Found in Ethiopia in 1994, a mysterious 4.4-million-year-old skeleton was proclaimed in 2009 as an early human ancestor that was an upright biped on the ground and a quadruped in the trees, proving that there was never a long-armed, arm-swinging ape in the human ancestry. Two leading experts on human fossils, William Jungers from Stony Brook University and William Kimbel from Arizona State University, debate what the fossils of Ardipithecus mean and how they have transformed our understanding of human evolution.
