FIGURE 2. The known fossil record of hominids, including S. tchadensis, also showing ourselves (top left) and the chimpanzee (top right).

Extinct species are indicated with the dates of the earliest and latest fossil evidence, but these are likely to increase and decrease, respectively, especially for the less well-known examples. Species are assigned to one of four categories, based on brain and cheek-tooth size, and inferred posture and locomotion (we are obligately bipedal; facultative bipedalism is the ability to walk or run on two legs, or as a quadruped, according to circumstances). A fifth category is for 'insufficient evidence'. The species marked with an asterisk were all unknown a decade or so ago, an indication of the paucity of evidence, until recently, of hominid evolution between 1 and 4 million years ago. This comparatively rich record contrasts with the earlier part of the hominid fossil record. There are likely to be many 'undiscovered' species in the fossil record between 7 and 4 million years ago, and in reconstructing the early stages of human evolution in particular the incompleteness of data should always be acknowledged.