Question

Ardipithecus ramidus had a smaller brain volume than modern common chimpanzees. how this is possible if Ardi’s species evolved from chimpanzees. What would be the best way to explain the pattern based on their evolutionary relationship?

Answer 1

Ardipithecus ramidus had a small brain, measuring between 300 and 350 cm³. This is slightly smaller than a modern bonobo or female common chimpanzee brain, but much smaller than the brain of australopithecines like Lucy (~400 to 550 cm³) and roughly 20% the size of the modern Homo sapiens brain. Like common chimpanzees, A. ramidus was much more prognathic than modern humans.

A. ramidus existed more recently than the most recent common ancestor of humans and chimpanzees and thus is not fully representative of that common ancestor. Nevertheless, it is in some ways unlike chimpanzees, suggesting that the common ancestor differs from the modern chimpanzee. After the chimpanzee and human lineages diverged, both underwent substantial evolutionary change. Chimp feet are specialized for grasping trees; A. ramidus feet are better suited for walking. The canine teeth of A. ramidus are smaller, and equal in size between males and females, which suggests reduced male-to-male conflict, increased pair-bonding, and increased parental investment. Thus we can predict that the fundamental reproductive and social behavioral changes probably occurred in hominids long before they had enlarged brains and began to use stone tools.