Our Favorite Ape: Homo sapiens

Terms to know:

• anthropoid - of or pertaining to monkeys and apes
• hominoid - of or pertaining to the great apes (including humans)
• hominid - member of family Hominidae

It is correct to say only that humans, apes, and monkeys all diverged from a common anthropoid ancestor, evolving into different species due to natural selection and the other factors that can drive macroevolution.

• Multiregional Hypothesis:
  Homo sapiens evolved in each of the regions where its fossils are now found from ancestral Homo erectus that migrated out of Africa about 1.5 million years ago.
  Advocates of this hypothesis consider H. erectus to be an early version of H. sapiens, and not a different species. Constant interbreeding between neighboring populations of this "archaic" Homo sapiens may have prevented reproductive isolation, resulting in our present-day races of Homo sapiens, rather than multiple species of Homo.
• "Out of Africa" Hypothesis:
  All H. sapiens now living evolved from a second major migration out of Africa that occurred about 100,000 years ago, and not from wandering Homo erectus. These later migrants replaced the descendants of the earlier H. erectus migrants.
  So far, DNA analyses have supported the "Out of Africa" ("Replacement") hypothesis. But as any hypothesis, this one is subject to further testing as new analytic methods become available.

What makes humans different from the other great apes?

• Brain size
  Hominoids of 6 million years ago had brains of about 400 - 450cm². This is about the same as modern chimpanzees. Modern humans' average brain volume is 1300cm². This threefold increase in volume is associated with cultural trends such as development of complex language
• Jaw shape
  Anthropoids and ancient hominoids have prognathic jaws: the upper and lower jaws protrude beyond the nose. Recall that the human face is paedomorphic with respect to that of other modern apes. The face is flatter, the prognathous jaws lost. Changes in dentition accompanied this change in jaw shape.
• Bipedal posture
  Ancestral anthropoids and some of the earliest hominoids walked on all fours, though--like modern apes--they could probably walk on their hind legs with some degree of balance and skill. Humans are different from all other apes in that the body posture is fully upright and locomotion is entirely bipedal. A number of skeletal and muscular modifications make this possible, but there is still a great deal of academic argument about why humans became bipedal.
• Reduced sexually dimorphic size differences
  In orangutans and gorillas, the male weighs about twice as much as the female. In chimpanzees and Bonobos, males weigh about 1.4 times as much as females. In humans, the difference is still less, with males averaging 1.2 the body weight of females. (Why do you suppose this is the case? HINT: social structure is a major selective factor here.)
• Key changes in family and other social structures
  • Gibbons are social, with a dominant male defending a group of females from other male rivals.
  • Orangutans are solitary and do not form permanent social groups
  • Gorillas are social, with a single "silverback" dominant male getting all the mating opportunities with the females in his band. Immature and subordinate males are allowed to stay in the group, but do not get many mating opportunities.
  • Chimpanzees are social and promiscuous. When a female comes into estrus, all males will attempt to mate with her, and she will mate with multiple males.
  • Bonobos will have sex with anyone who holds still long enough.
  • Hunter-gatherer human societies may be polygamous or polyandrous. But in most human societies, monogamy is the norm. (But is it biologically programmed?)
• Young are even more altricial than other great apes' young.
  Newborn humans are exceptionally dependent on their mothers, and parental care lasts longer after birth than in other ape species. This extended period, coupled with the enlarged brain, enhances learning and is one factor that contributes to the behavioral complexity of the human animal.