Disclaimer: this is in no way or form my work I am simply summarizing point made by Leslie C. Aiello and R.I.M. Dunbar in their article entitled Neocortex Size, Group Size, and Evolution of Language.
Throughout this paper Aiello and Dunbar present evidence to support their evolution language model for the hominin line. Their model holds that the close correlation between encephalization and group size serves as powerful indicator of the point in human history when language becomes necessary to maintain large group size. Aiello and Dunbar’s model also suggests that the necessity for both large groups and (at least) rudimentary language appeared early in the evolution of the genus Homo and began to rapidly increase in the second half of the Middle Pleistocene. Contrary to other research it provides no evidence for the relatively sudden appearance of a radically novel communication of the system in the Upper Paleolithic period about 40,000 years ago. This is just another example of the contentiousness prevalent in anthropology, specifically in this case human evolution.
Aiello and Dunbar’s model is based on a close statistically relationship that shows that there is a correlation between relative neocortex size, group size, and the amount of time devoted to social grooming among nonhuman primates. When extending these results to anatomically modern humans Aiello and Dunbar argue that the time required to service the relationships in the large groups predicted for modern human would have been too great to be sustained by methods normally employed by nonhuman primates (mainly grooming). Furthermore extant primates spend up to 20% of their day performing social grooming without compromising their time budgets. From this extrapolatory finding Aiello and Dubbar can conclude that language evolved as form of bonding mechanism in order to use social time more efficiently and the crisis point for the evolution of enhanced verbal communication must lie at some higher value perhaps 25-30 % of the day.
When examining the hominins line with use of a equation derived from the correlation between group size and encephalization Aiello and Dunbar find that Austrailopithecines have a grooming times below the limit found in nonhuman primates, the grooming times requirements for Homo neanderthalensis are similar to that found for both archaic and modern human which were extremely high, and finally the Homo habilis/ruldolfensis has an average grooming requirement (23%) that is close to the highest values observed in living primate populations (20% for the geladas). Reiterating part of the introduction this suggests that the evolution of the genus Homo was a gradual process that began in the second half of the Middle Pleistocene rather than more recently in the Upper Paleolithic. Lastly, it is noteworthy to point out how similar gelada vocalization is to human speech. Gelada vocalization possesses a variety of features once considered unique to human speech such as: fricatives, plosives, and nasals, labials, dentals and valears, as well as rhythmic, melodic, and conversational properties involving synchronized bout with intense emotional overtones.
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