Australopithecus sediba

Australopithecus sediba Temporal range: 1.977–1.98 Ma PreЄ Є O S D C P T J K Pg N ↓ Pleistocene
In situ cranium of "Karabo"[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Genus:	†Australopithecus
Species:	†A. sediba
Binomial name
†Australopithecus sediba Berger et al., 2010[2]

Australopithecus sediba is a species of Australopithecus of the early Pleistocene, identified based on fossil remains dated to about 2 million years ago. The species is known from at least four partial skeletons discovered in the Malapa Fossil Site at the Cradle of Humankind World Heritage Site in South Africa, one a juvenile male (MH1, the holotype), an adult female (MH2), at least one other adult and an 18-month-old infant.^[2][3] The MH1 and MH2 fossils were buried together, and have been dated to between 1.977 and 1.980 million years ago.^[4][5]

Over two hundred and twenty fragments from the species have been recovered to date.^[2] The partial skeletons were initially described in two papers in the journal Science by American and South African palaeo-anthropologist Lee R. Berger and colleagues as a newly discovered species of early human ancestor called Australopithecus sediba ("sediba" meaning "natural spring" or "well" in the Sotho language).^[2]

Discovery

The first specimen of A. sediba was found by paleoanthropologist Lee Berger's nine-year-old son, Matthew, on August 15, 2008.^[6] While exploring near his father's dig site in the dolomitic hills north of Johannesburg, on the Malapa Nature Reserve, Matthew stumbled upon a fossilized bone.^[6] The boy alerted his father of the find, who could not believe what he saw, a hominid clavicle and upon turning the block over; "sticking out of the back of the rock was a mandible with a tooth, a canine, sticking out. And I almost died", he later recalled.^[6] The fossil turned out to belong to a 4 ft 2 in (1.27 m) juvenile male, the skull of which was discovered in March 2009 by Berger's team.^[6] The find was announced to the public on April 8, 2010.

Also found at the Malapa archeological site were a variety of animal fossils, including saber-toothed cats, mongooses, and antelopes.^[6] Berger and geologist Paul Dirks speculated that the animals might have fallen into a deep, 100–150-foot (30–46 m) "death-trap", perhaps lured by the scent of water.^[6] The bodies may have then been swept into a pool of water rich with lime, and with sand at the bottom, making it possible for the remains to become fossilized.^[6]

Age estimates

The fossil was dated using a combination of palaeomagnetism and uranium-lead (U-Pb) dating by Andy Herries (La Trobe University, Australia), Robyn Pickering (U. Melbourne, Australia) and Jan Kramers (U. Johannesburg). U-Pb dating of the underlying flowstone indicates that the fossils are not older than ~2.0 Ma. The occurrence of species of animal that became extinct at ~1.5 Ma indicate the deposit is not younger than 1.5 Ma. The sediments have a 'normal' magnetic polarity and the only major period between 2.0 and 1.5 Ma when this occurred is the Olduvai sub-Chron between 1.95 and 1.78 Ma.^[5] As such, the fossils were originally dated to ~1.95 Ma. Recent dating of a capping flowstone illustrated this was not possible and the normal magnetic polarity sediments have since been correlated to the 3000 year long Pre-Olduvai event at ~1.977 Ma.

Morphology and interpretations

Matthew Berger displays the fossil he found on the Malapa Nature Reserve

Because of the wide range of mosaic features exhibited in both cranial and post cranial morphology, the authors suggest that A. sediba may be a transitional species between the southern African A. africanus (the Taung Child, Mrs. Ples) and either Homo habilis or even the later H. erectus (Turkana boy, Java man, Peking man).^[2] The cranial capacity of MH1, which has been estimated to be at 95% of adult capacity (420 ml/cc), is at the higher end of the range for A. africanus and far from the lower range of early Homo (631 ml/cc), but the mandible and tooth size are quite gracile and similar to what one would expect to find in H. erectus, so similar that if found in isolation without other skeletal remains could be classified as Homo based on tooth and mandible size. However, the cusp spacing is more like Australopithecus. Another interesting example of the wide range of interspecific variation of this species is that although MH2 is an adult, the tooth size of MH1, the juvenile, is larger than that of the adult. At the same time in Africa were the Homo ergaster (or early H. erectus) with a more human morphology and a larger cranial capacity (700-900cm³ ).

Regardless of whether Australopithecus sediba is an ancestor of early Homo or not, our understanding of the range of variation in early hominins has been greatly increased with the finding of these new specimens.

