Neandertal introgression, anatomically

I'm just finished with the Neandertal meeting in San Diego, so it's time to decompress a bit. And what better way to do it than some more Neandertal blogging!

It is worth mentioning the paper that finally came out this week about Pestera Muierii, Romania, by Andrei Soficaru, Adrian Dobos, and Erik Trinkaus. Here's the abstract:

The early modern human remains from the Petera Muierii, Romania have been directly dated to 30,000 radiocarbon years before present (30 ka 14C BP) (35 ka cal BP) ("calendrical" age; based on CalPal 2005) and augment a small sample of securely dated, European, pre-28 ka 14C BP (32.5 ka cal BP) modern human remains. The Muierii fossils exhibit a suite of derived modern human features, including reduced maxillae with pronounced canine fossae, a narrow nasal aperture, small superciliary arches, an arched parietal curve, zygomatic arch above the auditory porous, laterally bulbous mastoid processes, narrow mandibular corpus, reduced anterior dentition, ventral-to-bisulcate scapular axillary border, and planoconcave tibial and fibular diaphyseal surfaces. However, these traits co-occur with contextually archaic and/or Neandertal features, including a moderately low frontal arc, a large occipital bun, a high coronoid process and asymmetrical mandibular notch, a more medial mandibular notch crest to condylar position, and a narrow scapular glenoid fossa. As with other European early modern humans, the mosaic of modern human and archaic/Neandertal features, relative to their potential Middle Paleolithic ancestral populations, indicates considerable Neandertal/modern human admixture. Moreover, the narrow scapular glenoid fossa suggests habitual movements at variance with the associated projectile technology. The reproductive and scapulohumeral functional inferences emphasize the subtle natures of behavioral contrasts between Neandertals and these early modern Europeans. (Soficaru et al. 2006:17196).

The paper describes the provenience of the bones -- they are not newly found, but had been originally assumed to be Holocene in age. Recent radiocarbon dating placed them at around 30,000 years old, which makes them among the earliest modern Europeans.

The bottom line is that the bones are modern (i.e., not Neandertal), but they include features that are common in Neandertals. Almost all the other European bones of early Upper Paleolithic date also have Neandertal features. The number and frequency of such features in this earliest Upper Paleolithic sample are greater than in any later sample.

In other words, they look like they have genes from Neandertals. And those genes declined in frequency or effect over time.

Of course for any particular feature on any particular specimen, the story gets more complicated. Take the occipital bun on Muierii 1. It clearly is a projection of the posterior cranium, in the position of the occipital bun in Neandertals, it projects well posterior to inion and it has a fairly abrupt superior aspect. On the other hand, the projection is expressed on a much higher and shorter vault, and certainly doesn't look identical to a Neandertal bun.

But then, Neandertal buns are quite variable, which several Neandertals having no bun at all, and others exhibiting a variable morphology. The ontogeny of the trait probably relates to growth of the posterior brain, the timing of closure of the lambdoidal suture, and the relative bone growth rates of the parietal and occipital bones. Those developmental prerequisites almost certainly differed between skulls with a Neandertal-like cranial shape, and those with a higher, more rounded skull. So the same feature -- or at least, a result of the same developmental process -- may be manifested with different forms in different cranial contexts.

"Contextually archaic" is a nice phrase. It is describing anatomies that occur within modern humans, and that continue to occur within recent and (presumably) living people, but that have become very uncommon. They are far more common in archaic humans, but may have a slightly different pattern of expression, in many cases because the developmental process that generates such features depends on anatomical configurations or events that have themselves changed. So within the context of the sample, they are "archaic" -- reflections back upon earlier humans, in this case Neandertals.

"Neandertal features" certainly has a more intuitive meaning -- features that occur at their highest frequencies in Neandertals -- but it really doesn't convey a lot more information, except for the regional specificity of Neandertals versus all archaic humans elsewhere in the world. But of course since we have many more Neandertals than any other archaic specimens, these "Neandertal features" in some cases are simply "contextually archaic" features in the European context.

What is the point I am coming to? Many "Neandertal features" clearly are more common in early Upper Paleolithic people than in later Europeans, and they show a unidirectional trend toward lower frequencies over time. Some folks would argue that these features don't really demonstrate Neandertal-modern intermixture, because (a) you can't really prove that they are absent in archaic Africans, or they may even be there, although in lower frequencies than Europe; or (b) they are not really the same feature, but instead are consequences of different developmental processes or parallelism.

Why do I think these critiques have little force? Because at this point, we have enough early Upper Paleolithic specimens with such features to notice something very important about them: different specimens have different Neandertal features.

It's like a shotgun approach to Neandertal intermixture. These are not one or two things appearing in parallel, and they aren't chance resemblances in this small early Upper Paleolithic sample, when they almost all decline systematically in later samples.

So when we see each new specimen, like Muierii 1, carrying not only Neandertal features, but its own distinctive set of Neandertal features, that emphasizes the early role of genome-wide intermixture.

However, these traits co-occur with contextually archaic and/or Neandertal features, including a moderately low frontal arc, a large occipital bun, a high coronoid process and asymmetrical mandibular notch, a more medial mandibular notch crest to condylar position, and a narrow scapular glenoid fossa.

Each of these features occurs in other modern specimens, but not in the same combination. And every other specimen from the early Upper Paleolithic with Neandertal or archaic features has a different mix of them. If this phenomenon were the result of parallelism on modern humans entering Europe, or if it were a consequence of features retained from archaic Africans, we should not see this broad and altering mix of features in different specimens.

So bigger samples, adding specimens one at a time, really are important. They let us look at the pattern of variation in ways that test these evolutionary hypotheses.

References:

Soficaru A, Dobos A, Trinkaus E. 2006. Early modern humans from the Pestera Muierii, Baia de Fier, Romania. Proc Nat Acad Sci, USA 103:17196-17201. DOI link