Ardipithecus ramidus

Background of discovery

Age

4.4 to 4.2 million years ago

Important fossil discoveries

Hundreds of pieces of fossilised bone were recovered during 1992-1994, all from localities west of the Awash River, in Aramis, Ethiopia. The finds number over 110 specimens and represent about 35 individual members of this species. Most of the remains are dental, but some skull and limb bones were also found. A partial humerus (arm bone) indicates that this species was smaller than the average Australopithecus afarensis.

In 2005, the remains of 9 individuals were recovered from As Duma in northern Ethiopia. The remains mostly consist of teeth and jaw fragments, but also some bones from the hands and feet.

Some specimens discovered earlier in Kanapoi, Lothagam and Tabarin could also belong to this species.

Key specimens

• ARA-VP-6/1 teeth: This is the holotype for this species. It consists of teeth and jaw bone and was found in Aramis in 1993.
• 'Ardi' ARA-VP-6/500: A partial skeleton found in 1994, consisting of about 125 pieces, was described and published in 2009. It is the oldest known skeleton of a human ancestor. The individual is believed to be a female and is nicknamed ‘Ardi’. She weighed about 50kg and stood about 120cm tall.The skeleton was in extremely poor condition and it took the team 15 years to excavate, scan, make virtual reconstructions, assemble and then analyse. The results were hugely significant in terms of how we view the evolution of the earliest hominins and the physical appearance of the last common ancestor of humans and chimpanzees. The skeleton does not look much like a chimp or gorilla or have the expected 'transitional' features. Instead, it may well preserve some of the characteristics of the last chimp-human ancestor. Analysis of the skeleton reveals that humans did not evolve from knuckle-walking apes, as was long believed. It also indicates that chimpanzee evolution underwent high degrees of specialisation since diverging from the last common ancestor and thus these apes are poor models for understanding the appearance of this ancestor.

What the name means

The name is derived from the local Afar language. ‘Ardi’ means ‘ground’ or ‘floor’ and ‘pithecus’ is Latinised Greek for ‘ape’. The name ‘ramid’ means ‘root’ in the Afar language.

Distribution

Fossils belonging to this species were found in eastern Africa in the Middle Awash valley, Ethiopia. Additional fossils that may also belong to this species have been collected in northern Kenya.

Relationships with other species

This species position as a direct ancestor of humans is unclear and scientists are still debating where it should be placed relative to our direct line. The discovers think it was ancestral to Australopithecus - it is the only putative hominin in evidence between 5.8 and 4.4 million years ago - but others do not agree. Even if Ardipithecus ramidus is not on our direct line, it must have been closely related to the direct ancestor and probably similar in appearance and adaptation. It also offers new insights into how we evolved from the common ancestor we share with chimps.

This species was originally classified as Australopithecus ramidus in 1994, but was reclassified in 1995 because its discoverers believed it was distinct enough to be placed into a new genus, Ardipithecus.

Key physical features

This species was a facultative biped and stood upright on the ground but could move on all four limbs in trees. Features of the anatomy are extremely primitive.

Brain

• about 300-350cc, similar in size to modern female chimpanzees and bonobos

Body size and shape

• similar in size to modern chimpanzees. The most complete specimen, a female, stood about 120cm tall
• males were only slightly larger than females
• the body shape was more ape-like than humans, but differed from living African apes in a number of significant features

Limbs

• mix of primitive and derived features suggest this species was able to walk upright on the ground yet efficiently climb trees
• long powerful arms that were not used for weight-bearing or knuckle-walking as with quadrupedal apes
• bones in the wrist (particularly the midcarpal joint) provided flexibility and the palm bones were short. These features suggest this species was not a knuckle-walker and that the palms could support the body weight when moving along branches
• finger bones were long and curving, both features useful for grasping branches
• upper and lower legs bones (femur and tibia) have features consistent with bipedalism
• feet were relatively flat and lacked arches, indicating this species could probably not walk or run long distances
• they had grasping abducted toe characteristic of gorillas and chimps
• the foot was more rigid than chimpanzees with the bases of the four toe bones oriented to reinforce the forefoot when pushing off. Chimps have a highly flexible midfoot that improves their ability to grasp and climb but are less effective for propulsion when walking on ground

Pelvis

• has a mix of features useful for both climbing and upright walking and suggests the species still spent significant time in the trees
• shape of the upper blades (ilium) appear short and broad like Australopithecus afarensis, indicating that the gluteal muscles had been repositioned. This lowered the body's centre of mass so to balance on one leg when walking
• the lower pelvis is large and the angle of the ischial surface does not face upward as it does in humans and Australopithecus. These are primitive features that suggest this species had massive hindlimb muscles for climbing and did not walk like A. afarensis
• the sciatic knotch is similar in size and shape to later hominins. This is a derived feature and is not found in chimpanzees

Jaws and teeth

• much of the dentition is ape-like including relatively large canines and molars
• tooth enamel thickness is intermediate between that of chimpanzees and Australopithecus
• canines are less projecting and smaller than those of all other known apes and there is no evidence of honing. The base of the canines in both sexes are similar in size to female chimpanzees and male bonobos, but have shorter crown heights
• upper canines are shaped like diamonds, rather than the pointed shape seen in African apes, whch is a derived feature shared with Australopithecus afarensis . Lower canines appear to have less derived features.
• the jaw displayed significant forward projection compared to humans, but less than modern African apes
• pre-molars have derived features that are more advanced in the human direction
• canines (non-sharpened and small) and other teeth share features with Orrorin tugenensis

Skull

• skull rests atop the spinal column, indicating this species was bipedal, although it probably walked in a slightly different manner than humans
• the cranial base is short from front to back, indicating the head balanced on top of the spine
• the face is small and in a more vertical position than chimpanzees
• the ridge above the eye socket is unlike that of a chimpanzee

Lifestyle

Culture

There is no evidence for any specific cultural attributes, but they may have used simple tools similar to those used by modern chimpanzees, including:

• twigs, sticks and other plant materials that were easily shaped or modified. These may have been used for a variety of simple tasks including obtaining food.
• unmodified stones, that is stones that were not shaped or altered before being used. These tools may have been used to process hard foods such as nuts.

Environment and diet

Associated animal and plant fossils indicate this species lived a in relatively moist and heavily forested woodland. Fossil species include plants such as fig, palm and hackberry and animals like colobine and baboon-like monkeys, kudu, peafowl, bats, shrews, rodents, doves, eagles, owls and parrots.

The species, with its ape-like feet, probably spent considerable time in the trees looking for food and shelter.

Diet may have included nuts, fruit, leaves, tubers, insects and small mammals. They were probably more omnivorous than chimps (based on the size, shape and enamel of the teeth), and fed both in trees and on the ground. The evidence is inconclusive, but studies suggest the front teeth were regularly used for clamping and pulling, possibly reflecting a diet that included large amounts of leaves. Tooth enamel analysis suggests they ate fruit, nuts and leaves. Carbon-isotope studies of teeth show they ate woodland rather than grassland plants.