There has been significant effort to elucidate the evolutionary history of the hominoid species over the last five decades. Several approaches have identified a number of selective pressures that have triggered primate speciation over approximately 65 million years (my). One traditional approach is macroevolution, which explains phylogeny through comparative anatomy, fossil records, brain size, gestation period, social organization and mating patterns. Another type of approach is microevolution, which describes molecular changes through time, including genome size, chromosomal rearrangements, gene reorganization and immunological data.
Integration of results of these categorical studies plays an important role in the reconstruction of hominoid phylogeny. Attempts on estimating divergence times of each hominoid species are mainly based on morphometric analyses and molecular clocks. The hominoids are a fascinating species mostly because of their likeness to us humans, in terms of physical appearance, gestures, social structure and most importantly– intelligence. These species have been identified as the best model organisms to study human diseases and other physiological functions.
This paper discusses brain size, social organization and mating patterns in primates, in relation to the evolution of primate intelligence. II. Discussion Members of the Order Primates are the most-studied mammals in the animal kingdom, amidst the long-standing confusion in the classification of its species. This paper will describe aspects on the evolution of primate intelligence based on four (4) grades of primate evolution. The Superfamily Hominoidea is composed of several families, of which we will describe four (4) particular families or grades.
Grade 1 (Lemoroids), is composed of members of the Family Lemuroidea, which are small to medium-sized primates with long, furry tails and slender bodies to facilitate their time spent on treetops.
They have small hairy ears and small eyes and the second digit of their hind feet is enlarged for “grooming” purposes. These primates spend time both on trees and on land, with most of their activity performed during the day. Grade 2 (Tarsiers), consists of members of the Family Tarsiidae, which are small primates with round heads and extremely large eyes both facing forward. Read essay on ecosystem my role and responsibility
Their ears have minimal hair and their noses covered with hair. One particular feature of tarsiers is the differential length of their limbs—their hindlimbs are much longer than their fore limbs, a physical result of the elongation of their tarsals. These primates are active at night and spend most of their time on trees. Tarsiers forage on insects and capture their prey using their long digits that are equipped with toe pads. Grade 3 (Monkeys), is composed of members of the Family Anthropoidea, of which both Old World monkeys (Order Platyrrhini or flat-nosed) and New World monkeys (Order Catarrhini or down-facing nose) belong.
Old World monkeys are generally terrestrial because their tails have lost their prehensile ability. Instead, they have accumulated sitting pads around their tails and their thumbs are opposable, just like humans. On the other hand, New World monkeys spend more of their time on trees because they have prehensile tails. Their thumbs are usually in line with the rest of their fingers and move in a scissor-like manner for gripping. Grade 4 (Apes and Man), consists of members of the SuperFamily Hominoidea, better known as the great ape species—orangutan, gorilla, chimpanzee and humans.
These apes are large primates that have lost their tails and are known to be more intelligent than monkeys. They have prehensile hands capable of holding objects hence they spend most of their time foraging in the forest. Genetically, the orangutan, gorilla and chimpanzee are very similar to humans– apes have 48 chromosomes, wherein 2 chromosomes fused to generate a single chromosome in the human genome, to make 46 chromosomes. Protein comparisons show that chimpanzees and humans are 98% similar.
A. Brain size. The study of the structure and function of the primate brain has been motivated by the need to understand both biological and social aspects of living systems. Even the most primitive invertebrate animals have been known to have “brains” made out of simple networks of nerve cells found inside the anterior pole of the body. The brain has gradually increased in size as one traces the evolutionary tree to the most modern animals, hence it is imperative to investigate whether such changes were a result of adaptation or a need to developmentally diverge through time.
A classical view of brain evolution involved the brain/body relationship (Dubois, 1913) with an additional measurement depicted as encephalization quotient (Jerison, 1973). However, several investigators opposed the brain/body relationship and in turn suggested that the absolute brain size provided more significant information than encephalization quotients. It has lately been suggested that larger brain sizes were the result of a surplus of metabolic resources for further development of the brain as well as its maintenance, and not an effect of limiting factors such as growth (Schoenemann, 2004).
The hominoid brain size has been observed to increase in size across the Great Ape species, with that of the humans being 3. 1 times larger than the rest of the hominids (Rilling and Insel, 1999; Ho et al. 1980). The components of the brain—olfactory bulb, cerebellum, visual cortex and temporal and frontal lobes—have significantly increased in size in the humans, with a range of 1. 5 to 4. 7 times larger than the rest of the primates (Stephan et al. , 1981; Rilling and Seligmann, 2002; Schenker et al. 2005). It would then be interesting to determine what factors directly influence relative brain sizes. Recently, diet, life span and population density were associated to directly act on primate brain sizes, especially the neocortex (Walker et al. , 2006). Mechanisms behind such increase in size involves generation of a large brain which consequently programs the body to increase food intake and lengthen its life span to be able to substantially maintain and develop a bigger brain.
The brain has also evolved to accommodate changes for social interactions (Dunbar and Shultz, 2007). Sexual selection is also involved in the evolution of bigger brains sizes mainly through a negative feedback relationship with male competition for mates (Schillaci 2006). Sperm competition, as determined by measurement of primate testis, was determined not to play a role in primate brain evolution. In addition, monogamous mating, which involves more effort in social acuity and deception capabilities, is associated with larger brain sizes.
