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Elusive cranium of early hominin found

A 3.8-million-year-old hominin fossil reveals what the cranium of the oldest known Australopithecus species looked like, casting doubt on assumptions about how these ancient relatives of humans evolved.
Fred Spoor is at the Centre for Human Evolution Research, Natural History Museum, London SW7 5BD, UK, in the Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany, and in the Department of Anthropology, University College London, UK.

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Human evolution often captures the imagination, not least because some of our extinct hominin relatives are personified by well-preserved fossils with catchy nicknames. For example, a partial skeleton of Australopithecus afarensis is named Lucy, and an Australopithecus africanus cranium (the skull without its lower jaw) is called Mrs Ples. However, the oldest known species that is unambiguously part of the human evolutionary tree1, Australopithecus anamensis, has mainly languished away from the limelight because of its small and not particularly glamorous fossil record. Until now, A. anamensis was known only from partial upper and lower jaw bones, isolated teeth, a small part of the braincase and a few limb bones. These specimens were found in Kenya and Ethiopia and are between 4.2 million and 3.9 million years old2.

Writing in Nature, Haile-Selassie et al.3 and Saylor et al.4 report the discovery of a mostly complete 3.8-million-year-old cranium found in the Woranso-Mille area of Ethiopia. The fossil is of an adult, probably male, and was identified as A. anamensis mainly on the basis of the characteristics of its jaw and canine teeth. This cranium looks set to become another celebrated icon of human evolution.

A complete skull is not essential for a good understanding of the morphology of an extinct species. For example, A. afarensis had already been well documented from a large collection of fragmentary remains when the first skull from an adult of this species was found5. However, the newly discovered cranium of A. anamensis, casually named MRD after its collection number, MRD-VP-1/1, provides a wealth of information about A. anamensis by revealing for the first time what its full face and braincase looked like (Fig. 1).

Figure 1 | The skull of the hominin species Australopithecus anamensis. Haile-Selassie et al.3 and Saylor et al.4 report the discovery and dating of a 3.8-million-year-old cranium (grey) of A. anamensis, found in Ethiopia (collection number MRD-VP-1/1). Surface reconstructions (yellow) of fossils representing the only previously known parts of the skull region of this species are superimposed on the cranium. These 4.2-million-year-old fossils from Kenya2 are an upper jaw bone (collection number KNM-KP29283) and a lower jaw and ear bone (KNM-KP29281). The projecting cheekbones (blue arrow) create an apparent facial similarity to a 2.5-million-year-old specimen of the hominin Paranthropus aethiopicus8; however, the authors propose that this feature evolved independently in Paranthropus and A. anamensis. Both the Kenyan and Ethiopian fossils of A. anamensis are characterized, among other features, by protruding jaws (red arrow) and a small earhole (black arrow). Scale bar, 1 centimetre. The Kenyan fossils are presented at 97% of their original size to match the cranium size.

MRD offers insight into the shape of hominin skulls at an early stage of the better understood part of human evolution, from about 4.2 million years ago to the present. The new information will help scientists to determine which skull features are primitive (ancestral) and which are derived (evolved — that is, different from the corresponding feature in an ancestor); this, in turn, will affect inferences about the evolutionary relationships between species. The discovery will also trigger a re-evaluation of the sparse hominin fossil record from before 4.2 million years ago. Whether previously discovered fossils assigned to species of Ardipithecus, Orrorin and Sahelanthropus are all indeed part of the human evolutionary tree or are extinct apes is controversial1,6. MRD provides information that will advance this debate.

By comparing A. anamensis with other species, and including their new evidence, the authors generated evolutionary family trees in which A. anamensis was consistently placed as the most ancestral of all Australopithecus species and later hominins. This result confirms previous findings6, and reflects the fact that the cranium shows predominantly primitive features — including some in parts never documented before in A. anamensis fossils. MRD has a distinctly protruding face (Fig. 1) and a notably long and narrow braincase. The latter feature is remarkably similar in this respect to that of the 7-million-year-old cranium of Sahelanthropus7, and these two species both had a small brain. The new fossil has several features that are assumed by the authors to be derived rather than primitive. Most striking is the forward projection of the cheek bones, which creates a facial appearance reminiscent of much younger Paranthropus hominin species, particularly the 2.5-million-year-old Paranthropus aethiopicus8. The authors conclude that this facial characteristic evolved independently in A. anamensis and later species, but the resemblance might inspire alternative interpretations.

On the basis of previous comparisons in which only information about jaws and teeth was available for A. anamensis, it has been widely accepted that A. anamensis and A. afarensis were successively part of a single evolving lineage through time, and were represented in the fossil record, respectively, from 4.2 million to 3.9 million years ago, and from 3.8 million to 3.0 million years ago2,9. Thus, it has been argued that A. anamensis and A. afarensis should be considered a single evolutionary species9.

The MRD cranium now increases the number of A. anamensis features that can be compared with those of the other species to explore this issue further, and the authors present evidence that is not consistent with the two species being part of a single evolving lineage. First, they identify a number of features that are derived in A. anamensis but are primitive in A. afarensis. Second, with the shape of MRD as a basis, the authors conclude that a 3.9-million-year-old frontal bone (part of the forehead) from Ethiopia represents A. afarensis rather than A. anamensis. This attribution, along with the discovery of the 3.8-million-year-old MRD cranium of A. anamensis (dating evidence reported by Saylor et al.), provides a revised timeframe indicating that A. anamensis existed from at least 4.2 million to 3.8 million years ago, and A. afarensis from at least 3.9 million to 3.0 million years ago — so the temporal overlap between the two species was at least 100,000 years.

The model of a single, evolving lineage is certainly challenged by this new evidence, but more aspects will need to be considered. The isolated frontal bone attributed to A. afarensis might instead belong to Kenyanthropus platyops or Australopithecus deyiremeda, other broadly contemporary hominin species from eastern Africa10. Moreover, little is known about the face of early A. afarensis2,9, and in particular, whether it showed more similarities to the face of the MRD cranium than does the face of later A. afarensis.

One way in which Haile-Selassie and colleagues’ analysis of the fossil specimen stands out is in their use of wide-ranging digital reconstruction that corrects distortions of the fossil’s shape, and estimates missing parts. These digital methods are readily available and offer unique opportunities for research. However, many more shapes can be morphed and matched this way than would be possible with conventional methods, and care is needed to generate only the most realistic options. It is therefore essential that any digital reconstruction is carried out with detailed, first-hand knowledge of the original fossil, including how it is preserved and distorted.

This point is particularly relevant with respect to the forward-projecting cheekbones of the newly discovered fossil. After reconstruction, this area looks smoothed, with hardly any sign of the original bone surface. One prominent aspect of MRD where the reconstruction could be improved is the front of the upper jaw. Here, digital processing resulted in a less accurate idea of what the characteristic, strongly projecting subnasal area would have looked like before the fossil was broken.

MRD is a great addition to the fossil record of human evolution. Its discovery will substantially affect our thinking on the origin of the genus Australopithecus specifically, and on the evolutionary family tree of early hominins more broadly. This work demonstrates the importance that a single fossil can have in palaeontology, something we should remember when we get puzzled looks and sighs from our colleagues in the experimental biosciences regarding excitement about a sample size of n = 1.

Nature 573, 200-202 (2019)


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