Ardipithecus ramidus

Discoveries

Between 1992-1994, paleoanthropologist Tim White and his team found Ar. ramidus fossils that included 45% of a complete skeleton and the remains of 35 separate individuals.^[2]^[3] These fossils were dated to around 4.4 million years ago.^[1] In 2005, fossils from another 9 individuals were discovered.^[4]

Ar. ramidus was named after the word ramid, which means "root" in the Afar language of Ethiopia.^[5]

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Description

Walking

Ar. ramidus had adaptations that allowed it to walk upright (bipedalism) or to live in the trees (an arboreal lifestyle). We know it did not walk on its knuckles.^[6] It probably spent a lot of time in the trees and could not walk on two feet for very long.^[7]

It would have been less efficient at running and walking than members of Australopithecus or Homo.^[6]^[7] Still, its ability to walk on two feet makes it extremely important to human evolution. It could also leap like bonobos can.

Brain & skull

Ar. ramidus's brain volume was about 300 to 350 cc. The brains of modern female chimpanzees and bonobos are about this size.^[4] This is smaller than the brain of the Australopithecus afarensis individual nicknamed Lucy.^[3] It is only about 20% of the size of the modern human brain.^[3]

There are similarities between the skulls of Ar. ramidus, Australopithecus, and Homo. This provides evidence that these three genera are related.^[8]

Ar. ramidus lacked the large canine teeth that male chimpanzees use when showing aggression.^[1]

Body

Ar. ramidus was about the same size as modern chimpanzees.^[4] Males and females were similar in size, like with humans. This is different from Australopithecus, where males are much larger than females. (That is called sexual dimorphism.)

The proportion of their limbs to their bodies does not match humans or African apes; instead, it is similar to Old World monkeys.^[1]

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Diet

Based on analysis of fossil teeth, Ar. ramidus had a mostly plant-based diet.^[6] Still, it was probably more omnivorous than chimpanzees, eating a wider range of foods both on the ground and in the trees. These may have included nuts, fruits, leaves, tubers, insects, woodland plants, and small mammals.^[4]

Ar. ramidus did not have the large jaws it would need for heavy chewing (like eating roots). It also did not have the large incisors it would need to specialize in eating fruit, like chimpanzees do.^[1]

Some studies suggest that Ar. ramidus regularly used its front teeth to clamp and pull, which may mean their diet included a lot of leaves.^[4]