The adult human brain of today averages about 1,300 grams in weight, compared to an elephant’s 5,000 grams, and a whale’s 10,000 grams, both of which are much smaller than the human in terms of brain to body mass. The human brain has about 100,000 neurons per cubic centimeter and between 100 and 300 billion per brain. This is about the same number as there are stars in our galaxy. In comparison, rats have about 10,000 neurons per cubic centimeter, and monkeys just triple that. The human neocortex—cerebrum—has a surface area of 5.0 meters or 16 feet squared. In short, there was another great evolutionary leap from primates to humans. Part of the reason for the success of humans is that evolutionary mutations gave us much more myelin in ratio to nerve
cells which is protective to the nerve tissue itself and acts to prevent leak of electrochemical pulses with attendant errors of transmission, an important factor in intelligence. Gene modifications made that possible in humans, and the same genetic changes are not found in other primates which are more similar to the common ancestor than are humans. Myelin aids in the speed of nerve conduction, an attribute that differentiates high from low intelligence. It is safe to conclude that no other creature was ever capable of managing such a level of complexity or could provide the energy to produce and maintain it, or is ever likely to do so.
Myelin is relatively chemically inactive and uses less glucose than functional neuronal tissue. Thus, the lower energy use in more intelligent brains may be related to the higher percentage of myelin which somehow imparts greater efficiency to neuronal work—fewer energy-demanding neurons are required for any specific task than in the more primitive animals.
65 million years ago, the mammalian family was represented by the simple Didelphodon, a small, four-legged creature similar to the opossum of today. Over that time period, every mammal seen today evolved through the process of mutation accompanied with the pressure of natural selection. Evolution was an efficient system, utilizing genes and proteins contained in the molecular structure of preceding animals. In the last two million years, humans have added 50 billion neurons to the Homo erectus brain, and the human female pelvis was enlarged and redesigned by evolutionary forces to permit the passage of that enlarged cranium at delivery. In order to accomplish the phenomenal changes found in human brain size, complexity, and performance, it would have required an evolutionary velocity unknown previously. Assuming that Homo erectus was able to reproduce every 10 years (probably a considerably higher rate than actually occurred); in two million years, there would have been 200,000 generations possible to add 50 billion cells to the heritable brain structure. That means that every generation would have had to add 200,000 new neurons; every 200,000 years, 2.5 billion new cells would have been added.
Researchers have identified tiny changes in one amino acid on a single gene that have had a profound effect on speech processing in humans. It is clear that tiny changes in single genes can have very large effects on a species, and it is necessary to conclude that changes on the order required had to have happened in order to produce the modern human brain in an evolutionary very brief period of time. Those changes have yet to be fully identified or clarified. The book, Molecular Biology of the Cell, puts it this way:
“Humans–as a genus distinct from the great apes–have existed for only a few million years. Each human gene has therefore had the chance to accumulate relatively few nucleotide changes since our inception, and most of these have been eliminated by natural selection. A comparison of humans and monkeys, for example, shows that their cytochrome-c molecules differ in about 1 percent and their hemoglobins in about 4 percent of their amino acid positions. Clearly, a great deal of our genetic heritage must have been formed long before Homo sapiens appeared, during the evolution of mammals–which started about 300 million years ago)–and even earlier. Because the proteins of mammals as different as whales and humans are very similar, the evolutionary changes that have produced such striking morphological differences must involve relatively few changes in molecules from which we are made. Instead, it is thought that the morphological differences arise from differences in the temporal and spatial patterns of gene expression during embryonic development, which then determine the size, shape, and other characteristics of the adult.”
This, then, is how the human genome with around 25-26,000 genes, at most, is able to specify the creation, control, and maintenance of the human body containing trillions of cells, billions of carefully wired neurons, and hundreds of different cell types and to react of profound changes, both negative and positive, occasioned by small mutational events. Small collections of DNA mutations can and do have a very large effect on the final result. The remarkable phenotypic evolution of the human nervous system has a salient molecular correlate, i.e., accelerated evolution of the underlying genes, particularly those linked to nervous system development. The overall brain connectivity–like development of neurons and neural specificity–is remarkably similar in arthropod and vertebrate representatives and among mammals—humans and cows differ in only 11 of 437 amino acids that produce neurotransmitter receptors. Based on acetylcholine receptors, humans are very closely related to cows. It appears that the human system makes more efficient use of the genetic material at its proposal. continued…