EARLY UPRIGHT POSTURE
To help inform the answers, I needed to understand how we got to Homo sapiens in the first place. Ardi was a key link in our lineage from our common ancestor with the chimpanzees to the first human species which was probably Homo habilis. Ardi is the nickname for Ardipithecus ramidus("root ground ape").
This famous fossil was reported by Tim White in 1994 after its discovery in Ethiopia. It is thought by many to be the earliest hominid in the line to Homo sapiens, although Sahelanthropus tchadensis, Orrorin tugenensis, and Ardipithecus kadabba are also contenders for that distinction. Ardi is about 4.4 million years old. She climbed trees for food and safety reasons, helped along by her opposable big toe, but mainly spent her time walking upright between the trees. She had a small brain, closer to the size of a chimpanzee, and didn't make tools. She provided compelling evidence that bipedal upright posture proceeded the development of our large modern brain.
Click on links to other players in my journey below.