Brain Evolution in the Human Species
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Hominid evolution is marked by a very significant increase in relative brain size. Because relative brain size has been linked to energetic requirements, it is possible to look at the pattern of encephalization as a factor in the evolution of human foraging and dieting (Foley et al., 1991). Major expansion of the brain is associated with the Homo species rather than the Hominidae as a whole, where the energetic costs are likely to have forced prolongation of growth rates and secondary altriciality (Foley et al., 1991). Paleontological evidence indicates that rapid brain evolution occurred with the emergence of Homo erectus 1.8 million years ago and was associated with important changes in diet, body size, and foraging behavior (Leonard et al., 2007).
Extensive energy is required for brain growth and functioning. Parker (1990) analyzes intelligence and encephalization from the perspective of life history strategy theory, which is based on the premise that evolutionary selection determines the timing of major life cycle events-especially those related to reproduction-as the solution to energy optimization problems. Foley and Lee (1991) analyze the evolutionary pattern of encephalization with respect to foraging and dieting strategies. In considering the development of human foraging strategies, increased returns for foraging effort and food processing may be an important prerequisite for encephalization, and in turn a large brain is necessary to organize human foraging behavior.
Dietary quality is also correlated with brain size. Foley and Lee (1991) first consider brain size vs. primate feeding strategies, and note that folivorous diets (leaves) are correlated with smaller brains, while fruit and animal foods (insects, meat) are correlated with larger brains. Overall, the genetic costs of brain maintenance for modern humans are about three times that of a chimpanzee. The first dietary shift is seen beginning within the genus Homo, which began to include meat in the diet. It may be argued that meat-eating represents an expansion of resource breadth beyond that found in non-human primates (Foley and Lee, 1991). Therefore, Homo and its encephalization may have been the product of the selection of capable of exploiting energy- and protein-rich resources as the habitat expanded.
While the evolutionary causes of the enlarging human brain themselves are thought to have been due to factors that go beyond diet alone (increasing social organization being prime among the proposed factors usually cited), a diet of sufficient quality would nevertheless have been an important prerequisite. That is, diet would have been an important hurdle, or limiting factor, to overcome in providing the necessary physiological basis for brain enlargement to occur within the context of whatever those other primary selective pressures might have been. Leonard and Robinson (1994: add page numbers for direct quote) conclude:
These results imply that changes in diet quality during hominid evolution were linked with the evolution of brain size. The shift to a more calorically dense diet was probably needed in order to substantially increase the amount of metabolic energy being used by the hominid brain. Thus, while nutritional factors alone are not sufficient to explain the evolution of our large brains, it seems clear that certain dietary changes were necessary for substantial brain evolution to take place.
Fossil Hominid Skulls
Fossil hominid skulls provide direct evidence of skull evolution and information about diet, behavior, appearance and brain size, but unfortunately hominid skulls are relatively rare in the fossil record. The Ardipithecus ramidus skull is of particular interest because it predates known Australopithecines and thereby illuminates the early evolution of the hominid skull, brain, and face (Suwa et al., 200). The Ardipithecus ramidus skull exhibits a small endocranial capacity of 300-350 cc (similar to that of bonobos and female chimpanzees), small cranial size relative to body size, considerable midfacial projection, and a lack of modern African ape-like extreme lower facial prognathism (Suwa et al., 2009). It has a short posterior cranial base and lacks a broad, anteriorly situated zygomaxillary facial skeleton developed in later Australopithecus. This combination of features shows that Mio-Pliocene hominid cranium differed from that of both extant apes and Australopithecus (Suwa et al., 2009).
These and an additional feature of the skull hint that, despite its small size, the brain of Ardipithecus ramidus may have already begun to develop some aspects of later hominid-like form and function. The steep orientation of the bone on which the brain stem rests suggests that the base of the Ardipithecus ramidus might have been more flexed than in apes (Suwa et al., 2009).
