Human Evolution:

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.