Figure 1Evolutionary Changes of Braincase Shape from an Elongated to a Globular Shape. Scerri et al. (2018)

a new take on out-of-africa

One of the key features of science is that its findings aren’t set in stone. Bring forward a new body of evidence, & it’ll be reviewed and considered, and may just result in a particular model or view being changed.

I remember, back when I was in the 7th form (year 13), learning about how the human lineage went back about 12-14 million years to a species called Ramapithecus. (It’s now known as Sivapithecus, a genus of ape from India.) These days we believe that our hominin line parted company with that of the ancestors of chimpanzees, just 5-6 mya. However, up until now the general view has been that Homo sapiens arose from a single African population.

A paper published in June this year (Scerri et al., 2018) argues otherwise:

that Homo sapiens evolved within a set of interlinked groups living across Africa, whose connectivity changed through time.

Scerri & her colleagues put forward data from a range of sources to support this viewpoint: the ages/dates and physical appearance of sapiens fossils; the archaeological record (ie cultural evidence); and information from genetic studies.

Physical appearance

The researchers comment that our species probably dates back about 500 thousand years (kya), although the earliest fossils identified as sapiens date to 300 kya. (These are the Moroccan remains from Jebel Irhoud.) There are several features of skull & face that are viewed as typical of our species (& year 13 bio students should be able to list them!). These include a small & relatively fine-boned (gracile) face, a chin, and a large, rounded cranium. However, the researchers point out that the various African H.sapiens fossils show quite a lot of variation in these features.

Evolutionary Changes of Braincase Shape from an Elongated to a Globular Shape. The latter evolves within the H. sapiens lineage via an expansion of the cerebellum and bulging of the parietal. (Left) Micro-computerized tomography scan of Jebel Irhoud 1 (∼300ka, North Africa). (Right) Qafzeh 9 (∼95ka, the Levant). From Scerri et al., 2018).

While some palaeoanthropologists have placed the Jebel Irhoud remains, and others, in different species, Scerri’s team

view[s] H. sapiens as an evolving lineage with deep African roots, and therefore prefer[s] to recognize such fossils as part of the diversity shown by early members of the H. sapiens clade.

They note that this sort of variation within our species extended into the Holocene (~12 kya-present), suggesting that

Variation between populations in different regions and environments of Africa may have been shaped by isolation-by-distance and local environmental adaptations.

This would certainly support the observation that ‘modern’ characteristics appear to have developed in a mosaic fashion in the different sapiens (fossil) populations. The authors also note that H.naledi and H.heidelbergensis were contemporaneous with early sapiens, “raising the possibility of African archaic interbreeding.” Overall, their view is that our species is derived from an interlinked set of populations that fell in and out of contact with each other over time.

Cultural evidence

Here the authors present data about tool cultures – Fig.2 in the paper includes some stunning images of blades (and other tools) split off prepared stone cores. This technology, typical of the Middle Stone Age (MSA), is associated with sapiens fossils. It’s interesting that the authors say that the MSA kicked in at much the same time at sites across Africa (give or take 20,000 years or so), but that

Clear regionally distinctive material culture styles, typically involving complex stone tools, first emerged within the MSA.

North & South Africa both had “comparably distinctive [tool] industries”, some associated with other aspects of culture (beads, bone tools, & so on). Although, given the geographic separation between them and the differences in environment, surely it’s not unusual that the two regions showed such cultural distinctiveness? However, the research team go on to say that

the cultural record shows continued differentiation and derivation into the Holocene, supporting the biological evidence for variable population dynamics that did not result in wide-scale homogenization.

They also suggest new directions for archaeological research that might help advance our understanding of what was happening in Africa during the MSA.

Genetics

The starting point for most genetic studies of human origins has been to investigate the depth of present-day diversity between and within African populations.

These studies are often used to arrive at ‘split times’: the points in time when populations (or species) diverge from each other. They typically suggest a “deep African” origin for Homo sapiens. However, the authors’ summary of work in this area indicates that there’s quite a bit of variation in the conclusions of these studies, which have also been complicated by more recent gene flow. In addition, things like effective population size would change as geographically-dispersed populations fell in and out of contact with each other.   Scerri and her coauthors feel that their analysis

challenge[s] the view that the early prehistory of our species can be well approximated by population growth within a single lineage.

Instead, they state that

The genetic, fossil, and archaeological data discussed above indicate that H. sapiens evolved in highly structured populations, probably across many regions of Africa

and that, to clarify what was going on, there’s a need to look at environmental variability in Pleistocene Africa, both over time and in different regions. Their overview of large-scale ecological change in Africa during the Pleistocene is interesting, & would be useful for teachers and students to read (the paper is open-access, yay!).

In many ways the paper asks more questions that it answers, which highlights that our understanding of our relatively recent evolutionary past is far from complete.

 

Incidentally, I first came across this story when a New Scientist piece by the lead author was shared in a teacher group. Since I don’t have a subscription to NS I couldn’t read the whole thing, so looked for the original research paper, discussed here. But – when I went to share the link to the teacher page, FB rejected it! Apparently a link to a reputable journal triggered various filters. The ways of the FB algorithms are truly mysterious.

 

EML Scerri, MG Thomas, A Manica, P Gunz, JT Stock, C Stringer, M Grove, HS Groucutt, A Timmermann, GP Rightmire, F d’Errico, CA Tryon, NA Drake, AS Brooks, RW Dennell, R Durbin, BM Henn, J Lee-Thorp, P deMenocal, MD Petraglia, JC Thompson, A Scally, & L Chikhi (2018) Did Our Species Evolve in Subdivided Populations across Africa, and Why Does It Matter? Trends in Ecology & Evolution 38(8): 582-594

DOI:https://doi.org/10.1016/j.tree.2018.05.005

 

 

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