Australopithecus sediba is an extinct species of australopithecine recovered from Malapa Cave, Cradle of Humankind, South Africa.

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.

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.

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.

[1] 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.

[2] 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.

A large debris flow caused the remains to be deposited deeper into the cave along a subterranean stream, perhaps due to a heavy rainstorm.

[2][9][10][11][12] The fossil record of early Homo is poorly known and based largely on fragmentary remains, making convincing anatomical comparisons difficult and sometimes unfeasible.

[2] Phylogenetic analyses in 2023 based on craniodental morphology recovered A. sediba in an unstable, varied position among hominins, so the researchers concluded that adult skeletons of this species are required for appropriate classification.

However, MH1 has a smaller cranium, a transversely wider cranial vault, more vertically-inclined walls of the parietal bone, and more widely spaced temporal lines.

[1] However, such characteristics are also found in some A. africanus skulls from Sterkfontein Member 4, which Kimbel and Rak believed could indicate that these Homo-like attributes would have been lost in maturity.

Compared to patterns seen in modern great apes, such marked differences exceed what would be expected if these could be explained as due to sexual dimorphism or the juvenile status of MH1.

However, A. sediba seems to have had a highly mobile lower back and exaggerated lumbar lordosis,[20] which may have been involved in counteracting torques directed inwards while walking in the hyperpronating gait proposed for A.

[15] Like other australopithecines and early Homo, A. sediba had somewhat apelike upper body proportions with relatively long arms, a high brachial index (forearm to humerus ratio) of 84, and large joint surfaces.

It is debated if apelike upper limb configuration of australopithecines is indicative of arboreal behaviour or simply is a basal trait inherited from the great ape last common ancestor in the absence of major selective pressures to adopt a more humanlike arm anatomy.

The humerus has a low degree of torsion unlike humans and African apes, which (along with the short clavicle) suggests the shoulder blade was placed farther from the midline like in Homo, though it is positioned higher up the back like in other australopithecines.

The hand also features a relatively long thumb and short fingers, much like Homo, which could suggest a precision grip important in creating and using complex stone tools.

Dental microwearing analysis similarly suggests the two Malapa hominins ate hard foods, complexity values ranging between H. erectus and the robust P.

[30] While walking, A. sediba may have displayed hyperpronation of the ankle joint causing exaggerated transfer of weight inwards during stance phase.

Similarly, the attachments for the rectus femoris and biceps femoralis muscles in A. sediba are consistent with midline-directed strains across the legs, hips, and knees.

This mode of walking is unideal for modern human anatomy, and hyperpronators are at a higher risk of developing plantar fasciitis, shin splints, and tibial stress fractures.

It may have affected movement of the shoulder blade and the upper right quadrant of the back, perhaps causing acute or chronic pain, muscular disturbances, or muscle spasms.

Given A. sediba may have required climbing ability, the lesion's position near the insertion for the trapezius, erector spinae, and rhomboid major muscles may have limited normal movement patterns.

MH1 has the earliest diagnosed case of cancer for a hominin by at least 200,000 years, predating the 1.8- to 1.6-million-year-old SK 7923 metatarsal fragment presenting osteosarcoma from Swartkrans, Cradle of Humankind.

This agrees with the interpretation of the site as the base of a tall shaft, acting as a natural death trap that animals accidentally fell into.

MH2 bears evidence of bracing during injury, with loading to the forearm and hand and impact to the chest, perimortem fracturing identified on the right side of the body.

This may indicate the area featured a closed habitat as well as grasslands—judging by the home range of the cape fox, both existed within 20 km2 (7.7 sq mi) of the site.

[37] The coprolite of a carnivore from facies D contained pollen and phytoliths of Podocarpus or Afrocarpus trees, as well as wood fragments from unidentified conifers and dicots.

In modern day, the Malapa site is a grassland, and Podocarpus and Afrocarpus are found 30 km (19 mi) away in the Afromontane forest biome in the canyons 1,500–1,900 m (4,900–6,200 ft) above sea level in the Magaliesberg mountain range, where wildfires are less common.

Because A. africanus went extinct around this time, it is possible that South Africa was a refugium for Australopithecus until about 2 million years ago with the beginning of major climatic variability and volatility, and potentially competition with Homo and Paranthropus.