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The evolution of the neocortex from early mammals to modern humans.

Most of us are extremely interested in human behaviors and abilities. Because these behaviors and abilities are mediated by our brains, it is natural to wonder how the human brain is organized and functions. Understanding the human brain is challenging, largely because of its great complexity. As a result, much of neuroscience has been directed toward understanding simpler brains, such as those of rats and mice, with the expectation of discovering principles that apply broadly. But if some brains are simpler, how did our human brains become so complex? We start this discussion with the first mammals because that is when a novel brain structure emerged, the neocortex, the hallmark of the mammalian brain. While the neocortex is not actually new, as once thought, it did change dramatically from a thin single-cell layer of dorsal cortex in reptiles into a thick sheet of six-cell layers, each having a different functional role, in the first mammals. This new organization proved to be so useful and adaptable, as subdivisions and layers were modified in various ways, that the story of brain evolution is largely a story of the evolution of the neocortex. While the interesting modifications of the neocortex include those that allow echo-locating bats and tactile-driven, star-nosed moles to have their unique food-gathering behaviors, the focus here is on the brain changes that led from early mammals to modern humans.

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How do we chart the course of human brain organization? Usually patterns of evolution are deduced from the fossil record. For example, we know from the fossil record that modern horses evolved from a series of ancestors whose number of toes reduced from five to one. However, usually only bones are preserved in the fossil record and not soft tissue, especially not brains. Yet the inside of the skull does reflect the size and shape of the brain, so fossils can reveal brain size and sometimes the locations of major sulci, the folds in the brain that may hint about functional divisions. But fossils tell us nothing about the internal organizations of brains. Because of the limitations of the fossil record, most of what is known about brain evolution has been deduced by comparing the organizations of the brains of present-day mammals.

Brains are subdivided into functionally distinct parts, the subcortical nuclei and the cortical areas. Early investigators constructed theories of human-brain evolution by tacitly assuming that brains evolved by adding parts; they thought that some mammals living today have brains that had changed little from those of the first mammals and that others had added parts to various extents, thereby forming a series of mammals with increasing levels of brain complexity. According to this viewpoint, the study of a series of mammals from those with the simplest brains to those with the most complex ones would reveal the course of human-brain evolution. A suitable series might include a hedgehog (an insectivore with a small, relatively simple brain), a tree shrew (a squirrel-like mammal once thought to be a primitive primate), and a sequence of primate levels (prosimian, New World monkey, Old World monkey, ape, and human). These mammals do reflect a series of increases in relative brain size and complexity, and deductions based on this approach often may be correct. However, this approach provides us with no way of distinguishing between brain traits that are specializations of one line of evolution and those that reflect the ancestral condition. Hedgehogs, for example, are covered with sharp quills, but it would be a mistake to conclude from this that early mammals were covered with quills. Instead, we see that most mammals, including most insectivores, do not have quills; thus, it is logical to conclude that early mammals did not have quills.

This type of broad comparison is the essence of the cladistic method of reconstructing the brain features that characterized the brains of ancestors from first mammals to the recent past. A clade is any group of species that have all descended from a common ancestor, and a trait that is widely distributed in the clade is likely to have been present in the common ancestor. The cladistic approach can be a powerful way of determining when in a phylogenetic tree specific traits emerged, yet this method can be difficult to apply to the study of brains, especially for traits that are revealed only by costly and time-consuming experimental study, as there may be few studied species to compare. In addition, this method tells us nothing about the evolution of brain structures in hominid (bipedal) primates because we are the only living representatives. Fossilbrain endocasts provide information about our hominid ancestors, but only about brain size and shape.

WHAT IS KNOWN ABOUT BRAIN EVOLUTION IN MAMMALS?