Australopithecus afarensis is one of the longest lived and best known early human species. This species was found between 3.85 and 2.95 million years ago in Eastern Africa, surviving for more than 900,000 year, which is over four times as long as our own species (Stanyon et al., 1993). Australopithecus afarensis had both human and ape characteristics. It had a flat nose and strongly projecting lower jaw similar to that of an ape, a small brain that was usually less than 500 cc (about 1/3 the size of a modern human brain), small canine teeth like all other early humans, and a body that stood on two legs and walked upright (Stanyon et al., 19934). The presence of an ape sized brain in a fully bipedal hominid indicates that the development of bipedalism preceded the expansion of the brain.
Homo habilis first appeared around 2.5 million years ago and is the earliest genus of hominid with evidence of tool use. With a relatively large brain, 680 cc on average and up to 800 cc, Homo habilis is the first definite human ancestor. Homo habilis is considered to be the first member of the Homo genus for two main reasons: their large brain size and the presence of tools. The mean absolute endocranial capacity of Homo habilis is appreciably larger than the mean for australopithecine species; the Homo habilis species mean is 45.1% greater than the Australopithecus africanus mean and 24.8% greater than that of Australopithecus boisei (Tobias, 1987). There are also two major cerebral areas governing spoken language in modern man that are well represented in the endocranial casts of Homo habilis. In functional capacity, its possession of a structural marker of the neurological basis of spoken language, Homo habilis had attained a new evolutionary level of organization (Tobias, 1987).
Homo erectus appears to have evolved in Africa about 1.8 million years ago, placing them between Homo habilis and the earliest appearance of Homo sapiens. The main distinguishing features shared by Homo habilis and H. erectus include the increased brain size, the present of brow ridges, a shortened face, and the projecting nasal aperture (O'Neill, 2010). Compared to modern humans, the Homo erectus brain case was more elongated from front to back and less spherical (O'Neill, 2010). As a consequence, the frontal and temporal lobes of their brains were narrower, suggesting that they would have somewhat lower mental ability (O'Neill, 2010). The adult Homo erectus brain size ranged from around 750 to 1250 cm3, averaging about 930 cm3. While this was only around 69% of modern human brains on average, the upper end of the Homo erectus brain size range overlapped that of modern people (O'Neill, 2010).
Archaic forms of Homo sapiens first appear about 500,000 years ago and these skulls exhibit features of both Homo erectus and modern humans. The brain size is larger than erectus and smaller than most modern humans, averaging around 1200 cc, and the skull is more rounded than in erectus (Foley). Homo sapiens also had a much steeper forehead than in previous species, which hints that the brain itself had more emphasis on the forebrain (Park, 1999). This is a very interesting observation because this sector of the brain is responsible for planning and reasoning, movements of limbs, speech, and social conduct which modern day humans are much more advanced in (Park, 1999).
The most well-known late archaic humans were the Neanderthals. The brain size of Neanderthals was close to that of modern humans, and the structural organization of their brains was essentially the same as well (O'Neill, 2010). The average Neanderthal brain was actually somewhat larger than the brains of most people today, however the difference is minimal when people of similar body size are compared. A larger head and more compact body shape was potentially able to produce more body heat relative to the amount that is lost to the environment through radiation, therefore the larger head compared to body size was probably selected for by nature (O'Neill, 2010).
The cranial capacity of the average Homo sapiens is approximately 1400 cc, which is a significant increase compared to their predecessors (Brown et al.). Modern human brains are composed of many structures, each of which performs a specific set of tasks. However, all of these structures can be divided into three parts, or evolutionary steps, of the brain. The first part, known as the reptilian brain, is the portion that we share with all other vertebrates (Brown et al.). The second part is known as the mammalian brain, which we share with all other mammals (Brown et al.) The third part of the brain is known as the human brain, which defines what it is to be human (Brown et al.) .
Basal Metabolic Rate
With the availability of a wealth of new data on basal metabolic rate and brain size and with the aid of new techniques of comparative analysis, it has been shown that energetics is an issue in the maintenance of a relatively large brain, and that brain size is positively correlated with the basal metabolic rate in mammals, controlling for body size effects (Isler and Schaik, 2006). Brain tissue is energetically expensive and requires a significant amount more energy per unit weight than several other somatic tissues during rest (Mink et al., 1981). Therefore, the high proportion of energy necessarily allocated to brain tissue may constrain the response of natural selection to the beneficial impact of increased brain size on an animal's survival or reproductive success.
