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 things affecting human evolution.

Humans are members of the order Primates which consists of about 180 species (there are 17 different orders of mammals which diverged 80-65 million years ago). Primates are a relatively old order of mammals. Most of the synapomorphies of this order are associated with an arboreal way of life: flexible digits, forward facing eyes, vision as a primary sense. These traits may have played a role in the evolution of brain size in the lineage leading to humans. Humans are a member of the family Hominidae which 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 mybp the Hominoids split off from the other old world monkeys. The dates are rough and get changed now and then.

Relationship of humans to African apes (= chimps, gorillas) and orangutan DNA hybridization indicates that apes are our closest relatives. Human/chimp/gorilla relationships not proven but chimps are most likely our closest relatives. The molecular clock says ~ 5 million years ago the human-chimp line split.

While Chimp and gorilla have knuckle walking , the humans posses many traits associated with bipedality: vertebral column, shape of pelvis, angle of femur, foramen magnum at base of skull. Bipedality seems to be a major "innovation" which allowed humans to enter a 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 commonly considered 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 acknowledged these four steps, but put them in a different sequence during human evolution.

Origin of Homo sapiens: Australopithecus afarensis = first bipedal hominid, found in east Africa about 3.0-3.2 MYBP. Later forms became more slender (= "gracile"). Homo habilis and H. erectus (~1.5mybp) came later. The evolution of bipedalism may have freed the hands for us in other functions: carrying, tool use. The trends in the evolution of tool use (more types, more specific tasks, different types of materials, more efficient use of materials) seems to follow (lead??) the evolution of increase cranial capacity. These both seem to increase noticeably about 2 mybp. One theme that involves each of the different sequences of evolution is that there was some feedback that lead to the increase in cranial capacity, e.g., becoming bipedal creates selection pressure for a more elaborate brain to control motor function and to process incoming sensory information. This in turn would allow for more successful bipedalism, etc. The same argument could be leveled about culture leading to an increase in brain size, and vice versa, so the sequence cannot be resolved just on the logic of feedback loops alone.

Origin of "modern humans": Two alternative scenarios for origins: 1) humans originated in more than one site ("Multiregional" model). Evidence supporting this are modern Homo sapiens samples found in Asia and Africa 2) a single origin ("Noah's Ark" model: one origin and dispersal out from site of origin). Homo sapiens are 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 diversity in Africa, aslo suggsting an African origin. Recent re-analyses shown that the cladograms of mtDNA cannot support an African origin on statistical grounds. Moreover, some recent fossil finds have put humans outside Africa about 2.4 MYBP, but these may be due to early migrations. However, three independent, recent articles in Nature (March 31, 1994; vol. 368, pgs. 449-457) all support an African origin for humans; two are based on fossil analyses and one is based on DNA analyses of microsatellites (next lecture).

The analysis of the evolution of culture and civilization in humans clearly must be based in materials other than human bones alone. The evolution of tools is one reliable correlate (they are recognizable as being rocks reworked as tools and, being rocks, they preserve well). The patterns of tool form show some suggestive trends regarding civilization: through time more types of tools become apparent and there is less variation among specimens in the shape/form of a given tool (see figure). This has been interpreted as evidence for communication or "training", since 'word may have spread' on just how to improve that stone ax so that it can be used more effectively for certain tasks.

The spread of Homo out of Africa is presumed to have taken place about 1.5 MYBP by Homo erectus. This species seems to be on a trajectory of brain size and body size that looks anagenetic, whereas one lineage that 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 Homo, to the Neanderthal to the modern American family standing in their driveway 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 provided by more than one distinct morphological type of early humans present at the same time (see below). As time gets closer to modern humans, however (Homo erectus on up), a phyletic gradualist anagenesis is more easy to accept.

Once a big brain is achieved and this provides the intellect for an organism to anticipate its environment, the notion that an organism evolves in response to changes of the environment becomes too simplistic. Humans evolved the power to alter their environment so as to protect themselves from its abiotic pressures. This means that they are altering their own selective pressures and a dialectic emerges between the organism and the environment such that these cannot be separated. Other organisms do this (beaver dams, deciduous trees), but in humans this cycle is accelerating. The rest is history.