MH1 (left), Lucy (centre) and MH2 (right). Fossil remains described to date superimposed on a generalized australopithecine background. Australopithecus sediba skeletons approximatly 1.3 meters tall.

A. sediba compared to its ancestor species A. africanus on the whole is described by Berger et al. as more derived towards Homo than A. garhi, especially showing a number of synapomorphies taken to anticipate the reorganization of the pelvis in H. erectus, associated with "more energetically efficient walking and running". The femur and tibia are fragmentary and the foot is more primitive.^[2] Its cranial capacity is estimated at around 420–450 cm³ (26–27 cu in),^[2] about one-third that of modern humans.

A. sediba had a surprisingly modern hand, whose precision grip suggests it might have been another tool-making Australopithecus ^[7].

As opposed to the authors of the initial description, who interpreted both fossils as a possible transitional species between Australopithecus and Homo, other palaeoanthropologists are reluctant to do so. In an accompanying news article published with the initial descriptions in 2010, detractors of the idea that "Au. sediba" might be ancestral to the genus "Homo" (e.g. Tim White and Ron Clarke) suggest that the fossils could be a late southern African branch of Australopithecus, co-existing with already existing members of the Homo genus.^[8] This interpretation is based on the observation that the lower jaw, discovered by Friedemann Schrenk, of a 2.5 million year old fossil attributed to H. rudolfensis is the oldest known fossil ascribed to the Homo genus. This specimen is thus presumed to be older than the Australopithecus sediba fossils. The critics, however continue to ascribe to A. africanus the status of precursor of the Homo genus. Criticism has been raised in a news report on the find in Nature magazine that the authors of the initial description have failed to take the wealth of variation within A. africanus into account, prior to defining the finds as an independent species.^[9] Additionally, the basing of the description of the species largely on the skeleton of a juvenile specimen has been subject to criticism, given that there is no certain way of saying to what extent adults would differ from juvenile specimens. This has been a common practice in palaeoanthropology, with even the type specimens of Homo habilis and Australopithecus africanus being juvenile specimens.

See also

• Human evolution
• List of fossil sites (with link directory)
• List of human evolution fossils

References

1. ^ Juliet King (June 4, 2010). "Australopithecus sediba fossil named by 17-year-old Johannesburg student". Origins Centre. http://www.origins.org.za/news/entry/australopithecus_sediba_named_by_17-year-old_johannesburg_student/. Retrieved July 9, 2011. 
2. ^ ^{a b c d e f g} Berger, L. R.; de Ruiter, D. J.; Churchill, S. E.; Schmid, P.; Carlson, K. J.; Dirks, P. H. G. M.; Kibii, J. M. (2010). "Australopithecus sediba: a new species of Homo-like australopith from South Africa". Science 328 (5975): 195–204. doi:10.1126/science.1184944. PMID 20378811. 
3. ^ Ann Gibbons (2011). "A new ancestor for Homo?". Science 332 (6029): 534. doi:10.1126/science.332.6029.534-a. PMID 21527693. 
4. ^ African fossils put new spin on human origins story - BBC News - Jonathan Amos - Retrieved 9 September 2011.
5. ^ ^{a b} Dirks, P. H. G. M.; Kibii, J. M.; Kuhn, B. F.; Steininger, C.; Churchill, S. E.; Kramers, J. D.; Pickering, R.; Farber, D. L. et al. (2010). "Geological setting and age of Australopithecus sediba from Southern Africa". Science 328 (5975): 205–208. doi:10.1126/science.1184950. PMID 20378812. 
6. ^ ^{a b c d e f g} Celia W. Dugger; John Noble Wilford (April 8, 2010). "New hominid species discovered in South Africa". New York Times. http://www.nytimes.com/2010/04/09/science/09fossil.html?src=mv. Retrieved April 8, 2010. 
7. ^ Part ape, part human - National Geographic - Retrieved 10 September 2011.
8. ^ Michael Balter (2010). "Candidate human ancestor from South Africa sparks praise and debate". Science 328 (5975): 154–155. doi:10.1126/science.328.5975.154. PMID 20378782. 
9. ^ Michael Cherry (April 8, 2010). "Claim over 'human ancestor' sparks furore". Nature. doi:10.1038/news.2010.171. http://www.nature.com/news/2010/100408/full/news.2010.171.html. 

External links

• Australopithecus sediba at the Fossil Wiki
• Pictures of the find at National Geographic
• Skeleton Analysis at Science
• Skulls
• Video presentation explaining the find
• Free Images and resources about "Australopithecus sediba" for Educators and Students
• Guardian Science Notes