Evolution of the brain

The principles that govern the evolution of brain structure are not well understood.

Brain to body size does not scale isometrically (in a linear fashion) but rather allometrically. The brains and bodies of mammals do not scale linearly. Small bodied mammals have relatively large brains compared to their bodies and large mammals (such as whales) have small brains; similar to growth.

If brain weight is plotted against body weight for primates, the regression line of the sample points can indicate the brain power of a primate species. Lemurs for example fall below this line which means that for a primate of equivalent size, we would expect a larger brain size. Humans lie well above the line indicating that humans are more encephalized than lemurs. In fact, humans are more encephalized than all other primates.

Encephalization quotients (EQs) may indicate how much brain power a species has in comparison with other mammals. Primates lie at the top of this range with humans having the highest EQ score. EQ has a high degree of correlation with the ecological conditions of an animal such as its feeding behaviours and food it consumes. Leaf eating monkeys have lower EQ than frugivorous or omnivorous monkeys since they have to work harder to forage than monkeys which eat abundant, easy to find leaves.

Human brain size in the fossil record

The evolutionary history of the human brain shows primarily a gradually bigger brain relative to body size during the evolutionary path from early primates to hominids and finally to Homo sapiens. Human brain size has been trending upwards since 2 million years ago, with a 3 factor increase. Early australopithecine brains were little larger than chimpanzee brains. The increase has been seen as larger human brain volume as we progressed along the human timeline of evolution (see Homininae), starting from about 600 cm³ in Homo habilis up to 1500 cm³ in Homo sapiens neanderthalensis which is the hominid with the biggest brain size. The increase in brain size topped with neanderthals, since then the average brain size has been a shrinking over the past 28,000 years. The male brain has decreased from 1,500 cm³ to 1,350 cm³ while the female brain has shrunk by the same relative proportion.

However it is argued that another essential element of brain evolution in humans is rearrangement (Hoffman et al. 2004). Larger brains require more wiring, but more wiring can become inefficient (Hofman 2001). The brain has therefore become reorganized for efficiency. Furthermore the average body size of nethanderthals was larger which lead to bigger brain size (see Brain-to-body mass ratio).

Early history of brain development

From fossil records, scientists can infer that the first brain structure appeared in reptiles approximately 500 million years ago [confusing - there were certainly no reptiles, indeed no tetrapods (terrestrial vertebrates), and few, if any, vertebrates of any kind 500mya. Perhaps the words "early fishes" should replace reptiles]. The functions of the hindbrain found by fossil records included breathing, heart beat regulation, balance, basic motor movements and foraging skills. A trend in brain evolution according to a study done with mice, chickens, monkeys and apes concluded that more evolved species tend to preserve the structures responsible for basic behaviors. What this means is that evolution is the process of acquiring more and more sophisticated structures, not simply the addition of different structures over a long period of time. A long term study comparing the human brain to the primitive brain found that the modern human brain contains the primitive hindbrain region – what most neurologist call the protoreptilian brain. The purpose of this part of the brain is to sustain fundamental homeostatic functions. The pons and medulla are major structures found there. A new region of the brain developed about 250 million years after the appearance of the hindbrain. This region is known as the paleomammalian brain, the major parts of which are the hippocampi and amygdalas, often referred to as the limbic system. The limbic system deals with more complex functions including emotional, sexual and fighting behaviors.

The brainstem and limbic system are largely based on nuclei, which are essentially balled-up clusters of tightly-packed neurons and the axon fibers that connect them to each other, as well as to neurons in other locations. The other two major brain areas (the cerebrum and cerebellum) are based on a cortical architecture. At the outer periphery of the cortex, the neurons are arranged into layers (the number of which vary according to species and function) a few millimeters thick. There are axons that travel between the layers, but the majority of axon mass is below the neurons themselves. Since cortical neurons and most of their axon fiber tracts don't have to compete for space, cortical structures can scale more easily than nuclear ones. A key feature of cortex is that because it scales with surface area, "more" of it can be fit inside a skull by introducing convolutions, in much the same way that a dinner napkin can be stuffed into a glass by wadding it up. The degree of convolution is generally greater in more evolved species, which benefit from the increased surface area.

The cerebellum, or "little brain," is behind the brainstem and below the occipital lobe of the cerebrum in humans. Its purposes include the coordination of fine sensorimotor tasks, and it may be involved in some cognitive functions, such as language. Human cerebellar cortex is finely convoluted, much moreso than cerebral cortex. Its interior axon fiber tracts are called the arbor vitae, or Tree of Life.

