Week 2: The Earliest Hominids (our first 4 million years)
I. Climatic Overview of the Cenozoic Era
A. Epochs of the Tertiary Period1. Paleocene: 65-53 mya: Angiosperm forests now common for first time. Selection pressures for efficient foraging in trees begin to differentiate primates (only prosimian-like at this point) from other mammals:a. hands well suited for grasping & manipulating b. enhanced vision (depth perception, acuity, color) c. behavioral flexibility: learn & remember identity & locations of edible plants: most primates are selective feeders2. Eocene: 53-38 mya. NW & OW monkeys share last common ancestor. 3. Oligocene: 38-25 mya: Apes & OW monkeys share last common ancestor towards end of this period or early Miocene. 4. Miocene: 25-5 mya: Lesser & greater apes differentiate 20 mya. Last common ancestor between chimps and hominids lived between 8 & 6 mya: "a leggy Miocene midget" & arboreal biped emphasizing vertical climbing & standing on branches; probably had grasping feet; not a knuckle-walker (Wolpoff). Humans and chimps are "sister groups" meaning they share a more recent common ancestor than either shares with the gorilla. Earliest hominid fragment is a bit of lower jaw from Lothagam (Kenya), at about 5.7 mya. 5. Pliocene: 5-2 mya: Australopithecines, Homo. General cooling. Stone tool use, division of labor, & food sharing in Homo allow increased dietary selectivity despite decreased floral diversityB. Quaternary Period1. Pleistocene: 2 mya-12,000 ya (B. P.): repeated glaciations over much of N. hemisphere. Weather report: cold, windy, dry & dusty, but highly variable! next 2 million years 2. Holocene (Recent) 12,000-present: more stable & warmer conditions
II. How we differ from the apes
A. Walking (bipedalism): by 3.7 mya at Laetoli (& 4.4 mya at Aramis). Obligate bipedalism B. Precision grip; A. afarensis intermediate between chimps and Homo. C. Hairlessness (+/-); by H. erectus times? D. Dental1. Reduction of canines (beginning with afarensis) 2. Loss of diastema 3. Thicker enamel on cheek teeth (molars)E. Larger brains & handedness developing by 2 mya (symptom of lateralization of brain?) F. Tool use indispensable and complex (by 2.2-2.5 mya) G. Language & speech highly developed (by 100,000- 35,000 ya?)
III. The Early Hominids. Dart, 1924. "Osteodontokeratic culture." The Killer Ape?
A. Newest oldest hominids:1. Ardipithecus ramidus: 4.4 mya. Aramis, Ethi- opia. ('rami-' = 'root' in Afar. Smaller cheek teeth and thinner enamel than later A. a. suggest frugivory? (Modern chimps have thin enamel). Cranial base suggests upright stance and pelvis may indicate bipedalism. 17 individu- als? Same size orsmaller than most other aus- tralopithecines. Long forelimbs, big canines, small premolars and small deciduous molars; but overall size increase from late Miocene precursor (?). (White, Suwa, & Asfaw 1994) 2. Australopithecus anamensis: 4.2 - 3.9 mya (Meave Leakey, Tim White, Alan Walker), from two sites near Kenya's Lake Turkana. ('anam' = 'lake' in Turkana. Tibia (shinbone) articulation with knee indicates bipedalism. Looks alot like later A. afarensis.B. Australopithecus afarensis: 4.2-3.0 mya. Lucy (1974); Laetoli; Hadar (skull, 1994). Known only from E. Africa. Obligate biped.1. contrast with modern Akka. Apelike in skull form, premolar dentition, most limb proportions, and may still have spent a sig- nificant amount of time in trees; adept climber. 2. bipedalism & reduced canines (vs. enlarged brain or technology) as leading the way (along with probably size increase) to homin- ization.C. Australopithecus aethiopicus (2.5 mya), type specimen WT 17000 ("Black Skull"; Walker & Leakey 1988). 410 cc adult male (very small brain); but most powerful chewing apparatus until later A. boisei. "Dead end." D. Later Australopithecines: Robust vs. gracile lineages1. Gracile line: A. africanus. Descendant of afarensis? Known only from S. Africa (Trans- vaal). Mean brain size 440 cc; less massive face, smaller cheek teeth, beginnings of a forehead. 2. Robust line (A. [Paranthropus] robustus, A. boisei): petite size, massive jaws & teeth, specialized for low-quality tough (high fiber) vegetable diet. Sagittal crest. (But not strict vegetarians?) Coexisted with Homo for over 1 my (from 2.5 mya to 1.2 mya). Evolutionary dead ends. 3. Social organization of Australopithecinesa. Females outnumber males b. Females much smaller c. Early hominids probably lived in large kin- related multi-male groups with females who were not kin-related. A social pattern like that of lions, with females working comun- ally to support a few, big, lazy, territory- patrolling males, would fit what we know.4. Roots of modern human male-female differences in sexuality, including --a. intrasexual competition generally much stronger among males; b. men incline towards polygyny c. men experience almost universal sexual jealousy toward their mates d. men tend to be more aroused by the sight of naked women than are women by the sight of naked men e. physical characteristics correlating with youth are much stronger determinants of sexual attractiveness in women than in men f. men are more predisposed to desire a vari- ety of sexual partners for the sake of variety