored morphological traits to place Ceprano in the proper context.

The discriminant function analysis based on geometric morphometrics is shown on Figure 1 (left).

F1 seems correlated with time, with the Early Pleistocene samples (including Ceprano) on the left, Neandertals in the middle, and modern humans on the right. Even within the modern human "reds", the more recent Holocene samples dominate the right-most edge, suggesting that the process that brought us modern humans has continued within the sapiens lineage itself.

Neandertal specificity is captured by F2, with Neandertal groups aggregating in the bottom of the Figure.

Of particular interest are the non-red points (that are not assessed a priori as modern humans) that cluster, nonetheless, with moderns. These include: Singa from Sudan, LH 18 from Tanzania, Irhoud 1 from Morocco, and, most surprisingly, the quite old Steinheim skull from Germany. I also find it interesting that Rhodesian Man (Kabwe 1), often cited as a possibly African precursor to sapiens is not closer to the modern group than Sima de los Huesos 5, nor is the Dali skull from the far east particularly modern. Jinniu Shan, on the other hand is in the middle of the Neandertal cluster, which is interesting as it's from a region well to the east of the normally considered Neandertal range.

Turning to the morphological features, modern humans seem to be quite specific again compared to the other groups (red), and Omo II, the most archaic of the Omo skulls, earliest anatomically modern humans is assigned to the modern branch. In black are samples often assigned to Homo heidelbergensis, in blue the Neandertals.

Interestingly, 39 morphological traits are associated with modern humans (out of a total of 50), and this seems to confirm the quite distinctive nature of modern humans in comparison to other human groups that preceded them.

The full dendrograms are also quite interesting, expanding on the various modern human skulls that are summarized by letters in the above figure. Skhul 5 from the Levant joins Irhoud 1 and Omo II, and these are skulls that will usually be classified as modern but with substantial archaic traits.

In conclusion, I'd say that H. heidelbergensis seems to be a valid taxon but I don't quite see how a strong argument can be made that it gave birth to modern humans in Africa and Neandertals in West Eurasia. Both these species have a number of synapomorphies that cannot really be rooted on heidelbergensis variation in their respective regions. The emergence of Homo sapiens does appear to be a punctuational event, but whether this occurred as part of a migration or species-wide selection process remains to be seen.


PLoS ONE 6(4): e18821. doi:10.1371/journal.pone.0018821

The Stem Species of Our Species: A Place for the Archaic Human Cranium from Ceprano, Italy

Aurélien Mounier, Silvana Condemi, Giorgio Manzi


One of the present challenges in the study of human evolution is to recognize the hominin taxon that was ancestral to Homo sapiens. Some researchers regard H. heidelbergensis as the stem species involved in the evolutionary divergence leading to the emergence of H. sapiens in Africa, and to the evolution of the Neandertals in Europe. Nevertheless, the diagnosis and hypodigm of H. heidelbergensis still remain to be clarified. Here we evaluate the morphology of the incomplete cranium (calvarium) known as Ceprano whose age has been recently revised to the mid of the Middle Pleistocene, so as to test whether this specimen may be included in H. heidelbergensis. The analyses were performed according to a phenetic routine including geometric morphometrics and the evaluation of diagnostic discrete traits. The results strongly support the uniqueness of H. heidelbergensis on a wide geographical horizon, including both Eurasia and Africa. In this framework, the Ceprano calvarium – with its peculiar combination of archaic and derived traits – may represent, better than other penecontemporaneous specimens, an appropriate ancestral stock of this species, preceding the appearance of regional autapomorphic features.



Onur said...

A MCLUST and an ADMIXTURE analyses of the data would be fine.

eurologist said...

It's good to see the idea of a loose, but widespread heidelbergensis group supported by these data. As I have mentioned numerous times, all evidence suggests relatively easy gene flow between Europe, west Asia, and Africa until about ~300,000 years ago. After that time, the characteristic development of idiosyncratic Neanderthal features indicates severe isolation, which climate studies support. Also, very late heidelbergensis seems to have moved from eastern Europe/west Asia further into Asia ~250,000 to 200,000 years go based on a change in tool kits and fire usage - again in conformance with the east Asian fossils of that time.

There is no question the huge set of distinct AMH features is of African origin - but a select few of these features already start developing just before the Neanderthal side-step and thus persist in some late European samples (Steinheim, Ehringsdorf).

I wouldn't subscribe to a theory that heidelbergensis is the "ancestor" of both modern humans and Neanderthals. Clearly the latter, and through them and likely more directly in Asia some contribution - but the flow into Africa must have been ongoing in the preceding hundreds of millennia, so there is a significant impact on development in Africa - but my bet is, still, the larger genetic share is home grown, there. AMH characteristics are just too unique.