Humans would not have evolved if the ancestors of the African great apes had not. The ape fossil record begins 23 million years ago with the earliest putative apes, including Morotopithecus and Proconsul (Figure 1), from sites in East Africa, followed by many others throughout Africa, Europe and Asia. Although this record is fairly rich, it has done no better than DNA-based estimates at helping researchers to determine how living apes are related. Genetic studies estimate that gorillas split off from other apes about 9 million to 8 million years ago, and that the ancestors of bonobos and chimpanzees began evolving separately from the ancestors of humans 7 million to 6 million years ago.
Comparative anatomy, physiology, behaviour and genetics provide enough evidence for us to understand that humans are more closely related to chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) than to any other species, and vice versa. But the fossil record of hominins (species more closely related to humans than to chimps) preserves snapshots of the how the evolutionary path of our lineage differs from theirs. Unfortunately, the fossil record of chimpanzee and bonobo evolution is small enough to fit into a coat pocket, but the fossil evidence for human evolution is far greater: there are hundreds of specimens, including many nearly complete skeletons and many well-preserved skulls. Although the hominin fossil record is dominated by durable teeth — which reveal diet, age of death, pace of growth and much more — here we will focus, briefly, on the tales of two other significant human traits that are well documented in the hominin lineage: our big brains and our bipedal bodies.
Of course, humans are not the only animals to have extremely large brains for their body sizes (to be highly encephalized). Witness the octopus and the squid — members of the cephalopod class — and, among mammals, the toothed whales, or odontocetes. The African great apes also have large brains, but humans, as the sole surviving hominin, are considered to be the most encephalized. Nor are humans the only animals to walk habitually on two legs. Birds and many of their extinct dinosaur relatives are just some of the many bipeds that have roamed, and continue to roam, Earth. But although many primates, especially the African great apes, frequently walk on their hind limbs — particularly when carrying objects, while moving about the trees and during bouts of threatening or playing — humans are the only ones to be dedicated to this mode of locomotion” (read more).
(Source: Palaeontology Online)