The area of the brain with the greatest amount of recent evolutionary change is called the cerebrum, or neocortex. In reptiles and fish, this area is called the pallium, and is smaller and simpler relative to body mass than what is found in mammals. According to research, the cerebrum first developed about 200 million years ago. It's responsible for higher cognitive functions - for example, language, thinking, and related forms of information processing. It's also responsible for processing sensory input (together with the thalamus, a part of the limbic system that acts as an information router). Most of its function is subconscious, that is, not available for inspection or intervention by the conscious mind. Neocortex is an elaboration, or outgrowth, of structures in the limbic system, with which it is tightly integrated.

Brain re-arrangement

With the use of in vivo Magnetic resonance imaging (MRI) and tissue sampling, different cortical samples from members of each hominoid species were analyzed. In each species, specific areas were either relatively enlarged or shrunken, which can detail neural organizations. Different sizes in the corticol areas can show specific adaptations, functional specializations and evolutionary events that were changes in how the hominoid brain is organized. In early prediction it was thought that the frontal lobe, a large part of the brain that is generally devoted to behavior and social interaction, was the main player in the differences of behavior in hominoid and humans. This theory was dispelled by the test that showed that with damage to the frontal lobe both humans and homonoids show atypical social and emotional behavior meaning that the frontal lobe was not very likely to be selected for reorganization. Instead, it is now believed that other parts of the brain that are strictly associated with certain behaviors was where evolution targeted. The reorganization that took place is thought to have been more organizational than volumetric; whereas the brain volumes were relatively the same but specific landmark position of surface anatomical features, for example, the lunate sulcus suggest that the brains had been through a neurological reorganization. There is also evidence that the early hominin lineage also underwent a quiescent period, which supports the idea of neural reorganization.

Dental fossil records for early humans and hominins show that immature hominins, including australopithecines and members of Homo, reveal that these species have a quiescent period (Bown et al. 1987). A quiescent period is a period in which there are no dental eruptions of adult teeth; at this time the child becomes more accustomed to social structure, and development of culture. During this time the child is given an extra advantage over other hominoids, devoting several years into developing speech and learning to cooperate within a community. This period is also discussed in relation to encephalization. It was discovered that chimpanzees do not have this neutral dental period and suggest that a quiescent period occurred in very early hominin evolution. Using the models for neurological reorganization it can be suggested the cause for this period, dubbed middle childhood, is most likely for enhanced foraging abilities in varying seasonal environments. To understand the development of human dentition, taking a look at behavior and biology.

Genetic factors contributing to modern evolution

Bruce Lahn, the senior author at the Howard Hughes Medical Center at the University of Chicago and colleagues have suggested that there are specific genes that control the size of the human brain. These genes continue to play a role in brain evolution, implying that the brain is continuing to evolve. The study began with the researchers assessing 214 genes that are involved in brain development. These genes were obtained from humans, macaques, rats and mice. Lahn and the other researchers noted points in the DNA sequences that caused protein alterations. These DNA changes were then scaled to the evolutionary time that it took for those changes to occur. The data showed the genes in the human brain evolved much faster than those of the other species. Once this genomic evidence was acquired, Lahn and his team decided to find the specific gene or genes that allowed for or even controlled this rapid evolution. Two genes were found to control the size of the human brain as it develops. These genes are Microcephalin and Abnormal Spindle-like Microcephaly (ASPM). The researchers at the University of Chicago were able to determine that under the pressures of selection, both of these genes showed significant DNA sequence changes. Lahn's earlier studies displayed that Microcephalin experienced rapid evolution along the primate lineage which eventually led to the emergence of Homo sapiens. After the emergence of humans, Microcephalin seems to have shown a slower evolution rate. On the contrary, ASPM showed its most rapid evolution in the later years of human evolution once the divergence between chimpanzees and humans had already occurred.

Each of the gene sequences went through specific changes that lead to the evolution of humans from ancestral relatives. In order to determine these alterations, Lahn and his colleagues used DNA sequences from multiple primates then compared and contrasted the sequences with those of humans. Following this step, the researchers statistically analyzed the key differences between the primate and human DNA to come to the conclusion, that the differences were due to natural selection. The changes in DNA sequences of these genes accumulated to bring about a competitive advantage and higher fitness that humans possess in relation to other primates. This comparative advantage is coupled with a larger brain size which ultimately allows the human mind to have a higher cognitive awareness.

Further reading

• Falk, Dean (2011). The Fossil Chronicles: How Two Controversial Discoveries Changed Our View of Human Evolution. University of California Press. ISBN 978-0-520-26670-4.
• Raichlen, D.A., and J.D. Polk. 2012. "Linking brains and brawn: exercise and the evolution of human neurobiology." Proceedings of the Royal Society B: Biological Sciences 280. doi:10.1098/rspb.2012.2250
• Striedter, G. F. (2005). Principles of Brain Evolution. Sinauer Associates.