An interesting case of hybrid morphology (and genome): the Oase-1 jaw

In 2002 the robust jaw of an adult male human, Oase-1, was found in Peștera cu Oase (‘the cave with bones’) in southwestern Romania. It was dated to 38-42 ka, which falls among the oldest European early Upper Paleolithic human remains, together with Grotta del Cavallo (43-45 ka), Kent’s Cavern 4 in UK (41-44 ka), Mladeč in Czech Republic (34 ka), Vogelherd in Germany (32-33 ka)… and overlaps the late surviving neandertals.

The morphology of the Oase-1 mandible presents a very interesting hybrid combination of derived and archaic traits:

Within the range of early modern humans in the Late Pleistocene:

  • A key feature of this fossil is the tuber symphyseos, a prominent triangular bulge occupying the inferior half of the anterior symphysis. This is a derived early modern human feature, which is clearly present in Oase-1.
  • Apart from its overall proportions, other traits reinforcing its classification as modern human are: more mesial mental foramen, narrow lateral corpus, retromolar space absence, symmetrical mandibular notch and medially placed condyle.
Oase-1 jaw

Oase-1 jaw. Photo credit: Roberto Sáez

Out of the modern human range:

  • The ramus is exceptionally wide!
    • It is comparable to other similar wide rami among Middle Pleistocene specimens such as Arago 2, KNM-BK 67, Loyangalani 1, Mauer 1 and Tighenif 3.
    • Among later Pleistocene human remains, only Dar-es-Soltane 5 and Nazlet Khater 2 from northern Africa have a comparable ramus.
    • The ramus breadth indicates, by extension, a long temporal fossa and anterior positioning of the zygomatic bone. This pattern appears among African later archaic and early modern humans, although present among Middle Pleistocene specimens.
  • Distal molar megadontia: the molars become progressively larger distally with exceptionally large third molars.
    • The combination of large molar dimensions and the proportions along the tooth row align Oase-1 with both the neandertals and some preceding Middle Pleistocene specimens (Krapina 53 -large M2-, BOU-VP-16/1 and Irhoud 3 – large M1) and Early Pleistocene specimens (KNM-BK 8518 – large M3).
Oase-1 dentition

(L) Occlusal view of the Oase-1 dentition. Photo credit: Roberto Sáez. (R) Right mandibular molars from M1 to M3. Photo credit: Trinkaus E. et at. An early modern human from the Peştera cu Oase, Romania. PNAS September 30, 2003. 100 (20) 11231-11236; doi.org/10.1073/pnas.2035108

  • The mandible length and the cross-sectional symphyseal orientation are intermediate between late archaic and early modern humans.
  • The lingual bridging of the mandibular foramen is a characteristic feature of the late Middle and Late Pleistocene members of the neandertal lineage.
    • Moreover, in Oase-1 it is absent from the right ramus but present on the left one, as happens in half of the neandertal mandibles.
Oase-1 jaw

Oase-1 jaw. Photo credit: Roberto Sáez

Mixture between regional neandertal populations and in-dispersing early modern humans:

Fortunately, human DNA material could be recovered from Oase-1 and presented in 2015 with the following finds:

  • The analysis shows that an amazing 6 to 9% of the Oase-1 individual genome was derived from neandertals, more than any other modern human sequenced to date. The normal neandertal contribution is 1 to 3% of the DNA of present-day people in Eurasia.
  • The size of three chromosomal segments indicates that this individual had a neandertal ancestor as recently as four to six generations back.
  • However, the Oase-1 individual does not share more alleles with later Europeans than with East Asians, suggesting that the Oase population did not contribute substantially to later humans in Europe.
Oase-1 jaw

Oase-1 jaw. Photo credit: Roberto Sáez

Conclusion

Together with other fossils recovered from the site, Oase-1 is an extraordinary case to illustrate the transition from neandertals to early modern humans in Western Europe, and the complex population dynamics of modern human dispersal into Europe.

These dynamics are the result of the earlier emergence of modern humans from Africa, their dispersal over tens of millennia throughout Eurasia, and the geographically and temporally variable mixture of those dispersing populations with other regional human groups, including the neandertals and other human groups (species?) that became extinct.

References:

  • Trinkaus E. et at. An early modern human from the Peştera cu Oase, Romania. PNAS September 30, 2003. 100 (20) 11231-11236; doi.org/10.1073/pnas.2035108
  • Fu, Q. An early modern human from Romania with a recent Neanderthal ancestor. Nature 524, 216–219 (2015) doi:10.1038/nature14558

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