Brain, Evolution of Primate

Cognitive thought processes that arise from consciousness are depicted as being an exclusive human characteristic. Reflected in the metaphysical views from Aristotle (384–322 BCE) to Rene Descartes (1596–1650), the philosophical implications for our species result in an unbridgeable chasm between our species and the rest of the animal kingdom. These [Page 401]geocentric and anthropocentric depictions of our species would be irrevocably damaged by the theory of organic evolution by Charles Darwin (1809–1882). With evidence refuting the traditional view of our species held by both philosophers and theologians, speculation as to our relationship with other primates began a critical point of inquiry. Inquiry into this relationship by Thomas Huxley (1825–1895) provided critical evidence in support of his pithecometra hypothesis, whereby he stated that our species is closer to the great apes (orangutan, chimpanzee, and gorilla) than the two lesser apes (gibbon and siamang). In further support of Huxley's hypothesis via embryology, Ernst Haeckel (1834–1919) concluded that not only does our species resemble other primates, but we share a common ancestor in the form of the illusive “missing link” that was speculated to be found in Asia. These scientific inquiries supported Darwin's claim that our species differs from other primates in degree and not in kind. These differences in degree can be extended to anatomical, physiological, and cognitive functions of the primate brain. Such bold statements would require evidence from the fossil record.

Though Darwin and Haeckel differed regarding the geographic location of human evolution (Darwin supported Africa, whereas Haeckel supported Asia), the search for fossil evidence resulted in the successful discoveries of fossil hominids in many areas of the world. Discoveries and interpretations by Davidson Black (1884–1934), Robert Broom (1866–1951), Raymond Dart (1893–1988), Eugene Dubois (1859–1940), Donald Johanson (1943–), and the Leakey family (Louis, Mary, Richard, and Meave) have contributed crucially needed evidence to aid in reconstruction our species' phylogenic past. Though the fossil record is far from being complete, the accumulated evidence can suggest an evolutionary descent as speculated by Darwin. Utilizing cladistic analysis, our species' phylogeny is speculated as having the following relationship: Australopithecus afarensis gave rise to A.africanus, which then split into two branches, one leading toHomo habilis and the other leading to A. robustus/boisei. Our species, whose descent is from the Homo line, can trace its origin back to these primitive hominid forms.

Our relationship to other primates extends back further than these hominid forms. The discovery of the fossil Proconsul indicates a common ancestor dating back 18 to 20 million years ago. Further back in time, around 40 million years ago, there was a split between New World and Old World monkeys. An additional split between Old World monkeys and apes had occurred around 35 million years ago. Of the great apes, it is speculated that there was a split that was ancestral to the orangutan around 16 million years ago and the gorilla split from the chimpanzees around 8 million years ago. The most recent split between hominids, the third chimpanzee, and both chimpanzees (common and pygmy) was as little as 6 million years ago. Supported by both immunology and molecular methods, the DNA hybridization indicates and vindicates the fossil record, whereby the human primate is closer to the chimpanzee and gorilla than any other primate. In fact, our species shares 98% of our DNA sequence with our chimpanzee cousins. However, it must be noted that sharing98% DNA does not confer or equate that our species is exactly the same; a similar misconception applied to the Darwinian model of evolution erroneously states that our species descended from the chimpanzee and not the fact that our species shares a common ancestor. Just as cranial capacity does not confer complexity, the sharing of common genetic material does not confer exact characteristic and behavior shared by all primates. Our species does resemble the chimpanzee (and other apes) in both in anatomy and physiology, same dental formula (2–1–2–3), close embryonic development, and a degree of mental capabilities and behavior. However, does 2% make a difference, not only in morphological features, but in the degree of complexity of the brain? Regardless of the percentage held to be in common, the evolutionary differences among the primate brain (in terms of reorganization) is evident; yet it does produce an affinity between our species and our cousin, the chimpanzee. The exact circumstances surrounding the evolution of primate brain is a point of speculation; however, certain anatomical and physiological features that coincide with cranial expansion may give the necessary indications as what factors may be responsible.

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