ABSTRACT
Chapter 5 is the final one in the Introduction to Evolution section. Extensively illustrated, it begins with Sahelanthropus ("Toumai"), an ape-like hominin from 7 million years ago. With a downward opening, oval-shaped hole in the bottom of the skull leading to the spinal column, Toumai was likely bipedal. Walking on two legs is a defining characteristic of our tribe.
Other hominin species from the Miocene epoch are covered, followed by those of the Pliocene (2.6-5.3 million years ago). "Lucy", the popular celebrity in the hominin lineup, was of the Australopithecus afarensis species. Members of it are believed to have left their footprints in Tanzania, graphically demonstrating advanced bipedalism. They were among the earliest stone toolmakers.
The Homo genus emerged about 2.4 million years ago in the early Pleistocene, as Homo habilis. This marked a surge in encephalization, i.e., brain size corrected for body size. Encephalization steadily increased through the middle and later Pleistocene.
Two million years ago, several hominin species coexisted on Earth. Now there is one. Our portion of the tree of life more resembles a well-pruned bush than a tree, with a single branch emerging from the top. Most likely the reason traces to competition due to similar environmental needs.
Only one species produced an explosion of cultural innovations, drawing on a diverse toolkit of cognitive processes. That was Homo sapiens, or in English, "wise man".
