The split with apes is believed to have occurred in Africa. A possible
environmental change that reduced forests and increased plains may have
forced quadrupedal creatures (walking on all fours) to venture into the
open in search of food. In time, natural selection led to bipedal motion
(walking upright). Preserved footprints indicate that this occurred at
least 3 million years ago. The conversion from walking on all fours to
walking upright required a significant redesign of the body (longer legs,
a basin-like pelvis, a curved spine, an arched foot with toes, new
muscles) that would have evolved only if it resulted in major advantages.
The conversion to upright movement may eventually prove to be the key to
human intelligence. Although walking upright was a handicap in escaping
predators, there were substantial compensations.
The raised head allowed a greater range of vision. A larger larynx could
be accommodated, allowing the development of speech. Speech, in turn, led
to rational thought, a larger brain, and the development of culture. The
hands were free for carrying weapons and other objects. This may have led
to the opposable thumb, which allowed a precision grip rather than the
power grip of the apes. The precision grip increased dexterity and was
crucial to human cultural development. Thereafter, the brain, the hand,
and the eye evolved somewhat together.
Natural selection favored the ability to think, reason, and anticipate
events and actions, the primary advantage of humans over other species
Hominids spread from Africa into the neighboring continents of Europe and
Asia. From these African roots different human species evolved, although
only Homo sapiens has apparently survived to the present. Humans spread
eventually across oceans to Australia perhaps 60,000 years ago and by land
bridge to the Americas perhaps as early as 40,000 years ago.