Comparative gene expression studies in several anthropoid species have shown that enormous amounts of energy is metabolized by neurons of the neurocortex, mainly due to the significantly high numbers of glial cells in the frontal cortex of humans, as compared to that of other primates (Sherwood et al. 2006). However, brain regions responsible for cognition and language did not have substantially greater energy requirements for metabolism than in other primates, suggesting that the energy needed by the human brain in mainly for neuron expansion and projection of axons and dendrites.
At the genetic level, the gene microcephalin (MCPH1) was observed to have undergone positive selection across the hominoid lineage, resulting in a human-specific haplotype that is observed at an allele frequency of 0. 70 (Evans et al. 2006). This recent haplotype emerged 1. 1 million years ago (mya) and was distributed throughout the rest of the human race approximately 37,000 years ago, suggesting an admixture between early and modern humans. Human genome-wide scans have identified five sets of brain-specific genes that unfortunately, are not directly involved in the brain evolution.
However, there are less nucleotide changes in the human brain than that in the chimpanzee, suggesting that mutations are not responsible in the current features of the human brain (Shi et al. 2006). Recent comparative primate protein sequencing has shown evidence for human-specific accelerated changes in gene expression in the brain and the rest of the human nervous system (Olson and Varki, 2003; Varki, 2004). B. Social organization Primates are inherently social animals.
The study of their social structure may provide insights on behavior, including mate selection, that may indirectly affect the gene pool. Primate social groups distinctly show the degree of distribution of gene across a social group, as displayed by group fission, male dispersal and migration rates (Melnich 1987). The social brain hypothesis describes the strong connection of social activity and the brain size and intelligence (Walker et al. , 2006). Such association is mostly due to the need to specifically localize such activity to the neocortex (Dunbar and Shultz, 2007).
Screening of 11 New World monkeys showed that social activity is greatly associated to reproductive success and caring of their offspring (Garber and Leigh 1997). Reproductive efforts and brain development entails enormous metabolic energy from mothers that a limited number of offspring at 2-year intervals seems to be the best scenario for rearing more offspring. Care-giving through cooperation of other female primates has facilitated metabolic usage in several monkey groups. This also results in rapid growth, development and weaning of offspring.
In other species, paternal care has been observed, which saves on energy for caring of offspring, as well as prevention of predation and competition. C. Mating patterns Sexual selection of mating partners has shaped the gene pool and influenced the evolution of the primate species. Several studies have shown that the evolution of the primate brain is closely linked to sexual selection, which may be influenced by sperm or mate competition. Larger primate brains tend to follow monogamous mating patterns, which in turn, requires more effort for social interaction and deception (Schillaci 2006).
Monogamy includes at least one of three aspects– exclusivity of mating, shared parental care and association, and is a behavioral pattern displayed by individual organisms that are results of natural selection (Desbury, 1987). It has been shown the primates have mate preferences, but unlike humans, chimpanzees prefer older females, suggesting that the choice of younger females as mates is a derived human-specific mating feature (Muller et al. , 2006). Selection is also associated with the evolution of the anatomy and physiology of primates, including genital morphology and copulatory behavior (Dixson 1991; Dixson and Anderson, 2001).
In addition, the aspect of parental investment was observed as a prime goal for mating selection in primates (Geher et al. 2004). III. Conclusion The study of the structure and function of the primate brain has provided a better understanding of both biological and social aspects of living systems. We have described the evolutionary relevant associations of brain size, social organization and mating selection to the evolution of primate intelligence. A number of views on brain evolution include the brain/body relationship, encephalization quotients, absolute brain size and metabolic resourcing.
The significant increase in brain size shows that each brain region, such as the cerebellum, visual cortex and the neocortex, have developed intricate structures to facilitate compartmentalization of brain activity. Such delegation of regions for specific brain activity entails more energy requirements thus increasing the amount of food intake, but also progresses to a more capable brain for a bigger number of social, physiological and other biological capabilities. The life histories of the higher primates have been lengthened due to the brain’s evolution.
Primate brain evolution has also affected primate sexual selection by the perception of male competition for mates and the incorporation of monogamous mating among primates. The structured primate social activity has resulted in the reproductive success and caring of their offspring, cooperation among primates for care-giving, as well as paternal care. Alongside the evolution of the primate brain arise mate preferences or mate selection, which is also associated with visual acuity of the anatomy and behavior of primates.
The amount of new information on the evolution of the brain and its association to primate intelligence has increased in the past 2 decades. However, the complexity of the dimensions of primate intelligence remains unclear because certain aspects, such as genetic factors that have been validated to be differentially expressed in primates may in turn be directly correlated with mate selection and patterns, as well as social cultures.
In addition, there are also ecological and genetic factors that could influence variability in intensities of selection in different primate families which may have different effects on primate life histories. There is a need to integrate both micro- and macro-evolutionary data to unify the concept of primate intelligence. More importantly, the classification of primates should also be addressed, so prevent further confusion on primate taxonomy. The possible employment of DNA data for distinguishing taxonomic families and orders should be correlated with morphological classification of the primate species.