Searching for the roots of our lineage: 6 African skulls from the Late Middle Pleistocene

Last January, Lee Berger challenged me on Twitter to add Homo naledi to a picture I posted, which came from a paper by Aurélien Mounier & Marta Mirazón (2019). That was an image of five hominins from the African Late Middle Pleistocene. I found it interesting, so after getting their permission to modify the picture, here is the result! Now, I wanted to briefly describe the six specimens, and reflect on their place in the search for the roots of the human lineage.

From left to right, top to bottom:

Modified from figure 3 in Mounier & Mirazón Lahr (2019) – see reference

Omo II

Kibish rock formation, Omo River Valley, Ethiopia (1967). Age: 190-200 ka.

This cranium is quite different from the other one found at the same location: Omo I was thought to be the earliest Homo sapiens fossil until the reassessment of the Jebel Irhoud materials in 2017, which predated that record. However, Omo II is less rounded, longer and narrower than Omo I. Actually, like occurs in Jebel Irhoud, many features are outside the variation of modern humans while others are fully modern. This mosaic makes really difficult to classify this cranium and shows the variability of the African Middle Pleistocene human groups, among which many of them would have probably become extinct.

LH 18

Ngaloba, Laetoli, Tanzania (1976). Age: 120-200 ka.

Although it is large and quite modern, again we can check some archaic features like a long and oval shape with a low vault, sloping and relatively flattened frontal bone, a significant and continuous supraorbital torus, and very thick bones, combined with a gracile face and a rounded occipital region.

Florisbad

Florisbad, South Africa (1932). Age: c. 259 ka.

This cranium shows a modern, high frontal bone although its roundness attenuates laterally and has a sagittal depression. It also shows a large supraorbital development, and the face is broad. Its discoverer Thomas F. Dreyer assigned it to a new species Homo helmei (1935), to distinguish it from Homo sapiens and Homo heidelbergensis.

KNM-ES 11693

Eliye Springs, Kenia (1983). Age: 200-300 ka.

This is a large skull with modern shape in its upper and rear view, very slight supraorbital development and absent transverse occipital torus. But it also has some primitive features: flattened and elongated appearance, with maximum width in lower position, presence of sagittal keel and very robust face.

Irhoud 1

Jebel Irhoud, Morocco (1961). Age: 280-350 ka.

The human remains of Jebel Irhoud are considered the earliest representatives of our species since their reassessment in 2017. This proposal was based on the modernity of some features of the face (gracile checkbones and lack of projection), the morphology of the dentition and the jaw (despite its large size), although combined with other primitive features, such as the elongated and low skull shape and the supraorbital arches.

LES1

Lesedi chamber, Rising Star cave, South Africa (2013). Age: 241-335 ka.

The skull of Homo naledi looks primitive, with features that resemble to those in Homo habilis, suggesting a very deep origin of its lineage. The cranial capacity is 610 cc. The teeth are also primitive in the increasing size towards the back of the tooth row, but modern in their small size and structure, as in the gracile mandible. The most modern features of Homo naledi are found in the post cranial skeleton.

Interestingly, Florisbad and Homo naledi were contemporary humans, perhaps living in close proximity in the same region.

Modified from figure 3 in Mounier & Mirazón Lahr (2019) – see reference

What is archaic Homo sapiens?

Except for the Homo naledi skull, all the rest could be considered representatives of an archaic Homo sapiens lineage. What are the species representing the archaic humans in Africa, apart from Homo naledi, if not Homo heidelbergensis? Given that Homo helmei is usually abandoned, we may use Homo rhodesiensis – or possibly Homo sapiens itself! But first, the priority must be to clarify what the term ‘archaic’ means here.

The barrier between the so-called anatomically modern humans and the term archaic Homo sapiens is not clear, because of the poor record of fossils and the great morphological variability between them. It is difficult to geographically and temporally locate the origin of the ancestral populations of modern humans, and also to understand the evolution of their morphology. In my interview to Chris Stringer in 2017, he explains:

  • The pattern of anatomically modern humans is typically seen in the high and rounded skull, the small face, the chin, the lighter-built skeleton with a narrow pelvis, etc. This is most of the pattern that can be found in Africa back to between 150-200 ka (Herto 1 and 2, Omo 1).
  • Beyond 200 ka, there are specimens in the Homo sapiens line that do not yet show the majority of the modern features: Florisbad, Eliye Springs, Ngaloba, Jebel Irhoud… The complexity of the archaic pattern shows that it might have actually gone on for a long time in Africa: different specimens are showing different combinations of these modern human and archaic features.
  • There is growing evidence that the divergence of the Homo sapiens lineage with the Neanderthal lineage goes back a long way, and we need a term for Homo sapiens in Africa before we have the majority of modern human features. Here we can refer to archaic Homo sapiens in Africa for the early part of our lineage. Some other Middle Pleistocene fragmentary fossils (Ndutu, Salé, Thomas Quarry) could be even early Homo sapiens rather than Homo heidelbergensis.
  • The use of that term outside of Africa is meaningless. Sometimes the Dali material from China is called archaic Homo sapiens, probably referring to a slightly bigger brain than erectus, but other features in Dali are not found in the African specimens.

In 2019, Mounier & Mirazón Lahr built a model to estimate the morphology of a virtual skull representing the ‘first modern human’, the last common ancestor of today’s humans (this is the study with the picture from where this post begins). They propose a complex evolutionary process of Homo sapiens composed of several phases, although many did not contribute to the genetic and phenotypic structure of current human populations:

  • Between 350-200 ka, a first diversification with the development of different contemporary populations forming local of morphs of pre-sapiens.
  • Between 200-100 ka, a period of fragmentation and expansion, coalescence of groups and hybridization, which resulted in the emergence of morphologically derived populations: Herto, Skhul and Qafzeh are examples of that. But not all populations contributed equally to the ancestral lineage of modern humans. Local extinctions and founder effects would have influenced too.
  • The morphology of the virtual last common ancestor is closer to this last phase than to the earliest one. This indicates that anatomically modern representatives are yet to be found in older chronologies.

Further information: Reconstructing the cranium of “the first modern human” | Nutcracker Man 

Reference: Figures in this post are modified from Figure 3 in Mounier, A., Mirazón Lahr, M. (2019). Deciphering African late middle Pleistocene hominin diversity and the origin of our species. Nat Commun 10, 3406. Images of LES1 from de Ruiter, D. J. et al. (2019). Homo naledi cranial remains from the Lesedi chamber of the rising star cave system, South Africa. Journal of Human Evolution 132.

 

 

 

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