HUMAN EVOLUTION
Humans are too complex to be "understood" by any one field.Thus we will look at a few major steps in evolution and some of the thingsaffecting human evolution.
Humans are members of the order Primates which consists of about 180species (there are 17 different orders of mammals which diverged 80-65million years ago). Primates are a relatively old order of mammals. Mostof the synapomorphies of this order are associated with an arborealway of life: flexible digits, forward facing eyes, vision as a primarysense. These traits may have played a role in the evolution of brain sizein the lineage leading to humans. Humans are a member of the family Hominidaewhich is believed to have diverged about 5 million years before the present(mybp) from the other members of the Old world monkeys. At least 20 mybpthe Hominoids split off from the other old world monkeys. The dates arerough and get changed now and then.
Relationship of humans to African apes (= chimps, gorillas) and orangutanDNA hybridization indicates that apes are our closest relatives.Human/chimp/gorilla relationships not proven but chimps are most likelyour closest relatives. The molecular clock says ~ 5 million yearsago the human-chimp line split.
While Chimp and gorilla have knuckle walking , the humans possesmany traits associated with bipedality: vertebral column, shapeof pelvis, angle of femur, foramen magnum at base of skull. Bipedalityseems to be a major "innovation" which allowed humans to entera new "adaptive zone". The first human (Australopithecus afarensis)seems to have an angle between the femur and tibia (Upper and lower leg)that is intermediate to that of humans and gorillas.
The evolution of modern humans from our hominid ancestor is commonlyconsidered as having involved four major steps: evolving terrestriality,bipedalism, a large brain (encephalization) and civilization.There are (and have been) several competing hypotheses that have acknowledgedthese four steps, but put them in a different sequence during humanevolution.
Origin of hom*o sapiens: Australopithecus afarensis = first bipedalhominid, found in east Africa about 3.0-3.2 MYBP. Later forms became moreslender (= "gracile"). hom*o habilis and H. erectus(~1.5mybp) came later. The evolution of bipedalism may have freed the handsfor us in other functions: carrying, tool use. The trends in the evolutionof tool use (more types, more specific tasks, different types of materials,more efficient use of materials) seems to follow (lead??) the evolutionof increase cranial capacity. These both seem to increase noticeablyabout 2 mybp. One theme that involves each of the different sequences ofevolution is that there was some feedback that lead to the increase incranial capacity, e.g., becoming bipedal creates selection pressure fora more elaborate brain to control motor function and to process incomingsensory information. This in turn would allow for more successful bipedalism,etc. The same argument could be leveled about culture leading to an increasein brain size, and vice versa, so the sequence cannot be resolved juston the logic of feedback loops alone.
Origin of "modern humans": Two alternative scenariosfor origins: 1) humans originated in more than one site ("Multiregional"model). Evidence supporting this are modern hom*o sapiens samplesfound in Asia and Africa 2) a single origin ("Noah's Ark"model: one origin and dispersal out from site of origin). hom*o sapiensare believed to have originated ~100,000 - 200,000 years ago.
Paleontological evidence suggests a single origin in Africa.Molecular data shows low genetic diversity worldwide with the highest diversityin Africa, aslo suggsting an African origin. Recent re-analyses shown thatthe cladograms of mtDNA cannot support an African origin on statisticalgrounds. Moreover, some recent fossil finds have put humans outside Africaabout 2.4 MYBP, but these may be due to early migrations. However, threeindependent, recent articles in Nature (March 31, 1994; vol. 368, pgs.449-457) all support an African origin for humans; two are based on fossilanalyses and one is based on DNA analyses of microsatellites (next lecture).
The analysis of the evolution of culture and civilization in humansclearly must be based in materials other than human bones alone. The evolutionof tools is one reliable correlate (they are recognizable as being rocksreworked as tools and, being rocks, they preserve well). The patterns oftool form show some suggestive trends regarding civilization: through timemore types of tools become apparent and there is less variationamong specimens in the shape/form of a given tool (see figure). Thishas been interpreted as evidence for communication or "training",since 'word may have spread' on just how to improve that stone ax so thatit can be used more effectively for certain tasks.
The spread of hom*o out of Africa is presumed to have taken placeabout 1.5 MYBP by hom*o erectus. This species seems to be on a trajectoryof brain size and body size that looks anagenetic, whereas one lineagethat lead to Australopithecus robustus seems to be on another line.In a broad sweep of time, the notion of the chimp leading to the Australopithecine,to hom*o, to the Neanderthal to the modern American family standing in theirdriveway is a myth. There were lineages that diverged in a branching cladogram,some of which did not make it to the present. Evidence for this is providedby more than one distinct morphological type of early humans present atthe same time (see below). As time gets closer to modern humans, however(hom*o erectus on up), a phyletic gradualist anagenesis is more easyto accept.
Once a big brain is achieved and this provides the intellect for anorganism to anticipate its environment, the notion that an organism evolvesin response to changes of the environment becomes too simplistic. Humansevolved the power to alter their environment so as to protect themselvesfrom its abiotic pressures. This means that they are altering theirown selective pressures and a dialectic emerges between theorganism and the environment such that these cannot be separated. Otherorganisms do this (beaver dams, deciduous trees), but in humans this cycleis accelerating. The rest is history.
