March 28, 2013

Morphological and mtDNA analysis of Mezzena mandible

I've written about late Neandertals becoming more AMH-like, and a new study on an Italian specimen that postdates the arrival of AMH in Europe lends some further support to that idea.

The Mezzena jaw has Neandertal mtDNA and shares a number of morphological traits with other Neandertals, but its overall shape places it within the cluster of modern humans  (triangles; figure on the left). Notice also that Qafzeh 9 (Q9) and Shkul V (SV) are also within the cluster of modern humans, and Spy 1 (a Neandertal) is actually closer to modern humans than to other Neandertals.

From the paper:
The position on the scatter plot of our specimen of interest, Mezzena, has been calculated a posteriori. Unsurprisingly, the Mezzena mandible does not present any particular affinities with mid-Pleistocene specimens. It is most similar to AMHs being positioned within the H. sapiens cloud of points and the DFA classifies the specimen with modern humans (Table S7). Especially its shape is similar to that of Ohalo II and to a lesser extent to the recent modern human specimen China5. However, it should be noted that its position also indicates affinities with some Neanderthal specimens: the late Neanderthal Spy 1 and Saint-Césaire, the Near-East specimens Tabūn II and Amud 1, and to a lesser extent the classic Neanderthals La Ferrassie 1 and Guattari III (Figure 2).
According to the authors:
In this light, we can interpret the position of the Mezzena mandible which stands within the modern human shape space, while presenting strong shape similarities with some Neanderthal specimens. Such a conflicting taxonomical position is not surprising, considering the geological age of the mandible [30]. Indeed, numerous late Neanderthals such as Spy 1, Saint Césaire and the Near-East mandibles Amud 1 and Tabun II possess hints of a chin (i.e. tuber symphyseo) though not a true modern human morphology [37], [51]. Late Neanderthals lived in area where AMHs might have been already present [2], [23], [52], while the Levantine fossils are displaying a less derived Neanderthal morphology [35], [36].  
Therefore, in our view, this change in morphology of the mandibular chin among the fossils of Mezzena and other late Neanderthals could have been the result of a small degree of interbreeding with AMHs.
It would be interesting to sequence Mezzena to confirm the existence of AMH admixture.

PLoS ONE 8(3): e59781. doi:10.1371/journal.pone.0059781

Possible Interbreeding in Late Italian Neanderthals? New Data from the Mezzena Jaw (Monti Lessini, Verona, Italy)

Silvana Condemi et al.

In this article we examine the mandible of Riparo Mezzena a Middle Paleolithic rockshelter in the Monti Lessini (NE Italy, Verona) found in 1957 in association with Charentian Mousterian lithic assemblages. Mitochondrial DNA analysis performed on this jaw and on other cranial fragments found at the same stratigraphic level has led to the identification of the only genetically typed Neanderthal of the Italian peninsula and has confirmed through direct dating that it belongs to a late Neanderthal. Our aim here is to re-evaluate the taxonomic affinities of the Mezzena mandible in a wide comparative framework using both comparative morphology and geometric morphometrics. The comparative sample includes mid-Pleistocene fossils, Neanderthals and anatomically modern humans. This study of the Mezzena jaw shows that the chin region is similar to that of other late Neanderthals which display a much more modern morphology with an incipient mental trigone (e.g. Spy 1, La Ferrassie, Saint-Césaire). In our view, this change in morphology among late Neanderthals supports the hypothesis of anatomical change of late Neanderthals and the hypothesis of a certain degree of interbreeding with AMHs that, as the dating shows, was already present in the European territory. Our observations on the chin of the Mezzena mandible lead us to support a non abrupt phylogenetic transition for this period in Europe.

Link

3 comments:

eurologist said...

This is a pretty late Neanderthal: ~10,000 years after arrival of AMHs in Italy, there indeed should be a good chance of interbreeding.

I have previously argued that the lack of uniparental Neanderthal markers in extant humans could be due the fact that Neanderthal populations were dwindling right around contact (when AMHs also still had relatively low numbers for the first 3,000-5,000 years), and chances of encounters over multiple generations were probably slim. As always, we would expect almost all interactions to be between male intruders and female natives (Neanderthals). In that case, some mixed male men with sufficient AMH traits may have made there way out and joined AMHs, will this would have likely been very difficult for mixed females. As a result, no uniparental markers were inherited into future generations.

Grognard said...

Even if you exclude selection on mtDNA, which is not very sensible, you won't get any mtDNA or Y DNA from small introgressions in a small group it's just math.

Small groups will become homogenous very quickly. Introduce 5 new members into a tribe and the chance either male or female lineage remains intact after ten generations is vanishingly small, let alone 25k years. OTOH the tribe will end up with an average amount of nuclear DNA similar to the number of tribe members it integrated.


terryt said...

"Our observations on the chin of the Mezzena mandible lead us to support a non abrupt phylogenetic transition for this period in Europe".

That makes so much sense. As does this:

"As always, we would expect almost all interactions to be between male intruders and female natives (Neanderthals). In that case, some mixed male men with sufficient AMH traits may have made there way out and joined AMHs, will this would have likely been very difficult for mixed females"

As does this:

"Introduce 5 new members into a tribe and the chance either male or female lineage remains intact after ten generations is vanishingly small, let alone 25k years. OTOH the tribe will end up with an average amount of nuclear DNA similar to the number of tribe members it integrated".