Humanity has a new older sister. A fossilized skeleton of Ardipithecus ramidus or "Ardi" predates Lucy by over a million years. The discovery has led to new insights about human evolution, suggesting previously unknown relationships to our chimpanzee brethren.
Charles Darwin, recognizing the similarities between humans and chimpanzees, postulated that we might someday find fossils of a "missing link," a creature that represented the evolutionary break between humans and chimps. The discovery of Ardi, however, suggests that when we do find that evolutionary break, the fossils we find will not be a blend of human and chimpanzee.
Researchers discovered Ar. ramidus near Aramis, Ethiopia, and have dated it as 4.4 million years old, considerably older than Lucy, who at 3.2 million years old was considered humanity's oldest relation. It's not clear whether humans are directly descended from this particular hominid, but it makes it clear that bipedal hominids are considerably older than previously thought.
The paleobiologists studying Ardi identify hers as an "intermediate" form, one that is bipedal, but at the same time capable of walking on all forms and traveling through trees. Still, although she represents a point past hominids' evolutionary break with gorillas and chimpanzees, she is very different from modern apes. For example, Ardi's had flat hands and feet and flexible wrists, and engaged in a form of locomotion called palmigrady, which is a trait of ancient apes and unlike gorillas and chimpanzees, which are stiff-wristed knuckle-walkers. This suggests that gorilla and chimp ancestors developed their knuckle-walking long after their evolutionary break with hominids.
In a paper in the upcoming issue of Science, which outlines the discovery, researchers will explain what Ardi's dissimilarity from modern apes means for our picture of human and chimp evolution:
Humans did not evolve from chimpanzees but rather through a series of progenitors starting from a distant common ancestor that once occupied the ancient forests of the African Micoene.