The concept that humans evolved is one of the most contentious of all issues in the world of modern education. It flies in the face of the common-place and common-sense understanding of religious thinking people around the world. Beginning in this blogpost, the science of human evolution will be presented, and the profound philosophical and religious arguments about the subject will be left to the antagonists. There are a considerable number of interrelated changes found in modern-day humans that favor survival and even dominance in the biological world. Probably foremost of those beneficial traits is that of having a larger, more complex, more subtle, and more efficient brain.
Like other aspects of evolution, the brains of human beings did not spring whole and complete in a short time. What follows in this blogpost is a much abbreviated presentation of the highlights of evolution of nervous systems—note the use of the plural. If the reader wishes a much more detailed and complete exposition, consider doing the work to get through Jon K. Hass, Editor-in-Chief, Evolution of Nervous Systems, A Comprehensive Reference, four volumes, 2000 pages, Elsevier/Academic Press, 2007.
All animal life can be traced back to a common ancestor, and the great variety in nervous systems that exists is the result of imperfect copying of the genetic code from generation to generation as shaped by selection. With little or no lateral gene transfer across lines of descent, complex animal life represents a true hierarchy with a single beginning and multiple branching points. This phylogenetic structure means that nervous systems more or less resemble one another, with accumulated differences that reflect times of branching and rates of change. This relationship greatly simplifies the task of understanding the diversity that exists. As a result of more than a century of work from around the world, we have more accurate phylogenic trees and well-developed methodologies for reconstructing what evolved from what. With the advent of evo-devo, we also know how to link evolutionary changes in genes, anatomy, and physiology to evolutionary changes in behavior by means of both correlative and experimental analyses. As a result, we can construct scenarios of how evolution ‘tinkered’ with nervous systems to help adapt species to their environments for greater fitness, success, and a better rate of reproduction. We know a great deal about how nervous systems scale and how conserved sets of genes and processes are used in a frugal fashion to produce nervous systems that, to previous generations of evolutionary neuroscientists, seemed completely dissimilar.
The nervous system of any animal collects, coordinates, integrates, and disseminates diverse types of information regarding both the external and internal environments. Processing of this information leads to appropriate physiological and/or behavioral responses. The genes and the proteins they produce play crucial roles in embryological development and in evolution, and much is known about them, including the important facts that nature has conserved the DNA templates over eons of time and in a most parsimonious fashion.
Broadly integrative studies by a great many scholars have revealed that invertebrate nervous systems vary dramatically in size, complexity, and functionality, but still are built from highly conserved sets of genes. In that respect, they are quite similar to
vertebrate nervous systems, though the diversity is more extreme in vertebrata. It is well to note–at a fundamental level–that all animals engage in some degree of interaction with their environment where they identify and respond to external stimuli. Living creatures have–during their evolution–acquired an implicit knowledge of their environment. They have–by selective pressures in the flow of many generations–been shaped to respond sensibly to their usual environment. This biological process has led to phylogenetic (ancestral) cognition. Cognition is expressed in the body forms and functions that exactly fit their environment. For example: the wings of birds carry information about the density of air. Ever since their reptilian ancestors acquired wings, birds have been the experts in air transport. The flight muscles of insects are equipped for the rapid oscillations of their wings that are so much smaller than birds’ wings. The fins of fish have, during their formation, acquired knowledge of the density of water, and so are equipped for motion in their particular environment. The sensory functions of animals and their feeding behaviors reveal that during phylogeny, they have acquired the necessary knowledge about their usual sources of food.
An accurate way to understand the evolution of the nervous system and the brain is to see it as the addition of higher and higher levels of control. For a sexually reproducing organism to survive and to leave viable progeny, it must be able to control many different types of perceptions, i.e., sensed aspects of its environment. At a minimum, the animal must be able to find food, avoid predators, and to select an appropriate mate. As the environment changed and become more complex and less forgiving and with increasing numbers of enemies, life forms evolved apace with robustness and complexity. The possession of an evolving, and improving nervous system gives a considerable advantage to be able to perceive and to control the increasingly complex and dangerous aspects of the environment.
