Ardipithecus ramidus is a 4.4 million year old hominid, one of whose examples has yielded the most complete skeleton to date from that time period. As the date of the fossil is close to the date for the branching between the human and chimpanzee lines, it's likely that A. ramidus is quite close, morphologically, to the common ancestor of those two lines. A. ramidus has a brain the size of a chimpanzee's, is bipedal, and has some arboreal adaptations that are entirely different from the chimpanzee's. All that makes this quite possibly the most important anthropological find of the 20th century.
20th century? Yes. The fossil was first found in the early 1990s. It just took them a few years for those who found it to publish their results.
Science has devoted an entire issue, including 11 articles, to the find. The articles are currently available for free - with registration - at: