Overview:

Ardipithecus ramidus was first reported in 1994; in 2009, scientists announced a partial skeleton, nicknamed ‘Ardi’. The foot bones in this skeleton indicate a divergent large toe combined with a rigid foot – it's still unclear what this means concerning bipedal behavior. The pelvis, reconstructed from a crushed specimen, is said to show adaptations that combine tree-climbing and bipedal activity. The discoverers argue that the ‘Ardi’ skeleton reflects a human-African ape common ancestor that was not chimpanzee-like. A good sample of canine teeth of this species indicates very little difference in size between males and females in this species.

Ardi’s fossils were found alongside faunal remains indicating she lived in a wooded environment. This contradicts the open savanna theory for the origin of bipedalism, which states that humans learned to walk upright as climates became drier and environments became more open and grassy.

History of Discovery:

A team led by American paleoanthropologist Tim White discovered the first Ardipithecus ramidus fossils in the Middle Awash area of Ethiopia between 1992 and 1994. Since that time, White’s team have uncovered over 100 fossil specimens of Ar. ramidus . White and his colleagues gave their discovery the name Ardipithecus ramidus (‘ramid’ means ‘root’ in the Afar language of Ethiopia and refers to the closeness of this new species to the roots of humanity, while ‘Ardi’ means ‘ground’ or ‘floor’). At the time of this discovery, the genus Australopithecus was scientifically well established, so White devised the genus name Ardipithecus to distinguish this new genus from Australopithecus. In 2009, scientists formally announced and published the findings of a partial skeleton (ARA-VP-6/500), nicknamed "Ardi", first found in 1994.

How They Survived:

Ardipithecus ramidus individuals were most likely omnivores, which means they enjoyed more generalized diet of both plants, meat, and fruit. Ar. ramidus did not seem to eat hard, abrasive foods like nuts and tubers.

How do we know they were omnivores?

The enamel on Ar. ramidus teeth remains show it was neither very thick nor very thin. If the enamel was thick, it would mean Ar. ramidus ate tough, abrasive foods. If the enamel was  thin, this would suggest Ar. ramidus ate softer foods such as fruit. Instead, A. ramidus has an enamel thickness between a chimpanzee’s and later Australopithecus or Homo species, suggesting a mixed diet. However, the wear pattern and incisor sizes indicate Ar. ramidus was not a specialized frugivore (fruit-eater).  Ar. ramidus probably also avoided tough foods, as they did not have the heavy chewing specializations of later Australopithecus species.

Evolutionary Tree Information:

Over 100 specimens of Ardipithecus ramidus have been recovered in Ethiopia.  Even though it has some ape-like features (as do many other early human species), it also has key human features including smaller diamond-shaped canines and some evidence of upright walking. It may have descended from an earlier species of Ardipithecus that has been found in the same area of Ethiopia, Ardipithecus kadabba. 

Questions:

We don’t know everything about our early ancestors—but we keep learning more! Paleoanthropologists are constantly in the field, excavating new areas with groundbreaking technology, and continually filling in some of the gaps about our understanding of human evolution.

Below are some of the still unanswered questions about Ardipithecus ramidus that may be answered with future discoveries:

1. Does the pelvis of Ar. ramidus support the hypothesis that this early human species was bipedal? The pelvis was reconstructed from crushed fossils and, according to some scientists, is only suggestive of bipedalism.
2. What is the average size of male Ar. ramidus individuals? If more fossils support the original finding of relatively low sexual dimorphism, how does this relate to male and female size differences in other early humans at the base of our family tree -- and what does it mean?

References:

First paper:

White, T.D., Suwa, G., Asfaw, B., 1994. Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. Nature 371, 306-312.

Other recommended readings:

Gibbons, A., 2009. A new kind of ancestor: Ardipithecus unveiled. Science 326, 36-50.

Lovejoy, C.O., 2009. Reexamining human origins in light of Ardipithecus ramidus. Science 326, 74-74e8.

Lovejoy, C.O., Suwa, G., Simpson, S.W., Matternes, J.H., White, T.D., 2009. The great divides: Ardipithecus ramidus reveals the postcrania of our last common ancestors with African apes. Science 326, 100-106.

White, T.D., Asfaw, B., Beyene, Y., Hailie-Selassie, Y., Lovejoy, C. O., Suwa, G., Woldegabriel, G., 2009. Ardipithecus ramidus and the paleobiology of early hominids. Science, 326, 75-86.