Among mammals, increased brain size is often accompanied by an increased basal metabolic rate relative to body mass (Isler and Schaik, 2006). As a result, mammals tend to meet the energy costs of increased energy intake or reduced allocations in function as growth, reproduction, digestion, or locomotion (Haskell et al., 2002).
In the last three to four million years, brain volume within the hominid lineage has increased from less than 400 ml to roughly 1400 ml (Holloway). The first appearance of the Homo species is marked at the end of the Pliocene and the beginning of the Pleistocene, and the first clear increase in hominid brain size. Compared with the preceding hominids, the new lineage has a noticeably larger brain, as well as an obviously larger body (Lee and Wolpoff, 2003).
However, the brain size of the new hominid species is much larger than can be explained by the increase in body size. Throughout the Pleistocene, brain size continues to increase further to that seen in modern humans, but again this increase in brain size is not related to a mean change in body mass. Brain size increase is arguably one of the most distinct and significant evolutionary trends in Pleistocene human evolution (Lee and Wolpoff, 2003).
Although the reality of an increase in brain size is not a topic of disagreement, there are many assertions about the pattern of increase that are conflicting. For example, some have used brain evolution as a reflection of gradualism and continuity, whereas others claim that certain portions of the human lineage were characterized by statis (Lee and Wolpoff, 2003). It has also been though that the evolution of brain size in some geographical regions has proceeded at different rates than in others.
In previous studies, brain size evolution has been characterized by linear regression analysis using cranial capacity regressed against time as an independent variable (Lee and Wolpoff, 2003). However, regression analysis is not an appropriate method to examine questions of pattern changes over time because fossils often do not fulfill the minimum requirements that are needed for regression to be applicable, and regression as a method does not necessarily provide statistically valid information about patterns of change (Lee and Wolpoff, 2003).
With regard to brain reorganization, left-right cerebral hemispheric asymmetries are present in existing pongids (chimpanzees, apes, and gorillas) and the australopithecines, but neither the pattern nor direction is as strongly developed as in modern or fossil Homo species (Holloway). The appearance of a more human-like third inferior frontal convolution provides another line of evidence about evolutionary reorganization of the brain. None of the australopithecine endocranial casts show this region preserved satisfactorily (Holloway). In comparing Neanderthal brain casts to more recent Homo sapiens, there is no significant evolutionary change except their slightly larger brain size.
Mosaic Pattern of Evolution
A pattern of evolution in which different parts of the body evolve at different times is known as a mosaic pattern of evolution (O'Neill, 2010). In the case of humans, we essentially attained our modern form below the neck by at least two million years ago. However, our cranial capacity did not reach its current size until after 100,000 years ago. This process of the brain increasing in size over and beyond that explainable by an increase in body size has been referred to as encephelization (O'Neill, 2010). The overall increase in brain size was mostly due to a result of changes in particular regions of the cerebrum, where most high level brain functions occur. It is likely that nature was selecting for the mental capabilities needed to adapt rapidly to new environments, also causing the brain to be neurally reorganized for processing complex information (O'Neill, 2010).
Large brains are energetically expensive, and humans expend a large proportion of their energy budget on brain metabolism than other primates. The high costs of large human brains are supported, in part, by our energy- and nutrient-rich diets (Leonard et al., 2007). The human fossil record indicates that major changes in both brain size and diet occurred in association with the emergence of early members of the genus Homo, and with the evolution of early Homo erectus, evidence of an important adaptive shift has been found-the evolution of the first hunting and gathering economy, characterized by greater consumption of animal foods, transport of food resources to home bases, and sharing of food within social groups (Leonard et al., 2007). Therefore, improvements in diet quality with Homo erectus appear to have been important for fueling rapid rates of encephalization. In summary, major changes in diet, foraging behavior, and body size have had an important role on brain evolution in the Hominid species.
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