Biological evolution accompanies a linear increase in complexity. Look around you, it doesn’t take much time for us to realize that our world is quite complicated. The Earth hosts a variety of taxa (species, subspecies, and races), with the major taxa, which have evolved most recently, being the most complex. Humans, or homo sapiens, are in this bracket.
It is easy for us to identify, on the basis of our physical and mental attributes, functions and behaviors, factors that may set us apart from others, and contribute to our unique identity. By doing so, we have merely partaken in a significant evolutionary behavior, namely the ability to identify differences among ourselves, as opposed to other species, including members of our own taxa. Most often, we relegate our comparisons to distinctions based on the quantity, and quality of characteristics.
In this chapter, Sagan uses this notion as a medium towards the deeper understanding of our evolutionary history as a species by addressing the history of life in the gradual dominance of brains over genes.
The Book of Life
While a primitive notion on the complexity of an organism can be obtained by considering its behavior, and functions, we can do the same by considering the minimum amount of information contained within said organism. In other words, how much genetic material does the organism hold?
The book of life is written in the language of four alphabets, consisting of complex molecular structures called the nucleotides. DNA (Figure 1), or Deoxyribonucleic acid, the primary hereditary molecule, consists of about five billion pairs of nucleotides. In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes.
All the organisms on Earth share this genetic language composed of four alphabets, leading to the theory that we are all descended from a single ancestor, or a single instance of life some billion years ago.
“If there are approximately six letters in an average word, the information content of a human chromosome corresponds to five hundred million words. If there are about three hundred words on an ordinary page of printed type, this corresponds to about two million pages. If a typical book contains five hundred such pages, the information content of a single chromosome corresponds to four thousand volumes.” – Carl Sagan, The Dragons of Eden
Brains vs. Genes
The gradual dominance of brains over genes, the premise of this chapter, is shown to follow a practical necessity in the evolution, and survival of various species. The main working materials of evolution are mutations (Figure 2), heritable changes in the nucleotide sequences of our DNA.
Most mutations/genetic changes are too slow for any practical benefit (occurring over millions of years). Most mutations are also harmful, and in some cases recessive. In the end, what really counts are mutations in the gametes, the eggs and sperm cells, the agents of sexual reproduction.
“Large organisms such as human beings average about one mutation per ten gametes-that is, there is a 10 percent chance that any given sperm or egg cell produced will have a new and inheritable change in the genetic instructions that determine the makeup of the next generation.” – Carl Sagan, The Dragons of Eden
With such a high mutation rate to already cope with, a greater complement of DNA could suggestively result in unacceptably high mutation rates. Too much would go wrong, for anything to stay right! As Sagan postulates, there must be a practical upper limit to the amount of genetic information that the DNA of larger organisms can accommodate, thus by the same instance, necessitating the existence of substantial resources of extragenetic information to allow for their survival, and growth. Such information is contained in all “higher” organisms, except humans (who have invented extrasomatic knowledge, or information stored outside our bodies, like writing), exclusively in the brain.
Peering Into The Brain
The remainder of the chapter an exclusive tour de force of the brain. The corresponding questions, and their central arguments can be summarized as follows,
What is the information content of the brain?
There are two extreme opinions on brain function:
(a) The brain or the cerebral cortex (its outer layers) are considered to be equipotent; there is no localization of function, such that any part of the brain can substitute for another.
(b) The brain is completely hard-wired, and specific cognitive functions are localized in specific parts of the brain.
The actual answer lies somewhere between the two as has been portrayed by various experiments over the years. Modern-day neuroscience continues to address this question.
Is there a correlation between brain mass, and intelligence?
There isn’t a one-to-one between brain mass or size and intelligence in human beings, but there remains a statistical correlation nonetheless (namely in that there are upper and lower limits which may correspond to normal adult human brain function). It has been found that the correlation between brain size, and intelligence is much better than the correlation between intelligence, and adult body weight. Sagan presents the criterion of brain mass to body mass, with no consideration of behavior to provide an acceptable first approximation for intelligence.
What is the structure of the brain, or how is it packed?
The human brain contains about ten billion switching elements called neurons. There is contention among neurobiologists that neurons are the active elements in the brain, as evidence has been provided that specific memories and other cognitive functions are contained in certain molecules in the brain, like RNA or small proteins. An average neuron in a human brain is said to have been 1000-10,000 synapses. If we were to consider each synapse as equivalent to a on-off state, then the number of different states of a human brain is far greater than the total number of elementary particles in the entire universe. We are all truly unique!
The brain is basically a system of microcircuits, and a very dense on at that with an information content of ten billion bits per cubic centimeter. As Sagan points out, a modern computer able to process the information in the human brain would have to be about ten thousand times larger in volume than the human brain. Though the processing speed of a computer is far higher than the brain, the fact that our brain can do so many significant tasks so much better than the best computer is a testament to how tightly packed, and well-organized it is. We have yet to break this mystery, and come to a complete understanding of the brain, and its functions.
The End Game
Thus, having considered the quality and quantity of genetic material, and brain information in organisms, Sagan is able to compare the gradual increase through evolutionary time of both the genetic, and brain information of organisms, showing that somewhere around the Carboniferous period, a few hundred million years ago, there first emerged an organism with more information in its brain than in its genes. While Sagan refers this organism to be an early reptile, conflicting theories exist today in contention of this statement, and our earliest ancestor.
In conclusion, the evolution of the brain is a symbolic event in the history of life. The consequent bursts of brain evolution would result in the emergence of mammals, and of manlike primates. Nevertheless, there still remains much to the story.
“Mankind is poised midway between the gods and the beasts.”- Plotinus
As we will see in the following chapters, humankind’s naivety rests in its pride as the summit of the biological cycle, but while our authority on this Earth may be justified by the repeated insistence of our higher intelligence, and brain function, we are yet to realize that we are not so completely removed from the “beasts” themselves.
“…with all these exalted powers- Man still bears in his bodily frame the indelible stamp of his lowly origin.” – Charles Darwin, The Descent of Man