A partial lower jawbone and a number of teeth have been recovered from a surface outcrop of fossil-bearing sedimentary rock in the Ledi-Geraru research area, in the Afar region of Ethiopia. This region has long been associated with the fossils of early hominins. The jawbone has been assigned to Homo (species indeterminate) (Villmoare, et al., 2015). The age of the jawbone is constrained by stratigraphic and palaeomagnetic considerations to between 2.80 and 2.75 million years old (DiMaggio, et al., 2015). This means that LD 350-1 is at least 400,000 years older than the earliest previously-known fossil assigned to Homo. The findings are published as two articles in the journal Science.
What was our previous understanding of human origins?
The conventional view is that the first human species was Homo habilis (‘Handy man’). Discovered in 1960 and announced as a new species four years later, it is believed to have evolved from an australopithecine ancestor though which is disputed. Possibilities include the South African Australopithecus africanusand Australopithecus afarensis from East Africa (or its probable descendant species Australopithecus garhi). The famous ‘Lucy’ belongs to Australopithecus afarensis. In comparison to a modern human, Homo habilis was small brained and its limb proportions (short legs, long arms) were still very apelike. However, the skull was less massively-built than an australopithecine; the upper and lower jawbones were within the human size range; and the feet and thumb joints were humanlike (Conroy, 1997). Homo habilis is known in the fossil record from 2.33 to 1.44 million years ago (Kimbel, et al., 1997; Spoor, et al., 2007). It is then presumed to have given rise to Homo erectus. Although still small-brained in comparison to a modern human, the limb proportions of Homo erectus are similar to those of later humans. Homo erectus first appears in the fossil record 1.8 to 1.9 million years ago (Wood, 2011).
This conventional view has a number of problems. Firstly, a second species, Homo rudolfensis is now known to have been contemporary with Homo habilis. First proposed in 1972, it was confirmed as a separate species in 2012 (Leakey, et al., 2012). How Homo rudolfensis fits into the bigger picture is far from clear: some have noted similarities to an earlier hominin, Kenyanthropus platyops (‘Flat-faced man of Kenya’) and it is possible that Homo rudolfensis belongs in Kenyanthropus rather than Homo.
Another problem is that the earliest example of Homo habilis, a 2.33 million year old upper jawbone known as AL 666-1 from Hadar, Ethiopia might in fact be something other than Homo habilis. The oldest uncontested example of Homo habilis is only 1.9 million years old (Lieberman, 2007) and given that the species also persisted well after the appearance of Homo erectus an ancestor/descendant relationship seems unlikely. Instead, it has been suggested that the two species shared a common ancestor (Spoor, et al., 2007).
Finally, it has been suggested that the late australopithecine species Australopithecus sediba from South Africa, which dates to around 2 million years old (Pickering, et al., 2011) is a more plausible ancestor for Homo erectus than is Homo habilis (Berger, et al., 2010).
Could LD 530-1 be an australopithecine?
The date of 2.8 million years ago puts it just after the time of Australopithecus afarensis (3.9 to 3.0 million years ago) and before the late australopithecine species Australopithecus garhi (2.5 million years ago).Given that the Australopithecus garhi is thought to descendant of Australopithecus afarensis, LD 350-1 is in the right place at the right time to be a part of that lineage. In terms of size, both the jawbone and the teeth are within the Australopithecus afarensis range, albeit towards the lower end. However, most other respects, the mandibular and dental characteristics of LD 350-1 fall outside the range for Australopithecus afarensis. The dentition is also reduced (and therefore more humanlike) in comparison to Australopithecus garhi, which would appear to bump the latter from the lineage leading to Homo. Overall, LD 350-1 appears to be transitional between Australopithecus and Homo and is likely to represent the earliest-known example of the latter.
What are the implications if LD 350-1 is indeed Homo?
Models that posit an australopithecine ancestor for Homo from the period 2.5 to 2.0 million years ago (e.g. Australopithecus garhi or Australopithecus sediba) would be ruled out. Instead, Homo diverged from Australopithecus much earlier than hitherto believed. Notably, 2.8 million years ago coincides with a shift to a more arid climate in Africa, suggesting a link between climate change and the emergence of Homo. The fossil record of Ledi-Geraru records a shift to a more open habitat of grasses or low shrubs at around this time.
Reboot for Homo habilis
In a separate study, published in the journal Nature, Fred Spoor and colleagues (Spoor, et al., 2015) carried out a reconstruction of the 1.8 million year old Homo habilis holotype specimen OH 7. The results suggest that the species was larger-brained than previously believed, within the range of Homo erectus. It was also found that the dentition of OH 7 is more primitive than the 2.33 million year old AL 666-1, suggesting that the latter cannot be Homo habilis – but implying that the origins of Homo habilis go back even further. The study did not consider the affinities of AL 666-1 any further but speculated that it could be early Homo erectus. By 2.33 million years ago, the Homo lineage was already apparently diverse, with early human species distinguished from one another more by gnathic morphology than by brain size. The reporting of the early Homo jawbone LD 350-1 dovetails neatly with this study.
What species is LD 350-1?
It is more primitive than Homo habilis but nevertheless lies within Homo. Its describers did not assign a species to it, but it is likely that it will be eventually recognised as a new species within Homo; the earliest human species yet.
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3. DiMaggio, E. et al., 2015. Late Pliocene fossiliferous sedimentary record and the environmental context of early Homo from Afar, Ethiopia. Science, 5 March.
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