The evolutionary context of the first hominins

Drawing the line on human ancestors Several fossil discoveries in recent years have been hailed as early hominins — members of the clade that includes humans and human ancestors — including Ardipithecus , from the Middle Awash region of Ethiopia, Orrorin from the Tugen Hills of Kenya and Sahelanthropus from the Djurab desert of Chad. Bernard Wood and Terry Harrison sound a note of caution. The differences between modern humans and non-hominins such as chimpanzees and bonobos are considerable. But between 8 million and 4 million years ago, when the human line is thought to have emerged, the differences between hominids in general would have been smaller. In this Review, Wood and Harrison offer alternative interpretations for where the likes of Ardipithecus , Orrorin and Sahelanthropus might be accommodated within the tree of life — close to but not perhaps on the hominin line. The relationships among the living apes and modern humans have effectively been resolved, but it is much more difficult to locate fossil apes on the tree of life because shared skeletal morphology does not always mean shared recent evolutionary history. Sorting fossil taxa into those that belong on the branch of the tree of life that leads to modern humans from those that belong on other closely related branches is a considerable challenge.