Australopithecus sediba

Australopithecus sediba is an extinct species of australopithecine recovered from Malapa Cave, Cradle of Humankind, South Africa. It is known from a partial juvenile skeleton, the holotype MH1, and a partial adult female skeleton, the paratype MH2. They date to about 1.98 million years ago in the Early Pleistocene, and coexisted with Paranthropus robustus and Homo ergaster / Homo erectus. Malapa Cave may have been a natural death trap, the base of a long vertical shaft which creatures could accidentally fall into. A. sediba was initially described as being a potential human ancestor, and perhaps the progenitor of Homo, but this is contested and it could also represent a late-surviving population or sister species of A. africanus which had earlier inhabited the area.

MH1 has a brain volume of about 350–440 cc, similar to other australopithecines. The face of MH1 is strikingly similar to Homo instead of other australopithecines, with a less pronounced brow ridge, cheek bones, and prognathism (the amount the face juts out), and there is evidence of a slight chin. However, such characteristics could be due to juvenility and lost with maturity. The teeth are quite small for an australopithecine. MH1 is estimated at 130 cm (4 ft 3 in) tall, which would equate to an adult height of 150–156 cm (4 ft 11 in – 5 ft 1 in). MH1 and MH2 were estimated to have been about the same weight at 30–36 kg (66–79 lb). Like other australopithecines, A. sediba is thought to have had a narrow and apelike upper chest, but a broad and humanlike lower chest. Like other australopithecines, the arm anatomy seems to suggest a degree of climbing and arboreal behaviour. The pelvis indicates A. sediba was capable of a humanlike stride, but the foot points to a peculiar gait not demonstrated in any other hominin involving hyperpronation of the ankle, and resultantly rotating the leg inwards while pushing off. This suite of adaptations may represent a compromise between habitual bipedalism and arboreality.

A. sediba seems to have eaten only C₃ forest plants such as some grasses and sedges, fruits, leaves, and bark. This strongly contrasts from other early hominins which ate a mix of C₃ and abundant C₄ savanna plants, but is similar to modern savanna chimpanzees. No other hominin bears evidence of eating bark as part of regular diet. Such a generalist diet may have allowed it to occupy a smaller home range than savanna chimps. The Malapa area may have been cooler and more humid than today, featuring closed forests surrounded by more open grasslands.

The first fossil find was a right clavicle, MH1 (UW88-1), in Malapa Cave, Cradle of Humankind, South Africa, discovered by 9-year-old Matthew Berger on 15 August 2008 while exploring the digsite headed by his father, South African palaeoanthropologist Lee Rogers Berger. Further excavation yielded a partial skeleton for MH1, additionally including a partial skull and jawbone fragments, as well as aspects of the arms, fingers, shoulders, ribcage, spine, pelvis, legs, and feet. MH1 is interpreted as having been a juvenile male due to the apparently pronounced development of the brow ridge and canine roots, eversion of the angle of the mandible, and large scarring on the bones. However, anthropologists William Kimbel and Yoel Rak contend that these are unreliable methods of determining sex, and suggest that MH1 is female based on the lack of anterior pillars (columns running down alongside the nasal opening down to around the mouth) and a slightly convex subnasal plate, using methods of sex determination for A. africanus. MH1 was nicknamed "Karabo", which means "answer" in Tswana, by 17-year-old Omphemetse Keepile from St Mary's School, Johannesburg, in a naming contest. She chose this name because, "The fossil represents a solution towards understanding the origins of humankind."

Another partial skeleton, the adult MH2, was recovered by Lee on 4 September 2008 with isolated upper teeth, a partial jawbone, a nearly complete right arm, the right scapula, and fragments of the shoulders, right arm, spine, ribs, pelvis, knee joint, and feet. The pubic bone is broad and square, and the muscle scarring on the body is weak to moderate, which suggest that MH2 is female.

The presence of species which evolved after 2.36 million years ago and became extinct around 1.5 million years ago indicates the A. sediba layer dates to sometime within this interval during the Early Pleistocene. Uranium–lead dating of a flowstone capping the layer yielded a date of 2.026±0.021 million years ago. Using archaeomagnetic dating, the sediments have a normal magnetic polarity (as opposed to the reverse of the magnetic polarity in modern day) and the only time when this occurred during this interval is between 1.95 and 1.78 million years ago. In 2011, the flowstone was more firmly dated to 1.977±0.002 million years ago again using uranium–lead dating.

The cave networks around Malapa comprise long, interconnected cave openings within a 500 m × 100 m (1,640 ft × 330 ft) area. The Malapa site may have been at the base of an at most 30-metre-deep (98 ft) cavern system. The cave is at the intersection of a north-northeast and north-northwest chert-filled fracture, and the hominin remains were unearthed in a 3.3 m × 4.4 m × 3.5 m (11 ft × 14 ft × 11 ft) section on the north-northwest fracture. The layer was exposed by limestone mining in the early 20th century. The cave comprises five sedimentary facies A–E of water-laid sandstone, with A. sediba being recovered from facies D, and more hominin remains from facies E. MH1 and MH2 are separated vertically by at most 40 cm (16 in). Facies D is a 1.5-metre-thick (4.9 ft), lightly coloured layer overlying flowstone. Small peloids are common, but are fused into large and irregular groups, which indicate they were deposited in a water-logged setting. Peloids may represent faecal matter or soil microbes. The preservation state of MH1 and MH2 indicate they were deposited quickly, were moved very little, and were cemented soon after deposition in a phreatic environment (in a subterranean stream). There is no evidence of scavenging, indicating the area was inaccessible to carnivores.

This could all indicate that Malapa Cave was a deathtrap, with inconspicuous cave openings at the surface. Animals may have been lured by the scent of water emanating from the shaft, and carnivores to the scent of dead animals, and then fallen to their deaths. A large debris flow caused the remains to be deposited deeper into the cave along a subterranean stream, perhaps due to a heavy rainstorm. The chamber eventually collapsed and filled with mud.