Earliest Evidence of Genus Homo

Home

 
March 13, 2015

An American team working in the Afar region of Ethiopia has found the earliest evidence of our genus Homo, it was announced last week in the journal Science.

The find has been designated Ledi Geraru 350-1, Ledi Geraru being a part of the Afar near Hadar, where Lucy and so many other examples of Australopithecus afarensis have been found. This find is significant not only for its age but also for what it has to say about the evolutionary transition from Australopithecus to Homo. Dated reliably to between 2.75 and 2.8 million years ago, LD 350-1 is the earliest known member of the genus Homo.

Below, Donald Johanson provides a detailed technical analysis of this new find from Ethiopia and the extent to which it relates to specimens both younger and older, attributed variously to the genus Australopithecus or early Homo. Dr Johanson discovered Lucy in 1974 and numerous other fossils in the genera Australopithecus and Homo and is particularly well placed to comment.

Dr Johanson's comments:
It has now been nearly thirty-four years since the announcement of Lucy and the other Hadar fossil hominids as a new species, Australopithecus afarensis.  Like many developments in paleoanthropology this was controversial and attracted considerable debate.  Following a period of adjustment A. afarensis eventually joined other hominin species on the family tree as a bone fide taxon.  

Recognition of A. afarensis dictated a revision of hominin phylogeny. A. africanus, from South Africa was generally viewed as the ancestor to later hominins including the human genus, Homo.  I considered this species too derived in the direction of the robust australopiths and removed A. africanus from the ancestry to Homo.

A. afarensis is a more generalized species that was widespread over East Africa from Tanzania to Ethiopia, and perhaps as far west as Chad.  It  lived in a variety of habitats between 3 and 4 mya.  I postulated that A. afarensis was the common ancestor to both Homo and more recent species of Australopithecus.  This revised phylogeny was highly controversial.  After all, in East Africa Homo habilis and A. boisei fossils from Olduvai Gorge lived 1.4 million years after Lucy.  This was arguably a long stretch of time in which to draw evolutionary relationships. Then in 1985 a cranium dubbed the Black Skull was found west of Lake Turkana.   Sporting a blade-like sagittal crest, and carrying enormous molars (estimated from tooth roots) this was a robust and categorized as  Australopithecus aethiopicus.  Following a detailed analysis of the anatomical features of the Black Skull it was concluded that the crest and teeth linked it with A. boisei, but a host of features such as a projecting snout, large anterior teeth and so on connected it to A. afarensis.

Hence the Black Skull became viewed as an evolutionary bridge between Lucy’s species and A. boisei, and bolstered the proposition that A. afarensis was an ancestor to later Australopithecus.  Dated at 2.5 million years the Black Skull was situated approximately halfway between the latest occurrences of A. afarensis and the oldest appearance of A. boisei.

To test the other half of my recommendation that A. afarensis was also a precursor to Homo required the recovery of fossils in the two to three million year time period attributable to Homo.  This was not an easy task because fossils in general were relatively rare in this span of time.

Fortunately in 1994 during a strategic survey within the Hadar Formation a well-preserved palate including partial dentition was recovered from 2.35 million year old deposits.  A.L. 666-1 manifested classic Homo anatomy in its arch shape, dimensions and deeply arched palate.  It was not possible to assign it to a species, nor did it have affinities with A. afarensis materials, but it did narrow the gap between A. afarensis and Homo to roughly 600,000 years.

The recent announcement of a partial mandible from Ledi-Geraru, located some 21 miles northeast of where Lucy was recovered, sheds considerable light on the A. afarensis - Homo transition.  I was generously given the opportunity of examining the specimen at the National Museum of Ethiopia.  My initial impression was that it did not look like any A. afarensis jaw and I proffered the view that it should be assigned to the genus Homo.

Now after an exhaustive study, published on March 4 in the journal Science, the Homo status of the mandible has been confirmed.  The mandible possesses diagnostic Homo features not seen in A. afarensis including long, narrow molars, a rounded dental arch, and symmetrical premolars, just to mention a few. And, most importantly the LD 350-1 Ledi-Geraru mandible is linked to A. afarensis because of the presence of a receding chin.

Dated at 2.8 mya the LD 350-1 specimen nicely reduces the gap between A. afarensis and early Homo to a mere 200,000 years.  The antiquity of the mandible and its intermediate anatomy adds credence to the view that A. afarensis was indeed ancestral to Homo.

At the same time LD 350-1 was published, a computer driven reconstruction of the type specimen for Homo habilis (OH 7) from Olduvai Gorge was announced in the journal http://www.nature.com/nature/journal/v519/n7541/full/nature14224.html?WT...) " target="_blank">Nature andgenerated two unexpected results. First, a new estimate of brain size increased the cranial capacity from 680cc, to 800cc, firmly within the acceptable Homo range.    Second, the reconstructed mandible, is unexpectedly more primitive than anticipated. The more ape-like projecting face and parallel tooth rows, confirm affinities with A. afarensis.  

Adding further complication is the fact that two maxillas, A.L. 666-1 and OH 65 (same antiquity as OH 7) have rounded dental arches and occlude comfortably with the LD 350-1 mandible, but not the OH 7 mandible.  Admittedly a small sample, but it appears that there are two species of Homo at Olduvai, one H. habilis (OH 7) and Homo sp. (OH 65).

Assuming that A. afarensis is now an even stronger candidate for ancestry to Homo there are a number of phylogenetic implications.  An unlikely series of evolutionary events would be primitive (i.e. A. afarensis) evolving into more advanced (i.e. LD 350-1) to primitive (OH 7).  Evolution just doesn’t work that way. So it is more likely that A. afarensis gave rise to two Homo evolutionary lineages:  one first heralded by LD 350-1 and evolving into A.L. 666-1 and OH 65 and the other retaining more primitive mandible features but undergoing brain enlargement and evolving into Homo habilis (OH 7)

Conclusions:

1. LD 350-1 is an early species of Homo at 2.8 mya

2. the evolutionary history of Homo is more complex with at least two Homo lineages descended from A. afarensis.  

3. Australopithecus afarensis is the last common ancestor to more recent species of Australopithecus and the genus Homo.

4. Evidence for early Homo in eastern Africa significantly predates its appearance in South Africa.

5. H. habilis is more primitive than more ancient Homo specimens represented by A.L. 666-1 and LD 350-1 or its contemporary, OH 65.   H. habilis as defined by OH 7 may not be an ancestor to modern humans.  

6. A new species name has not been assigned to the LD 350-1 hemi-mandible, but based on its distinguishing anatomy is certainly deserves to be placed in a new species.