Gaps in the fossil record have limited our understanding of how Homo sapiens evolved. The discovery in Morocco of the earliest known H. sapiens fossils might revise our ideas about human evolution in Africa. See Letters p.289 & p.293
Modern Homo sapiens share certain skeletal features that can also be recognized in fossil remains. These include a high, rounded braincase (the part of the skull that surrounds the brain), a small face tucked beneath it, and small, separated brow ridges (bone ridges above the eye socket). Our understanding of episodes in human evolution is mainly based on fossils and the available DNA. However, gaps in our knowledge remain about when and where H. sapiens evolved from ancestral humans within the genus Homo.

On page 289, Hublin et al.1 report the earliest known H. sapiens fossils, and present an analysis of the size and shape of these remains. Accompanying dating evidence is provided by Richter et al.2 on page 293. The fossils, excavated with associated stone tools, provide crucial information about early steps in the evolution of H. sapiens.
Fossil remains indicate that early modern H. sapiens were present in Africa from about 200,000 years ago3, and these individuals had an anatomy similar to that of humans today. However, DNA analyses of living people and fossils4 suggest that our lineage diverged from that of our close relatives, the Eurasian Neanderthals and Denisovans, more than 500,000 years ago — considerably earlier than the first recognizable early modern H. sapiens. This could imply that earlier members of the H. sapiens lineage existed that had features pre-dating the emergence of the full suite of modern skeletal traits, and that instead had a preponderance of archaic (primitive), rather than modern, features. Until now, it has been difficult to identify such fossils.
Human fossils5 (Fig. 1) were recovered from Jebel Irhoud, Morocco, in northwest Africa, in 1961 and 1962, alongside stone tools described as 'Mousterian', a name given to artefacts associated with Neanderthal sites. Given the popular view at the time that modern humans had evolved from Neanderthal ancestors (a now falsified idea), these fossils were called African Neanderthals. They were estimated to be about 40,000 years old6. Size and shape analyses of the fossils in the 1970s7 indicated that one skull had a facial structure that was quite distinct from that of Neanderthals and more closely resembled that of H. sapiens. However, because it was thought to be a comparatively young fossil, it was not considered as a potential ancestor of later H. sapiens7.
Figure 1: Skull-shape differences.
Figure 1
Structural differences in ancient skulls can illuminate evolutionary steps. Replica casts of the original skulls are shown. a, A skull found in Sima de los Huesos, Spain, that is around 430,000 years old12 is thought to represent an early form of Neanderthal. The Sima cranium exhibits some traits observed in more-recent Neanderthals, such as the characteristic Neanderthal brow-ridge shape, but also retains some more ancestral features not seen in later Neanderthals, such as a broader face and smaller average brain size. b, An approximately 60,000–40,000-year-old skull16 from La Ferrassie, France, is an example of a late Neanderthal. c, Hublin et al.1 and Richter et al.2 report approximately 350,000–280,000-year-old fossils from Jebel Irhoud in Morocco that could represent an early stage in Homo sapiens evolution. The facial shape of a Jebel Irhoud fossil previously discovered at the site5 shows similarities to the structure of more-modern humans, such as the presence of delicate cheekbones. However, the shape of the braincase (the section of the skull enclosing the brain) is archaic in form, and has an elongated shape that is less globular than the structure of more-modern H. sapiens. d, An approximately 20,000-year-old H. sapiens fossil16 from Abri Pataud, France, has a globular braincase. Scale bar, 5 cm. Image: NHM London (CC-BY)
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A child's jaw was found at the site in 1968, and analysis of the teeth indicated a modern-looking growth pattern8. This was significant because modern humans mature more slowly and over a longer period than was the case for archaic humans such as Homo erectus and Neanderthals. Furthermore, this specimen was dated to approximately 160,000 years ago8,9. Because more-modern-looking human fossils had been found in East African sites of similar age10, the view persisted that the Jebel Irhoud fossils were marginal in their location in Africa and peripheral to the origins of H. sapiens.
The Jebel Irhoud excavations (Fig. 2) reported by Hublin et al. and Richter et al. have uncovered additional stone tools and human fossils, including a partial skull and a lower jaw. Analysis of these findings, along with fossils recovered during the 1960s, has allowed at least five individuals to be identified. These fossils accumulated in a layer dated to about 350,000 to 280,000 years old by the authors, who tested flint artefacts and a human tooth. Thanks to improvements in dating techniques, particularly in luminescence dating, this layer, from which all the specimens had been excavated, is now revealed to be approximately twice as old as previously thought.
Figure 2: Jebel Irhoud, Morocco.
Figure 2
A view of the site studied by Hublin et al.1 and Richter et al.2. When the site was occupied by early humans, it would have been a cave, but the covering rock and much sediment were removed by work at the site in the 1960s. Image: Shannon McPherron, MPI EVA Leipzig, License: CC-BY-SA 2.0
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The tools the authors discovered are assigned to the Middle Stone Age (about 300,000–40,000 years ago), and were found with fauna showing evidence of human modification and charcoal, perhaps indicating controlled use of fire. Similar Middle Stone Age artefacts have been described in southern and eastern African sites, although those artefacts are consistently younger than the ones from Jebel Irhoud. Given the secure dates for the fossils and tools, the Jebel Irhoud site represents the earliest known association of H. sapiens and artefacts from the Middle Stone Age. The Sahara Desert in northern Africa is inhospitable today, but faunal evidence and modelling of ancient climates suggest that there were times when it could have been crossed11, possibly enabling humans and their technologies to migrate across the continent. Alternatively, Middle Stone Age technologies might have arisen independently in multiple locations in Africa.
Hublin et al. used a shape-analysis statistical technique to compare the excavated fossils with those of ancient human relatives dated to between 1.8 million and 150,000 years ago, modern H. sapiens fossils from the past 130,000 years and Neanderthals. Facially, Neanderthals and most of the other fossil humans were clearly distinguishable from the Jebel Irhoud specimens, which were most similar to modern H. sapiens. The lower-jaw fossil from Jebel Irhoud also showed the greatest shape similarity to the jaw of modern H. sapiens, although it is much larger. The Jebel Irhoud material showed some structural variation, however, particularly in brow-ridge size, which is possibly related to within-species sex differences.
The Jebel Irhoud braincases retained some archaic features, such as an elongated shape and low height when compared with the braincases of H. sapiens fossils from within the past 130,000 years. Their external braincase shape was intermediate between that of archaic and more-modern-looking fossils, but was most similar to the late archaic H. sapiens skull from Laetoli in Tanzania10 and the early modern H. sapiens skulls from Qafzeh in Israel10. Their internal braincase shape was distinctive. Perhaps it represents a structure near the beginning of the trajectory that led to the evolution of the globular brain shape characteristic of H. sapiens during the past 130,000 years1.
We agree with Hublin and colleagues that the Jebel Irhoud fossils now represent the best-dated evidence of an early 'pre-modern' phase in H. sapiens evolution. These specimens probably constitute an early representative of the H. sapiens lineage that could illuminate the evolution of our species in a way equivalent to how the early Neanderthal Sima de los Huesos fossils12 from Atapuerca in Spain have provided insight into the development of Neanderthals.
The authors suggest that the Jebel Irhoud fossils could aid our understanding of H. sapiens evolution across the whole of Africa. The facial shape of two skulls looks like a larger version of that found in H. sapiens today, and Hublin and colleagues make comparisons with the approximately 260,000-year-old Florisbad fossil from South Africa10, often assigned to early H. sapiens. However, it seems increasingly probable that the delicate face of modern humans is inherited from non-sapiens ancestors in our family tree10. If so, such similarities between the Irhoud and Florisbad fossils could be parallel retentions of primitive ancestral features, rather than indications of kinship across Africa.


