Australopithecus sediba

The latest new hominin species to be announced has scientists hotly debating its validity as a species and its relationships to other hominins, in particular its relationship to our genus Homo.

Background of discovery

Age

This fossils of this species date to 1.95-1.78 million years ago. This does not represent the timespan for this species, merely a point in time for a limited number of fossils.

Important fossil discoveries

The first specimen was a right clavicle (collarbone) discovered by Matthew Berger, the 9-year-old son of palaeontologist Lee Berger, in Malapa, South Africa in August 2008.

Subsequent excavations in the cave deposits uncovered two partial skeletons. These were found close together and it is likely that they died about the same time and were entombed in sediment before their remains had fully decomposed.

On the basis of a combination of primitive and derived characteristics of the skull and postcranial, the discoverers announced it as a new species in the journal Science in April 2010. More fossils are in the process of being excavated.

Key specimens:

• Malapa Hominin 1 (MH1): this is the type specimen or holotype. It is considered to be a juvenile and is represented by a partial cranium (UW 88-50), partial lower jaw (UW88-8) and postcranial elements including a right clavicle (UW 88-1). The second molars are erupted and it is considered to have reached about 95% of adult brain size.
• MH2: probable female adult represented by partial lower jaw (UW 88-54), isolated teeth from the lower jaw (UW 88-54) and some partial postcranial elements including most of the right arm, ankle and knee joints and bits of the pelvis and shoulder blade. This is the species paratype (a specimen other than a type specimen that is used for the original description of a taxonomic group)

Distribution

All known specimens have been found at Malapa in South Africa. Malapa is about 15km from sites of Swartkrans and Sterkfontein.

What the name means

Australopithecus, means ‘southern ape’. It is based on ‘australo’, a Latin word meaning ‘southern’ and ‘pithecus’, a Greek word meaning ‘ape’. The name was originally created just for this species found in South Africa but several closely related species now share the same genus name.

The word sediba means ‘fountain’ or ‘wellspring’ in the seSotho language.

Relationships with other species

Due to the age and overall skeletal features, the discoverers believe this species descended from A. africanus. It also shares derived features with early Homo, more so than any other australopithecine species, suggesting that it is possibly ancestral to Homo (or a sister group to a Homo ancestor). Although the discoverers favour A. sediba being ancestral to Homo, they also accept that it may be an evolutionary dead end.

The origin of Homo and its direct ancestor among austropithecines is widely debated and remains unresolved. If this interpretation of the fossils is correct, these remains add to the debate by suggesting that Australopithecus africanus should again be considered a possible direct human ancestor. A. africanus was once believed to be a direct ancestor but numerous finds in the later 1900s caused many scientists to push it to a side branch in our evolutionary family tree.

Many other palaeontologists consider the ‘A. sediba’ fossils to be a chronospecies of A. africanus – meaning that the slight anatomical differences between the new fossils and A. africanus are due to changes over time within a species rather than them being from different species. This view makes the fossils merely an interesting side branch of our family tree but does extend the time range for A. africanus by almost half a million years.

Key physical features

This species is distinguished from others by a combination of primitive and derived features rather than single identifying characteristics (autapomorphies). A number of features suggest close relationships to A. africanus and possibly Homo.

Body size and shape

• similar to other australopithecines in body size and shape, standing about 1.2 metres tall

Brain:

• relatively small brain size estimated at about 420cc
• shapes of the right and left brain halves was uneven, as in Homo

Skull:

• minimal cresting compared to earlier australopithecines
• cranial vault is similar in shape to A. africanus
  face lacks the pronounced flaring zygomatics (cheekbones) of A. africanus but is otherwise generally similar in appearance
• derived facial mask due to the arrangement of the brow ridge, prominent nose, nasal ridge, eye sockets and less-flared cheekbones
• small cranium with transversely expanded vault
• slight postorbital constriction
• weakly arched supraorbital torus (browridge)

Jaws and teeth:

• overall, jaws and teeth display features generally similar to A. africanus
• lacks extreme postcanine megodontia of A. garhi and Paranthropusspecies and teeth are more similar in size to Homo species
• relatively closely spaced premolar and molar cusps as with other australopithecines
• jaw protrudes less than with earlier australopithecines
• the front of the lower jaw is nearly vertical and has a slight bony chin compared to A. africanus
• teeth differ from A. africanus in having weakly defined buccal grooves of the upper molars and smaller postcanines. These features are more derived towards Homo
• moderately developed canine fossa
• parabolic dental arcade
• relatively thick tooth enamel

Limbs:

• features are similar to other australopithecines
• relatively long upper limbs with large joint surfaces
• retention of primitive features on upper and lower limbs
• numerous features of hip, knee and ankle indicate this species was bipedal
• foot bones were primitive and like other australopithecines
• hands are curved like other australopithecines but more compact

Pelvis:

• similar pelvis to other australopithecines but with derived features in the ilium that anticipate the reorganisation of the pelvis and limbs as seen in Homo ergaster (African Homo erectus)

Lifestyle

Culture

There is no evidence of tool use or any other cultural elements. It is likely that this species lived in a manner similar to A. africanus and was adapted to a similar ecological niche. It probably used simple tools such as sticks found in the immediate surroundings and scavenged animal bones. Stones may also have been used as tools, however, there is no evidence that these stones were shaped or modified.

Environment and diet

The woodland environment of South Africa started to dry out about 2.5 million years ago, leading to the spread of savannah grasslands. A. sediba lived in a generally flat landscape with a patchwork of grasslands and woods.

Numerous bones of other animals were found in the cave deposits, including saber-toothed cats.

Although no detailed analysis has as yet been carried out on tooth wear or isotopes, it is likely that it ate fleshy fruits, young leaves and perhaps small mammals or lizards.