Australopithecus sediba

Australopithecus sediba is an extinct species of genus australopithecine. Remains of two individuals found in South Africa provide scientists with almost all they know about the hominins. A. sediba is of the phylum chordata, class mammalia, order primates, suborder haplorhini, infraorder simiiformes, family Hominidae, subfamily homininae, and tribe hominini.

The genus name Australopithecus means “southern ape.” It comes from the Latin word austral, which means “southern,” and the Greek word pithecus, meaning “ape.” The word sediba is from the seSotho language and means “fountain” or “wellspring.”

Hominins are primate species of the last fourteen million years. This is the point at which early human and chimpanzee species split. Hominins include modern humans, extinct human species, and immediate ancestors of modern humans. Hominids, a term previously used in a much broader sense and in many cases still misapplied in the 2020s, includes all extinct and modern Great Apes, which includes modern humans, bonobos, chimpanzees, gorillas, orangutans, and their immediate ancestors. The distinction was made after genetic research revealed the similarities and differences between species. Much debate has surrounded scientific classification of humans and other species. Twenty-first century genetic research has found that modern humans’ evolutionary closest living relations are members of the genus Pan: chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). The human and Pan lines branched about seven million years ago, while chimpanzee and bonobo lineages branched about one to two million years ago.

A. sediba dates to between 1.78 and 1.95 million years ago, long after the human and chimpanzee species split. The species has some characteristics that are more similar to humans than to other species and could offer clues about the ancestors of the genus Homo.

Background

The Cradle of Humankind is a region of caves about 30 miles (50 kilometers) from Johannesburg, South Africa, that is known for many archaeological finds, notably early humans and related species. The region is about 180 square miles (466 square kilometers) and has yielded more than a dozen major archaeological sites. The region’s name refers to its significance as the site where the earliest evidence of human life has been unearthed. Researchers have found humanin and animal fossils, tools, and evidence of early human-made fires. For this reason, the United Nations Educational, Scientific and Cultural Organization (UNESCO) named the area a World Heritage Site in 1999. Finds in the Cradle of Humankind date to at least three million years ago. The best-known fossil site is Sterkfontein, near Krugersdorp. It has yielded many significant finds.

Discoveries at the Cradle of Humankind were first made in the early twentieth century. Lime mining operations were active in the region at the time. Paleontologist Raymond Dart of the University of Witwaterrand made it a habit to look at the finds of quarry workers, who saved boxes of rocks containing any fossils they noticed. One day in 1924, a quarry manager at Taung gave him two boxes of fossils. One contained the skull of a young individual dubbed the Taung Child, who had apelike features. Dart looked at the opening in the skull that allows the spinal cord and brain to meet. The structure indicated that the child, who had been about three years old when he died, had walked upright like humans. He named the find Australopithecus africanus. Other scientists were sure he was wrong because they believed that humans first emerged in Asia or Europe. However, further finds cemented Africa as the birthplace of humankind.

Two of the most well-known finds were discovered at Sterkfontein. Paleontologist Robert Broom was likewise collecting fossils found by miners. In 1947 he found a skull at the site. He called it Mrs. Ples, though later it was deemed to be male. The A. africanus individual lived between 2.5 and 2.8 million years ago. Little Foot was also found in Sterkfontein, although not by Broom. Researchers believe the young male hominin fossil is an even older species.

Malapa Cave has also yielded finds. Paleoanthropologist Lee Berger found fossils of a young male hominin near the cave in 2008. The find consists of a collarbone, which was found by his son, Matthew, then nine years old, and a jawbone. These were named Karabo, which in the language of the Sotho people means “the answer.” About a month later, the researchers returned to the site. Berger saw a portion of a humerus in the wall of the pit. He leaned in to get a better look and two hominid teeth landed in his palm. Scientists later classified the fossils as a new species, A. sediba.

Scientists made a major find in 2015 in the Rising Star cave system. They unearthed more than 1,500 human fossils from at least fifteen individuals. This was another new species, which was dubbed Homo naledi. Scientists determined the species lived between 335,000 and 236,000 years ago, roughly when early Homo sapiens or modern humans lived.

Overview

Berger’s discoveries in Malapa Cave since 2008 have included several partial skeletons that offer a great deal of information about the species. All A. sediba specimens have been found at Malapa. Two of these A. sediba fossils were found close together. Researchers believe they died about the same time and were covered by sediment soon after death. They are called Malapa Hominin 1 (MH1) and Malapa Hominin 2 (MH2). MH1 is the holotype or type specimen of A. sediba. This refers to its status as the first specimen to be classified and named. MH2 is a paratype, another specimen representing the species.

MH1 is believed to be an older juvenile. The fossil consists of a partial cranium, a partial lower jaw, and some other finds, including a right collarbone. MH2, which is probably an adult female, consists of a partial lower jaw, some additional teeth, most of the right arm, ankle and knee joints, and portions of other bones, including the pelvis. Researchers speculate that they and many of the animals whose fossil remains have been found might have fallen into vertical shafts in the limestone landscape.

A. sediba stood about 3.9 feet (1.2 meters) tall. Its brain was not large, estimated to be about 25 cubic inches (420 cubic centimers). As in Homo, the right and left halves of the brain were uneven. The shape of the skull is less crested than earlier australopithecines, but the skull vault in which the brain was contained is shaped much like A. africanus. It does not have the prominent cheekbones seen in A. africanus, but the face resembles the other species generally. The jaws and teeth likewise are similar, but A. sediba’s jaw does not protrude as much as is seen in earlier australopithecines. The upper molars and postcanines resemble Homo in the definition of molar grooves and size of the postcanines. The smaller teeth could indicate the species had a different diet. The dental arch or arcade, the shape of the part of the jaw that supports the teeth, is parabolic, as is seen in Homo. The limbs are much like other australopithecines. The hands are similarly curved but smaller in A. sediba.

Researchers have found many similarities in A. sediba to earlier Australopithecus such as the length of the limbs and the narrow upper chest. The teeth share traits of earlier Australopithecus as well as humans, and facial features are more similar to those found in Homo. The broad lower chest likewise resembles the human form. The strength of the femur and humerus indicate that A. sediba moved more like humans, frequently walking upright. Changes in the pelvis related to upright walking appear to have evolved before other changes seen in later Homo species. The longer arms and smaller body suggest A. sediba may have lived at least part of the time in trees. Researchers did not find evidence of tools but, like other species, likely used sticks and animal bones.

A. sediba has also revealed new information about how upright walking evolved. Researchers say the legs and feet indicate that A. sediba put its weight primarily on the outer edge of the foot and walked with the foot inward. Scientists say this is not the way upright walking evolved in human evolution and indicates species that may have developed this skill in multiple ways.

Scientists believe A. sediba descended from A. africanus. The latter lived between 3.2 and 2 million years ago, while evidence of A. sediba ranges from 1.95 to 1.78 million years ago. The similarities with early Homo suggest that the species could be an ancestor of Homo or a sister group of a Homo ancestor. Early Homo species dating to about 2 million years ago include H. rudolfensis, H. habilis, and H. erectus. A. africanus was long ago believed to be a direct ancestor of Homo, but late-twentieth-century discoveries seemed to disprove this theory. Researchers say A. sediba opens the door to this earlier theory. A great many questions about the evolution of Homo and related families remain unanswered.

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