East of the Great Rift Valley, climate change, usually drought, led to different regions being isolated from one another for considerable periods. This may have led to the separate evolution of different species of hominid, since between five and two million years ago there was a number of these different species, some living at the same time and others being descendants. This diversification, with closely linked species living side-by-side for considerable periods before some died out and others continued to evolve may have been a significant evolutionary mechanism. This would further complicate the prediction of the relationship between the very few examples the archaeologists have discovered.

Returning to around 3 to 4 million years ago, a bi-pedal, probably vegetarian species, Australopithecus Afarensis, lived that may well have been our ancestor, or at least who lived at the same time as an, as yet undiscovered, early ancestor of ours. Rows of clearly bi-pedal footprints of Afarensis have been found in the African Rift Valley, close to a volcano that was erupting at this time. A layer of volcanic ash had fallen that was walked through by these ancient ancestors. Rain turned the ash into a material similar to concrete, and further layers of ash protected the casts so they survive today. The method of locomotion of Afarensis was unlikely to be the same as modern humans. The physical development of hominids to accommodate their new posture and lifestyle took hundreds of thousands of years. It was not a simple linear process, but fossil evidence shows various intermediate stages. Afarensis had evolved modifications to its pelvis, hip, knee and elbow for upright motion, but its shoulders, hands and feet were apelike, so it was probably as comfortable swinging in trees as on the ground.

Hominid Evolution - source Wikipedia p b works

A more recent species, Australopithecus Africanus, lived 2.5 to 3 million years ago and was likely to be a direct descendant of Afarensis. Two to 2.5 million years ago Homo Habilis appeared in East Africa, possibly as a descendant of Afarensis or Africanus. This was the earliest species of the genus Homo (the humans). The brain size of Afarensis was similar to modern apes, viz. 450cc. However, Homo Habilis had a larger brain size, approximately 750cc, about a third of the way between Afarensis and Modern Man’s at 1350cc.

Hominid Brain Size - source GoDaddy.com

Homo Habilis probably had also evolved a very important modification to the inner ear to enable them to balance properly upright. They started to eat some meat as well as fruit and nuts, and this variety and higher quality food no doubt had led to better survivability and greater reproductive efficiency. Their teeth were evolving to handle the switch from ape’s fruit diet to a harder one requiring grinding teeth. Being omnivorous was a significant advantage as it made the possible descendants of Homo Habilis very adaptable to the environment, being able to eat meat or vegetable food in any ratio. It is believed Homo Habilis began the craft of systematic tool making 2.5 million years ago, splitting stones to produce simple but sharp edged flints, which no ape had done before. The tools they made were used to cut meat, wood and plant material. These earliest toolmakers also appear to have been predominantly right handed, as we are today, but apes do not show this characteristic. So some of what we now consider to be modern human traits were evident very early on. Many present-day traits may be linked through genes developed in pre-history. For example, tendencies such as curiosity and young males wanting to “break away” and gain their independence may partly explain the capability for rapid geographic dispersal, enhancing survivability in rapidly changing climates and enabling the maximum exploitation of resources.