Fortunately, we can say quite a bit about the brains of early mammals. The fossil record indicates that most early mammals were rather small, cat-sized or less, and had small brains relative to body size. For uncertain reasons, brain size tends to follow body size, so it is helpful to consider how much larger or smaller a mammal brain is than is common for mammals of that size. Early mammals had smaller brains and less neocortex for their body sizes than most mammals do today, but some mammals today also have small brains and little neocortex relative to body size. These mammals include the tenrecs of Madagascar. Tenrecs are small, insect-eating mammals once classified as insectivores with moles and hedgehogs but are now recognized as members of a separate order in the superclade, Afrotheria. It is immediately apparent from the tenrec's brain that it has very little neocortex and that olfaction is the dominant sense, judging from the large proportion of the brain that is devoted to the olfactory bulb and piriform (olfactory) cortex (see Figure 1). Experimental studies reveal that the neocortex contains few cortical areas.

Most of these areas have sensory or motor functions. A primary visual area, V1, topographically represents the receptors of the eyes; a primary somatosensory area, S1, represents the receptors of the body; a primary auditory area, A1, represents tone frequencies; and electrical stimulation of a primary motor area, M1, evokes movements. Evidence suggests a second somatosensory area, S2, and perhaps one or two additional auditory, visual, and somatosensory areas exist as well. Other areas further evaluate stimuli and motivate behavior. Overall, the neocortex appears to have only ten to fifteen functionally distinct areas. Areas identified in tenrecs are widely found both in other small-brained mammals such as hedgehogs, opossums, and rats, and in mammals with larger brains, including us. Thus, we can conclude that early mammals had small brains with little neocortex and few cortical areas. Furthermore, these areas have been retained in nearly all descendent mammals, although they have been repeatedly altered in structure, connections, and size to modify their functional roles.

Primates represent one branch of a superclade that includes rodents, lagomorphs, flying lemurs, and tree shrews. Judging from what these mammals have in common, early members of this clade had more neocortex than early mammals, and the temporal lobe had expanded to include several additional visual areas. The fossil record indicates that early primates were characterized by orbital convergence, suggesting an increase in the importance of binocular and stereoscopic vision. Such a change would have aided them in their roles as nocturnal, arboreal predators of small invertebrates and vertebrates. Their brains were larger than those of early mammals and were similar in relative size and shape to present-day strepsirhine primates (lemurs, lorises, and galagos). This increase in size included a considerable expansion of the temporal lobe, and the temporal lobe of all extant primates is dominated by an array of visual areas. Early primates soon diverged into three major lines: the strepsirhine primates noted above, and a diurnal haplorhine line that led both to anthropoid primates (monkeys, apes, and humans) and to a tarsier line that reverted from a diurnal adaptation to become a highly specialized nocturnal predator with large eyes as a readaptation to dim light.

Galagos are small rat- to cat-sized primates with a brain that is smaller than those of anthropoid primates of a similar size, while having a number of brain features that characterize primates in general. All primates have a calcarine fissure that enlarges the visual cortex of the medial occipital lobe and a lateral sulcus that expands the cortex devoted to somatosensory processing (that is, the sense of body position, pain, temperature, and so on--rather than senses such as vision and hearing). The lateral geniculate nucleus of the visual thalamus isolates three specialized outputs from the eyes that are partially segregated in their relay to the primary visual cortex and in their distribution from V1 to additional visual areas (Figure 1). The second visual area, V2, is subdivided into band-like processing modules devoted to different V1 outputs. Dorsal and ventral processing streams of interconnected visual areas are specialized for action or object vision, respectively. The dorsal stream includes the middle temporal area, MT, which is devoted to processing visual motion signals, and visuomotor areas in the posterior parietal and frontal cortex for directing eye movements. The motor system has expanded to add dorsal, ventral, and supplementary premotor areas to M1. Somatosensory areas have been added to the parietal cortex, and the posterior parietal cortex contains areas related to visual guidance of reaching and hand use. The many features of galago brains shared with other primates indicate that early primates of perhaps 80 million years ago already had brains that were highly specialized in ways that differed from those in other mammals.

