While early efforts to codify a hierarchy of intelligence in animals were rife with personification and the attribution of thoughtfulness and intention where there may have been none, it has been recognized that some species perform complicated actions that require a "mind which has reached a stage of prediction" (Cope). This knowledge of oneself as a causative agent is the most elementary facet of the psychological capacity referred to as self-conception. (For the sake of convenience, this node will use the terms "knowledge of the self as a causative agent" and "self-concept" interchangeably.) A self-conceiving individual understands its effect on its immediate environment, and can mentally run through and choose from scenarios, based on their forecasted consequences. A convincing model for the evolution of this capacity examines the locomotion of the arboreal great apes and theorizes that, "self-conception evolved as a psychological mechanism to facilitate planning and execution of unusually flexible locomotor patterns" (Povinelli).
Experimentally, an animal that self-conceives can be recognized by its ability to recognize itself in a mirror, as opposed to reacting to a mirror image as it would to another individual. For instance, in one experiment, wild born chimpanzees, with no prior exposure to reflective surfaces, were kept in isolation for two days, and afterwards presented with mirrors. The chimpanzees initially responded by behaving socially towards the mirrors, but such responsiveness declined rapidly over a period of days, where after all the chimps showed unmistakable signs of self-recognition, "grooming parts of the body which would otherwise be visually inaccessible without the mirror, picking bits of food from between the teeth while watching the mirror image, visually guided manipulation of anal-genital areas by means of the mirror, picking extraneous material from the nose by inspecting the reflected image, making faces at the mirror, blowing bubbles and manipulating food wads with the lips while watching the reflection" (Gallup).
Orangutans show similar mirror self-recognition, as do humans, but gorillas, lesser apes, and monkeys do not (Povinelli). However, according to current phylogenetic understanding, the family of Hominidae contains the Genera Gorilla, Homo, Pan (chimpanzees), and, slightly less related to the others, Pongo (orangutans). That is, any common ancestor of the self-conceiving primates- humans, chimpanzees, and orangutans- must also be the ancestor of gorillas (Marks). Therefore, any model for the ultimate cause of the evolution of self-concept should be able to explain how the trait disappeared in gorillas, and how it persisted in humans.
In a typical forest canopy, a great ape must be able to climb tree trunks, cross gaps between trees, and acquire fruit and leaves located on thin branches. Due to its relatively large size, one of these apes cannot rest its weight on a single branch or vine, which would snap. And in locomotion the great apes have only a small margin of error, as due to their heavy weight falls are usually fatal. So, as opposed to arboreal monkeys, which walk or run quadripedally on top of their substrate, and the lesser apes, which divide their time about equally between orthograde brachiation (swinging along vines) and climbing, the arboreal great apes move through the trees using a mechanism called "clambering". Clambering makes use of great motility at both the hip and shoulder, and includes the suspension of body weight from the forelimbs and hind limbs. It is characterized primarily by the use of multiple supports (Cant), in a highly non-stereotyped- that is, non repetitive, technique that relies on an individual making subtle distinctions between the apparent position, length, and most importantly, strength, of branches. The orthograde (upright) position that a clambering animal usually occupies extends the inherent flexibility of this mechanism by allowing it to rotate safely around a trunk and vary its own orientation in space.
The arboreal great apes regularly negotiate a habitat that is unstable, unpredictable, and (due to gaps between trees) non-continuous, on a substrate that is breakable as a result of their body weight. This environment would have strongly selected for the ability to mentally simulate one's actions before actually performing them, for a personal agency that allows the individual to view itself as an object as well as a subject. The alleles generating this trait would increase in frequency in the ancestral population that this theory proposes. However, a species that deviates from the ecological conditions that led to the evolution of self-concept would need to somehow undergo continuous selection for the maintenance of that trait, or else genetic drift could reduce the trait's frequency. In this way, gorillas, which are almost entirely terrestrial, do not display self-conception. Their current locomotion consists of quadripedal knuckle walking, which is far less intricate than clambering. Humans, on the other hand, could have made evolutionary use of self-concept by incorporating it into the development of sophisticated tools, language, and cooperative gathering and hunting, so that the trait was not lost even after it was removed from its original context.
Bibliography:
- Cope, E.D. (1890). "The Evolution of Mind." American Naturalist, 24(286): 8999-913 and 24(287): 1000-1016.
- Cant, J.G.H. 1987. "Positional behavior of female Bornean orangutans (Pongo pygmaeus)." American Journal of Primatology, 12: 71-90.
- Gallup, G.G. Jr. (1970). "Chimpanzees: Self-Recognition." Science, 167: 86-87.
- Marks, J. 1991. "What's old and new in molecular phylogenetics. American Journal of Physical Anthropology, 85: 207-219.
- Povinelli, D.J., J.G.H. Cant (1995). "Arboreal Clambering and the Evolution of Self-Conception." Quarterly Review of Biology, 70(4): 393-421.
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