Charles Darwin in the 'Origin of Species' gave a lilechanism for the evolution of plants and animals in general, even though he had speculated about the origin of man. Four years later, he published the "Descent of Man", in which he speculated that like other animals, man too had evolved form pre-existing living forms. Darwin was greatly impressed by the similarities in the bodily structure of man and thegreat African apes, the Chimpanzee and the Gorilla.
From this, he speculated on the location of man's origins. In his own words, "In each great region of the world, the living mammals are closely related to the extinct species of the same region. It is, therefore, probable that Africa was formerly inhabited by extinct apes, closely allied tothe gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more than probable, that our early ancestors lived on the African continent than elsewhere". Darwin had noted that embryos, i.e. early unborn young ones, of different organisms pass through very similar stages. However, small alterations in the timings of events in early development might produce a substantial change in the mature organism; for example in many ways adult humans are like juvenile apes, their small faces and globular cranium bones enclosing the brain are indicative of this.
A crucial step in human evolution, enlargement of the brain, can be seen as a result of the slowing down of development in the embryo of an ape-like ancestor. Instead of stopping at birth, brain growth continues well into childhood, eventually producing a much larger and more complicated piece of mental machinery.