The Evolution of the Neocortex from Early Mammals to Modern Humans

Article excerpt

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.

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 presentday mammals.

Brains are subdivided into functionally distinct parts, the subcortical nuclei and the cortical areas. Early investigators constructed theories of humanbrain 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. …