One of the great issues in all of science is the search for the direct ancestor of modern humans. Many species of hominin have been put forward down the years and a recently uncovered speciesAustralopithecus sedibahas been claimed as the progenitor of all humans living today. However, tests have shown that this hypothesis is highly unlikely and that the search for our direct ancestor must continue.
Scientists have largely agreed thathomininsknown asAustralopithecus,are our most likelyancestors.Australopithecus afarensiswas found in the 1970s, and the first finds of this hominin are from the famous ‘Lucy’. However, in 2008 experts discovered evidence of another member of the Australopithecus family who was a very distant relative of the ape-likeLucy.
Australopithecus sediba, two fossils of which are shown on the left and right, are thought to have been a transitional species between older Australopithecus, like Lucy in the middle, and later Homo species. Image compiled by Peter Schmid courtesy of Lee R. Berger. (CC BY-SA 3.0)
Discovery of a new species
The 9-year-old son of a well-known paleoanthropologist Thomas Berger unearthed the jawbone and other bone fragments of a hitherto unknown species in Mapala Cave in South Africa. This caused an international sensation and immediately Berger and his colleagues claimed that thenew homininwhich they namedAustralopithecus sediba,after a local word for source,was the direct ancestor of anatomically modern humans.
The find was determined to be two million years old, this and its unique set of ape-like and advanced features gave some credence to the claims. According to a blog inDiscover Magazinethe assertion also reinvigorated ‘an old idea that the roots of human evolution were in South Africa’.
However, not everyone was convinced in the academic community, ‘primarily because the numbers didn’t seem to add up’ reportsCosmos Magazine. According to the study inScience Advances, theAustralopithecus sedibafossils ‘postdates the earliest Homo by 800,000 years’. This meant that the dating ofAustralopithecus sedibawas all wrong if it was to be our progenitor.The speciesfound first by Berger’s son was considerably younger than the earliest evidence we have of theHomogenus.
Matthew Berger moments after the discovery of the clavicle of Australopithecus sediba at the Malapa site. Photo by Lee R. Berger (CCBY-SA 3.0)
WasAustralopithecus sedibaour ancestor?
The discovery of thenew species in South Africaresulted in a great deal of debate, especially as many other discoveries ofAustralopithecus sedibawere made at Mapala Cave. There are many studies published on the hominin fossils found in Mapala Cave, but according toDiscover Magazine, none came to definite ‘conclusions but leaving the door open on its potential as our distant ancestor’.
The majority of researchers did not accept that humans are descended from the species found in South Africa because the hominin remains are younger than the oldest fossils of theHomogenus. To many, it did not make sense to claim that remains that were younger than the earliest knownHomocould be from its ancestor.
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Highly unlikely to be our ancestor
The controversy led to a team from the University of Chicago, developing a test to determine, once and for all if the evidence did show thatAustralopithecus sedibawas the ultimate origin of modern humans. Under lead researcher Andrew Du Ph.D., they developed statistical models that calculated the probability that this genus ofAustralopithecuswas the ancestor of subsequent humans. After testing a number of hypotheses, they established that Berger’s theory onhuman originswas highly unlikely.
According toScience Advances, the University of Chicago team found that an ‘ancestor’s fossil horizon that is at least 800,000 years younger than the descendant’s fossil horizon is unlikely (about 0.09% on average)’. This meant that the fossils were simply not old enough to come from our progenitor, as many had suspected.
However, this is not going to settle the controversy. Even the lead author of the study is quoted byInverseas ‘stating that it is ‘difficult for science to conclusively settle a debate’.
On balance, Lucy is top contender
Science Dailyquotes Zeresenay Alemseged, PhD, the Donald M. Pritzker Professor of Organismal and Biology and Anatomy at University of Chicago and co-author of the study as stating:
“Given the timing, geography and morphology, these three pieces of evidence make us think afarensisis a better candidate than sediba. One can disagree about morphology and the different features of a fossil, but the level of confidence we can put in the mathematical and statistical analyses of the chronological data in this paper makes our argument a very strong one.”
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The Hidden Treasures of Ethiopia’ exhibit at Houston Museum of Natural Science featuring a model of “Lucy”. (Jason Kuffer/CC BY NC ND 2.0)
In recent years there have been a great many other discoveries that would seem to reject the idea that the fossils found in Mapala Cave are from our ancestor. These include the discovery ofHomo Nadeliwhich was also discovered by a team led by Berger. Then there is the discovery in north-eastern, Ethiopia of a fossil that is almost 3 million years ago and this is held by some to be the oldest remains from a member of theHomogenus.
The American university’s statistical study shows thatAustralopithecus sediba, probably lived after the emergence of our earliest ancestors, by up to a million years. This and recent discoveries especially inEthiopiaalmost rule out the possibility that the hominin species found inSouth Africain 2008 is our ancestor. This means thatAustralopithecusafarensis, or ‘Lucy’ is the most likely candidate to be our progenitor, but even this is not definite.
Top image:Skull of Malapa hominid 1 (MH1) from South Africa, named “Karabo”. The combined fossil remains of this juvenile male is designated as the holotype for Australopithecus sediba. Source:CC BY-SA 4.0