This paper presents a model for the nature and adaptive significance of intelligence and language in early hominids based on comparative developmental, ecological, and neurological data. We propose that the common ancestor of the great apes and man displayed rudimentary forms of late sensorimotor and early preoperational intelligence similar to that of one- to four-year-old children. These abilities arose as adaptations for extractive foraging with tools, which requires a long postweaning apprenticeship. They were elaborated in the first hominids with the shift to primary dependence on this feeding strategy. These first hominids evolved a protolanguage, similar to that of two-year-old human children, with which they could describe the nature and location of food and request help in obtaining it. The descendents of the first hominids displayed intuitive intelligence, similar to that of four- to seven-year-old children, which arose as an adaptation for complex hunting involving aimed-missile throwing, stone-tool manufacture, animal butchery, food division, and shelter construction. The comparative developmental and paleontological data are consistent with the hypothesis that the stages of development of intelligence and language and their neural substrates in our species recapitulate the stages of their evolution.
Two competing philosophical paradigms characterize approaches to the evolution of the human mind. One postulates continuity between animal and human behavioral capacities. The other assumes that humans and animals are separated by major qualitative behavioral and mental gaps. This paper presents a continuity model that suggests that expanded human mental capacities primarily reflect the increased information processing capacities of the enlarged human brain including the enlarged neocortex, cerebellum, and basal ganglia. These increased information processing capacities enhance human abilities to combine and recombine highly differentiated actions, perceptions, and concepts in order to construct larger, more complex, and highly variable behavioral units in a variety of behavioral domains including language, social intelligence, tool-making, and motor sequences. Environmental input, including self-generated input, interacts with mental constructional capacities to assure that developing humans acquire species-typical and culturally-specific behavioral patterns. This mental constructional model is compatible with our current understanding of differences between human and non-human primate brains, of human brain plasticity, and of the minimal genetic differences between humans and chimpanzees.
Dental reduction has been sufficiently widespread among human populations to render the phenomenon of reduced tooth size worthy of scientific explanation. One of the most controversial models invoked to explain structural reduction in organisms is referred to as the "probable mutation effect" (PME). According to this model, structures no longer functional owing to ecological or cultural changes will experience a relaxation of selection pressure, permitting an accumulation of mutations in the population that inevitably will result in the reduction in size or the loss of the concerned structure. Although the PME continues to be offered as a viable explanation of human dental reduction, it is based upon several premises that modern dental clinical experience fails to support. Known enzyme defects resulting from mutations, factors predisposing to dental infections, and the deleterious effects of teeth that are too large or too small reveal that the PME does not logically account for the reduction of tooth size. Given such information, this paper proposes models of dental reduction based upon natural selection, which, unlike the PME, are testable in both modern and archaeological populations. The integration of clinical and skeletal data permits a more thorough understanding of dental reduction in the hominid fossil record.
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