Discovering the Things that Make Us Human: Evolution of the Brain

Biology 202
1998 First Web Reports
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Discovering the Things that Make Us Human: Evolution of the Brain

Libby OHare

There is a powerful sentiment among many people that human beings are the most intelligent and complex animals on earth. Our ability to use language is matched by no other species. It makes sense to assume that whichever characteristics of the human brain arent possessed by other animals, specifically primates, our closest evolutionary relatives, are the ones responsible for the acquisition, production, and comprehension of language. As the relatively new field of neuroscience develops, scientists are beginning to study the evolutionary relationships of brain organization and function in an effort to understand the regions of the brain responsible for language, abstract thought, and self-expression. This discipline, known as Paleoneurology, examines the development of the hominid brain (1). Paleoneurology has its roots in the fields of neurology, paleoanthropology, and primatology. There are two methods used by Paleoneurologists to study brain evolution: the examination of fossil skulls and endocasts, and comparative primate neuroanatomy (2). The study of the evolution of the brain is useful, because it allows for an understanding of the origins of the structural differences between humans and other animals--in effect, what makes us human.

According to the fossil record, the first brain structure appeared in reptiles around 500 million years ago. The functions of this hindbrain included breathing, heart beat regulation, balance, basic motor movements, and foraging skills (1). An interesting trend in brain evolution is that more evolved species tend to preserve the structures responsible for basic behaviors. In other words, evolution is the process of acquiring more and more sophisticated structures, not simply the addition of different structures. Therefore, the modern human brain contains the primitive hindbrain region, often called the protereptilin brain (1), and it is the seat of fundamental homeostatic functions. The major structures found in this region of the brain are the Pons and Medulla. About 250 million years after the appearance of the hindbrain, a new region known as the paleomammalian brain arose. This region consists of the hippocampus and cerebellum and is often referred to as the limbic system. As might be expected, this region is associated with more complex functions such as emotional, sexual, and fighting behaviors (1). The newest and most intricate area of the brain, the cerebrum, was first developed around 200 million years ago. The highly convoluted surface of the cerebrum is called the neocortex. Here we find the location of the higher cognitive functions, like language, thinking, and information processing (1).

A second set of observations in Paleoneurology comes from comparative neuroanatomy. These observations center on the size of brains across different species. Currently there is a debate as to the usefulness of brain size in understanding behavioral differences. Those opposed to the reliance on this method point to the lack of evidence for a significant relationship between brain size and intelligence (3). However, there does appear to be some evidence that brain enlargement involves an increase in neuron connections: It has been established that among mammals the number of brain cells remains remarkably constant, what increases is their degree of interconnectedness, the cell-density drops and the complexity of the neuron network increases (1). There is one standard that both camps of the size debate agree on: the development of folds in the neocortex was a significant factor in brain evolution, in that folding permitted a larger surface area for the organization of complex behaviors: Clinically an expansion in the prefrontal cortex would afford an increased capacity to relate internal and external experience and thus to identify ones inner feelings with those of other beings. It also plays a fundamental role in relating past, present, and future in regard to looking ahead making possible both anticipation and choice (1).

The current belief that humans are intellectually superior to all other animals rests on the major observation of Paleoneurology. Lateralization of brain function, which is a result of the cerebrum being divided into two halves, is unique to humans. It is believed that this specialization allows the brain to take both digital and analogic approaches to any subject (3). The left hemisphere controls the digital functions like rational, verbal, and analytic perception and thinking. The right hemisphere, or analogic hemisphere is responsible for creative thinking (3). The ability to combine these two types of thinking results in the skill that William Calvin calls sequential muscle control (4). Essentially, lateralization means that the human animal is capable of the integration and execution of complex sequences of behavior, like those involved in language production(1).

Several hypothesis have been offered to account for the presence of lateralization in the human brain. Holloway asserts that when our ancestors made the move to bipedal locomotion the nervous system was reorganized, through the process of natural selection, to operate in co-ordinated and sequential ways (1). Calvin suggests that the ability to throw overhand was the impetus for lateralization in the brain: Throwing has the advantage that it is one-sided (in terms of brain function/hemisphere), that it stress the system for speed, and that it has a major payoff (in terms of natural selection) when used for hunting (4). To truly appreciate the contributions of Paleoneurology to our understanding of the brain and behavior, we must remember to temper our beliefs about human intellectual superiority with a bit of humility. The dolphins brain is slightly larger than ours, and its neocortex has a greater degree of folding. According to the data and observations just presented, this means that dolphins are more intelligent than humans. Currently, this conclusion is not fully accepted across the field. However, regardless of which species turns out to be the brightest, work with dolphins could provide a glimpse of what is to come in the evolution of the brain.

WWW Sources

1) http://citd.scar.utoronto.ca/ANT 3032/Henderson/Intro.htm

2) BRAIN EVOLUTION AND NEUROLINGUISTIC PRECONDITIONS --Wilkins and Wakefield

3) Brain matters--a neurological journey guided by Tommy St¿ckel

4) The Throwing Madonna --Essays on the Brain

5) http://comp9.psych.cornell.edu/Psychology/News/Brain_Evolution

6) http://www.acsiom.org/cgi-bin/nsr/web_fet...b=neuro&doc_id=35281&query=Paleoneurology

 

 

Comments made prior to 2007

I would like to correspond with anyone having knowledge about the following:
1) What is the relationship between working with the hands and our ability to make decisions if any?
2) Assuming we learned to comminicate using rudementary signs with the hands, what is the function of that part of the brain at present ... Jim Archer, 30 March 2006