Overview:

Ardipithecus kadabba was bipedal (walked upright), probably similar in body and brain size to a modern chimpanzee, and had canines that resemble those in later hominins but that still project beyond the tooth row. This early human species is only known in the fossil record by a few post-cranial bones and sets of teeth. One bone from the large toe has a broad, robust appearance, suggesting its use in bipedal push-off.

History of Discovery:

When he found a piece of lower jaw lying on the ground in the Middle Awash region of Ethiopia 1997, paleoanthropologist Yohannes Haile-Selassie didn’t realize that he had uncovered a new species. But 11 specimens from at least 5 individuals later, Haile-Selassie was convinced he had found a new early human ancestor. The fossils—which also included hand and foot bones, partial arm bones, and a clavicle (collarbone)—were dated to 5.6–5.8 million years old. One of the specimens, a toe bone, is dated to 5.2 million years old; this fossil has features of bipedal walking. Faunal (fossil animal) evidence from the site indicated that the early humans there lived in a mixture of woodlands and grasslands, and had plenty of access to water via lakes and springs. 

In 2002, six teeth were discovered in the Middle Awash at the site Asa Koma. The dental wear patterns confirmed the early human fossils were unique and not a subspecies of A. ramidus. Based on these teeth, paleoanthropologists Yohannes Haile-Selassie, Gen Suwa, and Tim White allocated the fossils in 2004 to a new species they named Ardipithecus kadabba (‘kadabba’ means ‘oldest ancestor’ in the Afar language).

How They Survived:

Instead of eating mostly fruit and soft leaves like chimpanzees, there is evidence that Ardipithecus kadabba ate a variety of fibrous foods.

How do we know they ate fibrous foods?

The back teeth of Ardipithecus kadabba are larger than a chimpanzee’s, but its front teeth are narrower. This evidence suggests this species did most of its chewing in the back of its mouth. This type of chewing would focus on hard-to-eat foods like fibrous nuts.

Evolutionary Tree Information:

Scientists originally considered Ardipithecus kadabba to be a subspecies of the later Ardipithecus ramidus, then renamed as its own distinct species based on dental differences.

Questions:

We don’t know everything about early humans—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 kadabba that may be answered with future discoveries:

1. Was Ar. kadabba routinely bipedal? So far, the evidence for Ar. kadabba’s upright walking comes from an single toe bone that dates to 5.2 million years old and was found 10 miles away from the other Ar. kadabba specimens.
2.  If Ar. kadabba walked upright, what was its gait like?
3. DId bipedalism independently develop in the Ardipithecus lineage? Or is Ar. kadabba somehow related to Orrorin tugenensis and Sahelanthropus tchadensis, two earlier human species?
4. How is the Ardipithecus lineage related the Australopithecus lineage?
5. How big were the average male and female Ar. kadabba individuals? Was there a high level of sexual dimorphism in this early human species?  

References:

First paper:

Haile-Selassie, Y., Suwa, G., White, T.D., 2004. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science 303, 1503-1505.