Source: Dissertation Abstracts International, Volume: 60-12, Section: A, page: 4497.

Director: Andrew P. Hill.

Habitual bipedal locomotion is currently considered the essential shared, derived character in defining the Hominidae. In contrast, before 1951 members of the Hominidae were defined using a strict gradational scheme that arbitrarily set some lower limit on cranial capacity below which hominid status was denied. The roots of this arrangement can be found in the pre-evolutionary organizing principle called the Great Chain of Being. All living things were arranged linearly along the Great Chain with the simplest organisms at the bottom and the most complex (i.e. perfect) at the top. Despite the advent of evolutionary thinking in the late 18th century, vestiges of the old system remained. A temporal component was introduced to the previously fixed Great Chain and the universe was viewed as an unfolding divine plan rather than the result of a single creation event. An outgrowth of the temporalized Great Chain was the principle of orthogenesis, which viewed the evolutionary process as directed and goal oriented.

Until 1925 apes and humans were considered two distinct grades of organization along the predetermined pathway towards Homo sapiens. Furthermore, it was assumed that grade classification mapped directly onto phylogeny, hence the terms human and ape were synonymous with the categories of hominid and pongid, respectively. Initially, paleontological discoveries supported this "natural dichotomy" since all known fossils could be comfortably categorized as apes or humans. When Raymond Dart described Australopithecus as an ape, nobody disagreed, and for most scientists this mandated that the new species was also a pongid and placed accordingly on the phylogenetic tree. Since evolution was seen as a directed process, it was to be expected that independent lineages would strive for the same goal. Australopithecus was therefore viewed as an evolutionary dead end, its human-like features merely paralleling the evolutionary changes occurring in the actual human lineage that remained to be discovered.

The australopithecines could not be accepted as human ancestors until a new perspective on the evolutionary process appeared. The "new synthesis" was developed by biologists, geneticists and paleontologists during the 1930's and 1940's but did not infiltrate the anthropological community until the early 1950's, most notably at the Cold Spring Harbor Symposium on the Origin and Evolution of Man in 1950. The adoption of evolutionary theory more in-line with that of biologists (sensu lato) led to the rejection of many formerly held "laws" such as orthogenesis and the irreversibility of evolution. Anthropologists now looked to adaptive shifts in variable populations rather than relying on typological grades in classifying groups of organisms. Additionally, this new theory embraced the idea of mosaic evolution in which early members of a lineage could display only a rudimentary likeness to later descendants. Within this new framework, small-brained bipeds were viewed as representatives of an early stage of human evolution.