Humans are primate mammals; closely related to chimpanzees and gorillas, and distantly to monkeys and lemurs. Fossils imbedded in sedimentary layers dating back to the early Cenozoic Era confirm what Darwin, comparative anatomy, and DNA analyses tell us -- our closest relatives are chimpanzees, and our most distant ones are lemuroids.

Most primates live in trees and share anatomical characteristics that favor arboreal existence. Forward-facing eyes have overlapping lines of sight, enabling accurate distance perception when moving rapidly through trees. Long fingers and toes can grasp branches with all four limbs to prevent falling. Long arms, short bow-legs, and a concave spine curvature enhance safety in leaping between trees by enabling all four grasping appendages to aim for a particular branch.

A precursor to the primates was a squirrel-like mammal, Plesiodapis, that lived about 60 mya (million years ago) and used sharp claws on all four feet to get around in trees. The earliest known primate was a lemuroid, Notharctus, that lived 55 mya in North American forests (dates herein are all approximate and rounded off). Most lemuroids retain primitive divergent lines of sight for peripheral vision, but which inhibit accurate distance perception and compel slow movement through trees. Aegyptopithecus, a monkey with overlapping lines of sight and ability to move rapidly through trees, evolved 35 mya. Proconsul, a primitive chimpanzee, appeared 24 mya. Sivapithecus, an early orangutan (closely related to the African gorilla) lived 12 mya in Asia and Europe. Humans emerged in Africa about 4 mya as a successful divergent line from arboreal chimpanzees, whose DNA shows a 98 percent match with humans.

Two scenarios are possible for the early evolution of humans. In one, shrinking forest areas due to climate changes may have caused overcrowding in the trees that forced many arboreal chimpanzees out and onto a ground environment for which they were not well adapted. Another had the chimps so well-adapted to a stable tree environment that births continually exceeded deaths and created overpopulation stresses which compelled some chimps to abandon their former Eden. Most died on the ground, but the few that endured were those who had experienced chance mutations rendering them unfit for a tree environment, but transforming them to humans well suited for ground survival. Short arms with retained grasping fingers, long straight legs with a walking foot and short toes, and a reversed lower spine convex curvature enabled upright posture and efficient locomotion on the ground. One negative mutation that humans bore was shrinkage of the fangs to short canine teeth that were useless in combat against predators or prey. However, the concurrent mutation of an enlarged brain able to think of using a weapon (a rock, branch, or large bone), more than compensated for the loss of fangs.

The earliest human may have been Australopithecus, a short omnivorous biped who used rocks and bones 4 mya as tools for combat or digging, and who had a brain size (cranial volume) of 500 cc. Paranthropus, a herbivorous human, had a 530 cc. brain size 3 mya and huge molars with a skull crest-bone to anchor additional powerful jaw muscles needed for chewing through tough cellulose cell walls of plants. It used antelope horns to dig for roots, and from fang marks on a skull, served as prey for leopards.

The genus name 'Homo' designates tool-making humans. Homo habilis, with a 750 cc brain size, chipped rocks 2 mya to form sharp edges or points for cutting or stabbing. Homo ergaster, with a brain size of about 800 cc, first used fire for warmth, light and protection against predators about 1.6 mya. Homo erectus had a brain size of 1,000 cc, and was the first human to leave Africa for Asia and Europe 1 mya. Homo heidelbergensis (1,300 cc brain size) lived 0.7 mya. Homo Neanderthalensis, with a 1,600 cc brain size, evolved about 200,000 years ago, and was larger and stronger than modern humans (Homo sapiens), who first appeared with an average 1,400 cc cranial capacity more than 100,000 years ago.

Although brain size alone may not correlate directly with intelligence, why Neanderthals died out about 25,000 years ago while sapiens lived on remains a mystery. Was it disease, warfare, inbreeding, interbreeding, or inability to cope with rapidly changing environmental conditions due to natural global cooling and warming shifts that weakened the Neanderthal genetic pool to the point of extinction? We do not know.