One of the most important fossils in the study of human origins is Ardipithecus ramidus, a fossil pre-human discovered in 1994, in what was once a forest in the Aramis region of northern Ethiopia. This primary reason this fossil is so important, and is one of my favorites, is because it bridges the gap between the late fossil apes of the Miocene Epoch (25 million years ago to 5 million years ago) and our direct human ancestors– which gives us the first detailed evidence of the earliest example of upright walking.
Sometime during the late Miocene, between 7 and 5 million years ago, our distant ancestors went from a very generalized locomotion pattern of using all four limbs to get around (quadrupedalism) to walking on two legs (bipedalism). What Ardipithecus ramidus tells us is that this change did not happen overnight.
For researchers working in the human origins field, the defining characteristic of any species in our evolutionary line, is bipedalism. Furthermore, the adaptations for it are easy to spot in the fossil record (see this page for a complete description). Apes have very long and narrow hips while humans have wide hips that are very short, top to bottom. Beginning around 3.7 million years ago, the human pattern becomes ubiquitous.
The hip structure of Ardipithecus ramidus is exactly intermediate between these two evolutionary adaptations. The top of the hip is wide and short, while the bottom is stretched, like that of an ape. What this means is that it could spend as much time in the trees as it could on the ground and was content being both quadrupedal and bipedal.
Ardipithecus ramidus is a stark example of a transitional form between Miocene apes and forms that eventually led to us– and belies one of the continual talking points of supporters of young earth creationism and intelligent design: that there are no such forms in the fossil record.
Additionally, this fossil points back to one of the most fascinating and growing areas of the study of fossil humans: the search for the last common ancestor of apes and the line that led to us. Was the last common ancestor partly bipedal, like Ardipithecus? Did this transition take place in the ancient forests of northern Ethiopia, or elsewhere? Given what we know of evolutionary patterns, it is possible that Ardipithecus ramidus is not a direct ancestor to later humans but represents one of several forms that evolved these adaptations?
More fossil discoveries will help us fill in this time period and provide a clearer picture of how and when this transition took place.