EVOLUTION OF THE REASONING HOMINID BRAIN
by
Dr. Herman T. Epstein
Abstract
"Cognition is readily seen to be connected to evolution through plots of the ratio of cranial capacity to body size of hominids which show two regions of sharply increasing ratios beginning at 2.5 and 0.5 million years ago - precisely the critical times inferred by the author from his study of tools. A similar correlation exists between current human brain growth spurts and the onsets of the Piagetian stages of reasoning development."
Commentary on "Archeology and cognitive evolution" by Thomas Wynn in Behavioral and Brain Sciences vol 25, pp 389-402
The first goal of the author's article is stated to be "to make a case for the relevance of archeological contributions to...the evolution of cognition." His analysis focuses on spatial cognition.
If the author could be satisfied with a more general aspect of cognition, there is a more readily demonstrated one based on changes in a parameter interpretable as directly revealing increases in brain structural and cognitive complexity.
Evidence for such brain changes come from the ratio of brain or cranial size to body size. Any increase in brain size disproportionately greater than the corresponding increment in body size reflects acquisition of novel brain structures and their derivative functioning.
The data in table 1 and figure 1 are taken from Tobias (1987), Hofman (1983), Bauchot et al (1972), Stephan et al (1981), and Jerison (1973). Although the body weights increase fairly steadily from 37 kg to 68 kg, the ratio remains constant over two long time periods. Thus, there are two periods with transitions to significantly increased ratios: from about 11.5 cc/kg for Australopithecines to 17.2 cc/kg for Homo erectus and from 17.2 cc/kg for Homo erectus to 22.9 cc/kg for Homo sapiens. Those increases in brain size are greater than needed just to sense and control any increased body weight.
Any greater-than-proportional increases in brain size should signal increases in the complexity of brain structure and functioning. Such an inference also pertains to the fact that the ratio for Australopithecines is substantially greater than that of P. troglodytes, indicating a significantly augmented brain functioning for the first of the hominids.
The first of the hominid transition periods starts at about 2.5 million years ago and the second about 500,000 years ago; these are just the two transition points described by the author based mainly on findings about tools.
Making tools can be transmitted by showing the novice what to do, so it takes only a copying capacity. For that reason, the cognitive status of Kanzi needn't be very great so the cognitive level associated with the first transition might be presumed to reflect as reaching no more than what the Piagetians call concrete reasoning.
The author gives the spatial cognition turning points as 1.5 million years ago and about 500,000 years ago. The first of these is during the span of constant Homo erectus ratio so that it is not related to any great additional brain change. But, the second coincides with the spurt in relative brain size during the transition from erectus to sapiens. The first point (1.5 million years ago) would then be likely to show a minimal effect compared with that of the second point which is likely to be part of a substantial increase in complexity of structure and thought.
Two main categories of increments in the ratio are: (1) brain changes associated with physical properties of individuals and (2) changes associated with functional or cognitive aspects. The physical aspects would be likely to be manifested in enhanced agility and/or enhanced manual dexterity. Cognitive aspects could start from overall properties such as the reasoning stages described by Piaget to specific properties such as the spatial cognition used by the author. Accepting the assignment of Australopithecines to the concrete reasoning level, the Homo erectus level could be that of formal reasoning, while Homo sapiens could be that of post-formal reasoning.
It is useful to remind ourselves of the caution stressed by Churchland (1986) that currently observable behaviors and/or functions may be far-derived from the ones on which evolutionary selection could have acted.
There is precedent for asserting that large increments in brain size are correlated with substantial increments in cognitive levels because the Piagetian stages are correlated agewise with the stages of rapid brain growth we have discovered: 2-4, 6-8, 10-12, and 14-16/17 years. Thus, the brain stages occur at the onsets of those Piagetian stages (Epstein 1974a, 1974b, 1986,1980, 1999).
In present humans, a cranial capacity of about 900 cc is reached by about age 2 years; this birth to age 2 period is called the sensori-motor stage during which the senses and motor activities become reasonably functional. After this there is a large increase in cranial capacity reached by age 4 years. This 2-4 year period is what the Piagetians call the pre-operational stage when children cannot yet reason logically about directly experienced matters, but begin to think about things in symbolic terms. The hominid's brain/body plateau of 11.5 cc/kg corresponds to a cranial capacity of about 950 cc, making it similar to the 900 gm stage in present humans.
"The Great Leap Forward" (Diamond, 1993) took place about 60,000 years ago when the hominid suddenly became able to deal with complex or abstract problems. Planned agriculture replaced gathering, planned hunting replaced scavenging, abstract paintings appeared (as in the Lascaux caves), and awareness of the individual as a member of a group which had regular properties such as dying replaced just noting when an animal was dead. The Leap originated when some of the hominids discovered how to make use of the new arborization to become able to reason abstractly, that made possible the strikingly novel functions described by anthropologists. Such functions cannot be selected for because they can be taught, so there is neither need nor basis for selection for networks genetically programmed for higher intelligence provided that persons involved already have the evolutionarily augmented networks. It would be similar to the difficulty of selecting dogs for retrieving newspapers. Thus, humans remain dependent on instruction and experience for acquiring higher reasoning functions. From that time on, education became the means of spreading the new competencies so education became, and has remained, the main activity of human maturation.
FIGURE LEGEND
Figure 1. Ratios of cranial capacity to body weight for the hominids plus the value for P. troglodytes. There are plateaus for Australopithecines and for H. erectus, along with a possible plateau starting with Homo sapiens.
BRAIN AND BODY DATA FOR HOMINIDS (Hominid series)
REFERENCES
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