Ardipithecus ramidus

The species in brief

Ardipithecus ramidus is the best-known of the earliest probable human relatives. It lived about 4.4 million years ago in Ethiopia’s Middle Awash valley, in wooded country near streams, and is known from more than 100 fossils from at least 36 individuals. The most complete is a skeleton nicknamed Ardi.

The reason Ardi mattered is that she did not look like what scientists had expected. For decades the expectation had been that the earliest human ancestors would look like chimpanzees that had begun to stand up. Ardi did not. Neither chimpanzee nor human, she showed that the common ancestor of the two lines was probably a different sort of animal altogether.

Discovery

The first fossils were found in 1992 by a team led by Tim White, Berhane Asfaw and Gen Suwa at Aramis, in the Middle Awash region of Ethiopia’s Afar rift: a few teeth, spotted by Gen Suwa, and then pieces of a skeleton. In November 1994 the graduate student Yohannes Haile-Selassie found a hand bone, and the team later exposed further parts of a single, fragile skeleton. The bones were so crushed and soft that it took years to excavate them, record them, and rebuild the skull and pelvis. The first description of the species was published in 1994, and the full study of the skeleton appeared in a package of papers in Science in October 2009. Science named it the scientific “Breakthrough of the Year” for 2009.

The skeleton, ARA-VP-6/500, is about 45 percent complete. It is a female, found with parts of the skull, teeth, hands, feet, pelvis and limbs.

A body that was neither ape nor human

• Walking. The upper pelvis was short and wide, like a human’s, and set up for walking upright. But the lower part of the pelvis was still large and ape-like, a sign of strong climbing muscles.
• Feet. The big toe stuck out sideways and could grasp, as in apes, but the bones of the rest of the foot were stiffer, which helps with walking. The foot is a combination that is unlike anything alive today.
• Hands. The hand was less specialised than a chimpanzee’s. It lacks the long fingers and the knuckle-walking features of African apes, so the earliest human ancestors may never have been knuckle-walkers.
• Face and teeth. The canines were small and diamond-shaped, with little difference between males and females. In apes, large male canines are used to threaten rivals. Their reduction in Ardipithecus may point to less aggression between males, and perhaps to more cooperation.
• Brain. About 300 to 350 cubic centimetres, similar to a chimpanzee’s.
• Size. About 1.2 m (4 ft) tall and 50 kg (110 lb).

Food and habitat

Carbon in the tooth enamel shows that Ardi ate mainly food from forest and woodland plants, and little from grasses. She seems to have been an omnivore that took fruit and other foods both in the trees and on the ground. The fossil animals and plants found with her point to a mosaic of woodland and grassland, with lakes, swamps and springs nearby. That contradicts the old idea that our ancestors stood up because they were forced out of forests onto open savannah. Upright walking seems to have started in woodland.

The debate

Not everyone is convinced. The team led by C. Owen Lovejoy, who studied the pelvis, argued the skeleton shows an early human ancestor. Others have objected that the skeleton was so crushed that its reconstruction is uncertain. Some have argued that the way the skeleton was put together exaggerates features that look like later humans, or that Ardipithecus may be closer to a side branch than to our direct line. The researchers who worked on it reject that. The broader point, that the earliest stages of human evolution do not resemble a living chimpanzee, is widely accepted.

Where it fits

Ardipithecus ramidus is younger than Sahelanthropus, about 7 million years old, and Orrorin, about 6 million. It comes after an older Ethiopian species, Ardipithecus kadabba, which lived about 5.8 to 5.2 million years ago and is known mostly from teeth and a few bones. Ardi lived before Australopithecus anamensis and afarensis, and it is possible that something like her was ancestral to them. The fossils between 4.4 and 4.2 million years ago are still scarce.

Where to see it

The original fossils belong to Ethiopia and are kept at the National Museum of Ethiopia in Addis Ababa. Casts and reconstructions of Ardi have been shown in several museums worldwide.

Sources

• Smithsonian National Museum of Natural History, Human Origins Program. Ardipithecus ramidus.
• White, T. D. and others (1994). Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature 371: 306 to 312.
• White, T. D. and others (2009). Ardipithecus ramidus and the paleobiology of early hominids. Science 326: 64 to 86.
• Lovejoy, C. O. and others (2009). The pelvis and femur of Ardipithecus ramidus: the emergence of upright walking. Science 326: 71e1 to 71e6.