We want to find a simple logical progression for primate and human evolution, but evolution was a very messy process. ~ American anthropologist Brenda Benefit
Proconsul was a hominoid that lived 23–5 MYA. There were at least 2 Proconsul species, possibly 4. The species had distinct body sizes. One was as small as a gibbon. Another had the girth of a female gorilla.
The frugivorous Proconsul was built for an arboreal existence. The forest was so dense that the canopy was closed. A Proconsul could travel through the trees without ever touching the ground.
Most of Proconsul‘s features were monkey-like, including its brain and gait. But it had much of a human hand: similar thumb joint and overall proportions, thus able to manipulate small objects precisely – the same as humans. When a chimp tries to pick up a small object, it has to hold the item between its thumb and the side of its hand, rather than between fingertips, as Proconsul did, and as humans do now.
Proconsul was a hominoid offshoot. It is therefore only illustrative of hominid radiation and the numerous adaptations that would result in hominins.
Independent evolution of similar features is a common phenomenon, but it seems to be particularly rampant in primates. ~ Spanish paleontologist David Alba
Weighing around 33 kg, Pierolapithecus lived 13 MYA. Its limbs were different from modern apes. Its hands lacked the long, curved fingers that allowed easy suspension in trees. Its pelvis gave greater stability for moving using its forelimbs. Pierolapithecus illustrates a transition in lifestyle between living in trees and on the ground – the 2 different directions arboreal apes and hominids took.
A gibbon-sized (4–5 kg) hominoid lived 11.6 MYA that had a mosaic of ape and hominid features. Pilobates was a cautious climber, fond of fruit.
14–8 MYA in Africa apes and hominids evolutionarily parted ways. (The date range owes to a discrepancy between genetic mutation estimates (earlier) and the fossil record (later). Further, that there was a common ancestor is likely a simplification. DNA data indicates that hominids diverged from apes 14 MYA. But genetic molecular clocks are somewhat unreliable in assumptions about mutation rate. Statistical modeling based upon both fossils and genetics points to divergence 8 MYA.) Orangutans were the first to split from the clade that descended to humans, followed by Dryopithecus, an extinct ape; then gorillas, and finally chimpanzees and bonobos.
Dryopithecus lived in Africa and Eurasia ~12.5 MYA, during the Late Miocene. It had a large brain and delayed development. Dryopithecus spent most of its time in the trees, where it ate fruit and soft leaves.
The supposed last common ancestor of apes and hominids was small, weighing some 5.5 kg. Small size continued until the arrival of Homo erectus, long after hominids had taken to living on the ground rather than in the trees.
The snapshots of evolutionary events do not make an altogether coherent movie, but the picture of hominid emergence is fairly clear: progressively erect posture, changes in the brain, loss of body hair, and refinements in manual dexterity.
Hominoids underwent several adaptive radiations, producing a great abundance and variety of species. Climate change greatly reduced hominoid habitat through the Late Miocene and was probably responsible for the drop in the diversity of hominoids. ~ Roger Lewin & Robert Foley
Climatic pulses spurred adaptation as well as foreclosing some species to extinction. As habitats and food resources became more variable, more generalist species emerged, better able to cope with changing conditions. Hence erratic climate was a primary driver of hominid descent.