Sunday, 8 March 2015

On The Origins Of Us

While reconstructions vary, it is certain that Homo habilis was more
apelike than human in morphological terms. However, its cranial
capacity differentiates it from its ancestors
Ours is a unique species. We are bipedal and walk with a fully erect posture. We have the largest brain to body mass ratio. We are sentient and our minds are capable of solving complex puzzles both logical and practical.

Yet despite all our unique characteristics we can be linked to the rest of the animal kingdom without even stepping outside our own genus. Homo sapiens is unique, but his ties to his ancestors are reflected in the more primitive members of Homo.

Homo heidelbergensis was anatomically very similar to us, but with a reduced cranial capacity. Homo erectus was even more reduced and had a prominent brow ridge. Its body may have been covered in fur. Homo habilis and Homo gautengensis are increasingly ape-like.

Homo is a recent genus and so knowing when it evolved is important if we are to understand the transition from ape ancestor to hominids, with anatomies and mental capacities comparable to our own. Previously the oldest known fossil of Homo, specifically Homo gautengensis, was 1.9 million years old. Our models of how human evolution has progressed have been based upon this figure. Now a discovery from the Afar region of Ethiopia is set to push back this date by nearly a million years - a massive leap in terms of human evolution.

Discoveries in 2013 by a team of researchers led by scientists from the University of Arizona uncovered the left frontal portion of a jaw, complete with five teeth in the Ledi Geraru research area of Afar. While a jaw fragment may not sound like much, it contains a lot more information than one might expect. Teeth at the very least are diagnostic of the species they come from - particularly for mammals - and the shape of the jaw itself can provide information regarding the shape of the skull.

The analysis revealed advanced features, specifically slim molars, symmetrical premolars and an evenly proportioned jaw that distinguish early species on the Homo lineage, like Homo habilis or Homo gautengensis, from their apelike ancestors the australopithecines. The primitive, sloping chin, however, linked the Ledi-Geraru jaw to earlier hominids like Australopithecus.

The 2.8 million year old jaw fragment from Ledi Geraru - the oldest record thus
far of Homo and nearly a million years older than the previous record holders
'In spite of a lot of searching, fossils on the Homo lineage older than 2 million years ago are very rare,' said Brian Villmoare from the University of Nevada. 'To have a glimpse of the very earliest phase of our lineage's evolution is particularly exciting.'

Filling the gap between the australopithecines and the first members of the genus Homo, the Ledi Geraru jaw is a fantastic new find. Its age may even shed light on the factors which triggered the origins of our genus in the first place. 2.8 million years ago Africa underwent a sudden burst of aridity. Changes in vegetation and climate may have forced the australopithecines to adapt to new sources of food, changing their anatomy and resulting in the rise of Homo. That said, it is early days.

'We can see the 2.8 million year aridity signal in the Ledi-Geraru faunal community,' said research team co-leader Kaye Reed from the University of Arizona, 'but it's still too soon to say that this means climate change is responsible for the origin of Homo. We need a larger sample of hominin fossils, and that's why we continue to come to the Ledi-Geraru area to search.' We should keep our eyes trained on Ledi Geraru as it yields new discoveries about how we came to be a unique species.