Here is a very simple synopsis of the graduated steps in the early development of nervous systems:
1. crude nervous reactive system (single-cell amoeba haphazard movement)
2. primitive intercom system with crude memory (single-cell paramecium coordinated movement)
3. clustered cells had no more intelligence than single cells
4. multi-cellular animals lacking a nervous system but possessing the genes for one and neurosynapse proteins (sponges)
5. development of neurons and synapses—simplest true nervous system [decentralized nerve net and simple receptors] (cnidarians, e.g. hydra)
6. increased development of the anterior end of the nervous system and concentration of sensory cells into a primitive central nervous system
7. development of neurons and their specialization internalized into a CNS—one billion years ago (earthworms and planarians—which have CNS genes shared with humans)
8. development of a great diversity of nervous systems (e.g. mollusks)
9. Arthropods (insects and crustaceans) developed a nervous system made up of a series of ganglia, including a brain—or supraesophageal ganglion–connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly and well developed, complex sense organs.
Worms are the simplest creatures to have a central nervous system and to gain the power to have more complex behavior. Mollusks show a great diversity of nervous systems including some such as bivalves that have no cephalization. Arthropods [insects and crustaceans] have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly. Although miniscule by human standards, the range of abilities made possible by the brains of insects is impressive; these creatures show a remarkable range and variety of behaviors for locomotion, obtaining food, mating, and aiding in the survival of their offspring. Insects can crawl, hop, swim, fly, burrow, and even walk on water. The brain of the Leafcutter ant is programmed such that it harvests leaves and brings them into its nest where the leaves are used to cultivate indoor gardens of edible fungus. Honeybees have nervous systems that enable them to live in social communities where there is a strict division of labor.
There is extensive evidence that all vertebrate brains are amazingly similar at very early stages of development and are often strikingly similar to phylogenetic changes observed in invertebrates. Evo-devo scientists have found that several conserved genes–notably Pax6–are critical to eye development in both invertebrates and vertebrates. Vertebrates have complex sense organs and exhibit complex behaviors that require a complex, high energy consuming nervous system. Cephalization in vertebrates and the distinction between CNS and PNS [Peripheral Nervous System] is dramatically increased and is much more highly complex than that found in the urbilaterians, worms, and mollusks.
The basic pattern of the CNS is highly conserved throughout the different species of vertebrates and during evolution. During the long evolution leading to the bird and especially the human brain, nervous systems changed in five principle ways: 1. They became increasingly complex, 2. They became progressively centralized, 3. They tended toward cephalization, 4. The size, number, and variety of the elements of the brain increased; and 5. There developed an increase in plasticity, i.e., the brain’s ability to modify itself as a result of experience to make memory and the learning of new perceptual and motor abilities possible and progressive. The major trend that can be most readily observed in vertebrates is towards a progressive telencephalization: In the reptilian brain that region is only an appendix to the large olfactory bulb, but it represents most of the volume of the mammalian CNS. In the human brain, the telencephalon (that region farthest forward, consisting of the cerebral hemispheres) covers most of the diencephalon and mesencephalon (brain stem); and those oldest parts of the vertebrate brain still deal with reflex coordination and life preserving vegetative functions. The newer neocortex region interneurons receive, store, and retrieve information all the while weighing possible responses, a dramatic step beyond reflex function. The allometric–measuring relative growth rate–study of brain size among different species shows a striking continuity from rats to whales. However, with humans, the relative size of the brain does not fit the trend curve”. The human brain is disproportionately larger even than the brains of other non-human primates, including our closest relatives, the bonobos and chimpanzees. Researchers found that the genes of the human brain had gone through an intense amount of evolution in a relatively short amount of evolutionary time, a process that outstripped the evolution of the genes of the other animals. continued…