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ANTHROPOID PRIMATES AND HOMINIDS

Early anthropoid primates were African monkeys. One line, somehow, got to South America and diversified into the platyrrhine or New World monkeys to occupy a range of ecological niches. Their brains reflect the basic primate plan of cortical organization, while expressing a number of specializations. The other line, the catarrhine or Old World monkeys, is generally as large or larger than the largest of New World monkeys, and some of them have become highly terrestrial. The brains of Old World macaque monkeys clearly have a large number of auditory, visual, and somatosensory areas. They also have expanded the number of sensorimotor regions of the posterior parietal cortex that interact with an expanded number of premotor areas in the frontal cortex used to guide and plan behavior. Apes emerged as a major and initially successful branch of anthropoid primate evolution, but much of the radiation died out. We know relatively little about the large brains of the few lines of apes remaining today.

The first bipedal hominids diverged from a chimpanzee-like ancestor about six million years ago, and those hominids led to a sequence of human ancestors with ever-increasing brain size. Early Australopithecines had brains only slightly larger than present-day African apes, but hominid brains increased rapidly in size during the last two million years from the 500-800 c[m.sup.3] of Homo habilis, to the 700-1200 c[m.sup.3] of Homo erectus, to the 1200-1400 c[m.sup.3] of modern humans. The transformations in brain organization that occurred over that time are largely uncertain, but humans have areas that are not present or well-developed in monkeys.

Most notably, a large portion of the temporal lobe and parts of the frontal lobe of the left cerebral hemisphere are specialized for language in humans, and regions of the parietal lobe of the right hemisphere are specialized for spatial reasoning and related functions. Part of the ventral temporal lobe is specialized for recognizing individual faces, something very important for us as a highly social species. Our frontal lobes have areas that promote an understanding of the intentions of others and an appreciation of the consequences of our actions. Motor guidance and planning systems have differentiated to mediate an intuitive sense of tool use and an ability to acquire any number of impressive motor skills. Human brains are asymmetric because brain systems have developed differently in the two hemispheres. Similar but less-pronounced asymmetries in the brains of great apes and the fossil endocasts of the brains of our hominid ancestors indicate that the tendency of each cerebral hemisphere to specialize differentially has a long history.

We can make some assumptions about how human brains are organized simply because they have grown so large. Large brains are larger mainly because they have more neurons. Large size creates a connection problem because more neurons mean more and longer connections as neurons are spaced farther apart. To maintain short transmission times over longer connections, axons need to be thicker. Thus, as brains become larger, they need to devote proportionally more of their mass to connections. However, they do so much less than predicted from a model of maintained connectivity and axon function. This discrepancy implies more modularity in organization so that short, local connections can be favored over long connections. As a notable example, the differing functional specializations of the two hemispheres of the human brain greatly reduce the need for long connections between the hemispheres. The possibility of more modularity in the human brain is supported by present evidence for a large number of areas (more than fifty). The number of areas in the human neocortex is far from determined, but theoretical estimates suggest as many as 150. In addition, many areas in other primates are functionally subdivided in a manner that reduces the number of intrinsic connections, and this tendency should be especially pronounced in the human brain.

Thus, the evolution of the human brain was characterized by a great expansion of the brain, especially the neocortex, and a substantial increase in the number of functional areas. Brain functions were greatly enhanced as the greater number of areas allowed additional processing steps and the specialization of areas for new functions.

For Further Reading

Allman, J. Evolving Brains. New York: W. H. Freeman & Co., 1999.

Clark, LeGros, W.E. The Antecedents of Man. Edinburgh: University Press, 1954.

Jerison, H. J. Evolution of The Brain and Intelligence. New York: Academic Press, 1973.

Kaas, J. H. "Why Is Brain Size So Important: Design Problems and Solutions as Neocortex Get Bigger or Smaller." Brain and Mind, 1: 2000. 7-23.

Kaas, J. H. and T. M. Preuss. "Human Brain Evolution." Fundamental Neuroscience, 2nd ed. Ed. L.R. Squire. San Diego: Academic Press, 2003. 1147-1166.

Jon H. Kaas is Distinguished, Centennial Professor of psychology at Vanderbilt University. His major interest is in how sensory and motor systems are organized in mammalian brains, especially in primates. He is an elected member of the National Academy of Sciences and of the American Academy of Arts and Sciences.
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Title Annotation:brain research
Author:Kaas, Jon H.
Publication:Phi Kappa Phi Forum
Geographic Code:1USA
Date:Jan 1, 2005
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