Chapter 12: Humans, Human Brain and Evolution
Evolution of Humans
• Darwin’s theories tell us all living creatures been and continue subject to
• Evolutionary theory provides important insights into behaviours that also been
subject to selection pressures.
• Few researchers applied principles of evolution to human behaviour. This new
research is very controversial.
• Level of acceptance not yet achieved for evolutionary psychology, which
attempts apply principles of adaptation and selection to human behaviour. Lack
of acceptance of evolutionary psychology attributed to relative newness and
difficulties that are inherent whenever new theory challenges accepted social
views. We do not like thinking of ourselves as animals and being subject to laws
• Discovery of evolutionary theory generally credited both Charles Darwin and
Alfred Russel Wallace. Development of theory was made independently.
• Darwin became intrigued with understanding how species emerged and how
geographic isolation on islands produced such variety of features in what he
presumed were same species.
• Wallace explored Amazon River became intrigued with the relationships between
geography of particular location and its effects on specific characteristics of
species that inhabited that niche.
• Important insights that allowed them produce the theory evolution. Ideas include
o Carolus Linnaeus observed commonalities in structure among related
o Charles Lyell and William Smith supported notion earth was far older than
previously thought. Primitive forms occurring in older strata of the earth.
Lyell argued that length of time required for these event occur suggested
the earth much older than previously been thought.
o Malthus suggested populations grow exponentially until they surpass food
supply, leads to struggle for existence.
Historical Theory Evolution
• Historical theory of evolution can be summarized by three terms: variation (all
individuals vary, resulting in differences in morphology), inheritance
(differences in morphology can be passed from one generation to next), and
differential reproduction (these individual differences morphology result
variations in success in environment, terms of survival and reproduction).
• Darwin suggested mechanism underlying these changes natural selection.
Natural selection requires all individuals unique and that characteristics give an
animal reproductive advantage result in magnification of these traits in population. When trait results reproductive advantage and selected for, called
adaptation. Trait to be adaptation must be inherited from one generation to
• Darwin eventually became dissatisfied w/ natural selection as only possible cause
or (force) evolution therefore proposed another type of selection: sexual
selection. Natural selection – competition among individuals for survival to
reproduce determines which genes remain in gene pool and which genes
disappear from face of planet. Sexual selection - competition among individuals
for reproduction determine gene’s fate.
- Intersexual selection – one sex chooses a mate from among members of
other sex on basis of specific traits.
- Intrasexual selection – members of same sex compete for partners of
o Original theory of evolution referred as historical theory of evolution because of
changes to theory that has occurred since inception.
o Traits are passed on by genes and the original source of variation is random
mutation of these genes. Current version of evolutionary theory referred as
modern synthetic theory of evolution or modern synthesis.
o Modern synthesis based on what known about genes DNA (deoxyribonucleic
acid), chromosomes, and population biology.
o Genes, made of DNA, assort in pairs and located on chromosomes, which simply
strings of genes. Human have twenty-three pairs of chromosomes. When egg is
fertilized, sperm contributes twenty-three chromosomes to egg’s twenty-three
chromosomes, resulting in twenty-three pairs of chromosomes.
o There is more the same in genetic makeup of individuals than different. These
similarities allow for there to be common features such as eyes and bipedal gait
(walking upright on two legs), among members of our species.
o Your genotype is entirely your genetic composition, and barring exposure certain
chemicals or radiation, genotype is invariant during lifetime. Phenotype is
interaction of your genotype w/ environment which you develop.
o Only genotype passed on to your children.
o Natural selection operates on phenotypes, only genotypes transmitted to
generation to generation.
o Genes make proteins not traits or diseases.
o There is often more than one form of given gene; different forms called alleles.
o When say found gene for Alzheimer’s disease, mean is we found one allele of
gene that, in interaction w/ environment, produces some of symptoms associated
w/ Alzheimer’s disease.
o As genes assort in pairs, dominant genes always expressed whenever present.
Recessive genes expressed only when there no dominant genes present.
o Both alleles are same, say that an individual is homozygous for that trait.
o When two alleles diff., say that individual is heterozygous for that trait. o Very few traits completely determined by single gene. When multiple genes
affect a trait, say trait is polygenic. Eye color is affected by at least three different
genes on two different chromosomes.
o Any population, there is genetic variation. Variation comes about by random
mutation and/or recombination of DNA. Mutation occurs when there is change in
genotype due to an error in replication of DNA. Rate of mutation is typically
slow, most mutations are harmful to organism, resulting in reducing ability to
o Evolution works by selecting individuals who are better able to survive and
reproduce, thus passing their genes on to offspring.
o Central tenet of modern synthesis is certain environments select certain
phenotypes and phenotypes are expression of genotype interacting w/
o Environment cannot typically change the genotype. Environment factors may
affect rates of mutation, do not provide direction for mutation to occur.
Environment does not produce adaptations.
o Genes occur at level of individual, evolutionary change occurs level of
populations. Populations evolve by changes in gene frequency that are brought
about by natural selection, random genetic drift, and gene flow. Small diff. in
genes can result large changes in genetic makeup of population in relatively short
period of time if provide tangible survival and/or reproductive advantages for
those who possess the gene.
o Genetic drift, tendency for isolated populations to depart from original genetic
composition of population. Drift away from original genetic composition of
population most likely result of extensive interbreeding (offsprings resulting
from related parents) and adaptation to isolated environment. Gene flow
movement of genes through population results form mating.
o Species represent diff. gene pools rather than fundamentally unique groups.
Species judged as such by genotypes, not their phenotype. When judge speciation
basis of phenotype, can often be misled because there is often great variation in
phenotype among individual of population.
o Three major diff. between modern synthetic theory of evolution and historical
evolutionary theory. First, modern theory recognizes traits are result of genes that
are inherited from one’s parents and interact w/ environment. Second,
recognition that there are mechanisms other than natural selection that effect
evolutionary change. Third, recognition that what we call species only diff. in
gene pools of populations.
Evolution of the Nervous System
o Brain tissue soft and quickly deteriorates under natural conditions, thus leaving no
o Researchers examine fossilized skulls to obtain clues regarding brains of our
o Direct observation of skulls of our fossil ancestors, can see an increase in skull
size and in size of brain.
o Must be cautious about direct implicating brain size w/ increased intelligence. o Tend to have one of larger brains for size of our body.
o Human brain has almost 3.2 times amount of cortex than other species.
o Another way examine skull of animals make endocast, or mol