Lecture 23. The Evolution of Hominids
1. Hominoids: Apes and Humans
i. the living apes are chimpanzees, gorillas, orangutans and gibbons (apes have larger brains than monkeys, and lack tails)
ii. gorillas and chimpanzees are the closest ape relatives to humans; humans and chimps appear to be more closely related than are chimps and gorillas
2. The 2 major characteristics that set humans apart from the apes:
I. Larger brain size
II. Bipedalism = walking upright on the hindlimbs - has led to anatomical changes in all parts of the body:
- foot (no opposable big toe)
- longer leg bones
- knee with straight hinge
- short, bowl-like pelvis
- S-shaped backbone (C-shaped in apes)
- skull on top of vertebral column instead of at the top (different angle of attachment of the spinal column to the skull (foramen magnum)
3. The family Hominidae (Hominid = upright walking primate) See Figure
i. Hominids diverged from the chimpanzees 5-6 mya
ii. Ardipithecus ramidus - 4.4 million years old - the most primitive known hominid, considered to be near the base of hominid phylogeny (discovered in 1994)
ii. Australopithecus anamensis - ~ 4 million year old species which appears to be a "missing link" between Ardipithecus and "Lucy" (published in 1995)
iii. Australopithecus afarensis - "Lucy" skeleton found in 1974, aged at ~ 3.6 my old
- creature 3.5' tall; walked fully upright
- skull more apelike than human (forward thrust jaw; brain size ~ 400 cc (modern human brains = 3x that size)
- some A. afarensis found since date back to 3.9 mya
- "Son of Lucy" = A. afarensis skeleton found in 1992, dated at 2.9 - 3 mya, therefore the species is known to have persisted for almost 1 my, with little morphological change
iv. 2-3 mya there were several distinct hominid species living sympatrically in eastern Africa
4. Evolution of the genus Homo
i. a population of Australopithecus africanus or a very similar species appears to have diverged to form the genus Homo ~ 2.5 mya (Homo then lived concurrently with Australopithecus for ~ .5 my)
ii. major changed during the evolution from Australopithecus to Homo:
1. increase in body size
2. doubling in brain size
iii. oldest Homo = Homo habilis (~ 2 my old) - wide ranging in the dry savannahs of Africa
- used tools
iv. Homo erectus also evolved in Africa, then migrated north and east to eastern Asia (reached central China 1.7 - 1.9 mya)
- used fire; hunted wild animals; made a variety of stone tools
- survived in Eurasia until 250,000 mya, but in Africa it was displaced by Homo sapiens ~ .5 mya
5. Evolution of Homo sapiens ( = "wise man")
• decrease in size of teeth
• increase in brain size (up to ~ 1500 cc, then decrease)
- language
- social interactions
ii. Principle of Neoteny in human evolution? (juvenile stages of ancestors become the adult features of descendants)
- small face, vaulted cranium and large size of brain in proportion to body size, no opposable big toe, foramen magnum under the skull, distribution of hair
- retain mental flexibility - humans are learning animals
iii. Homo ergaster ~ 1.6 my old fossils from East Africa, close to H. erectus
iv. Homo antecessor - 800,000 years old, found in Spain
- face as in modern humans, but also some primitive features, such as a prominent brow ridge and some dental features
v. Homo heidelbergensis - 0.5 my old, found in Africa and Europe
vi. Homo neanderthalensis - widespread in Europe and Asia 130,000 - 30,000 yrs ago
- brain larger than in modern humans
- lived concurrently and sometimes sympatrically with H. sapiens, but unlikely that the two interbred
vii. Homo sapiens
- more sophistocated tools; explosion of art, symbolism, and cultural complexity
- inhabited Asia, and entered North America via the Bering Land Bridge
vii. Hypotheses for the evolution of Homo sapiens: (Fig. 34.35)
Multi-Regional (= Regional Continuity) Hypothesis: all modern human groups are descendants of Homo erectus or H. ergaster, but populations evolved in different regions, exchanging enough genes with neighbors to all remain the same species, eventually H. sapiens
Monogenesis (= "Out of Africa" or "Single Origin") Hypothesis: H. sapiens descended from a single ancestral population that emerged in one location (probably Africa)