We currently lack data about human connections around and across the Sahara at this time, and it is not known how isolated the Jebel Irhoud population would have been. Moreover, similarities between the Jebel Irhoud material and fossils13,14 from Zuttiyeh and Tabun in Israel are a reminder that corridors on the African periphery 300,000 years ago might have periodically linked northern Africa and western Asia.
Hublin and colleagues suggest that clear-cut boundaries in H. sapiens evolution, such as the descriptions of fossils as 'archaic' or 'anatomically modern', are likely to fade as the fossil record improves. They are probably right, although their evidence adds to the picture of an extended temporal overlap of archaic and more-modern-looking forms across the continent that includes the dating of the primitive species Homo naledi in South Africa to about 300,000 years ago, as reported15 last month. Perhaps additional dating studies will clarify the extent of the overlap and the processes that might have led to the evolution of modern humans. The authors propose that the globular brain shape of present-day humans could have evolved comparatively recently, making this a potential defining characteristic of human modernity. Given the likelihood that both brain size and shape evolved independently and in parallel along the Neanderthal and H. sapiens lineages over a period of at least 400,000 years, this might also imply that cognitive differences could have developed between the two species during that time.

Notes

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Change history

  • 14 June 2017

    The fossil samples from which the authors deduced their results have been clarified.

References

  1. 1.
    et al. Nature 546, 289–292 (2017).