Chapter 12: Humans, Human Brains, and Evolution
➢ Evolution of Humans
Alevel of acceptance has yet to be achieved for evolutionary psychology, which attempts to apply
the principles of adaptation and selection to human behavior.
Due to its relative newness and difficulties that arise when a new theory challenges the social norm
However, evolutionary psychology gives insight into how our environment has affected the
development of our species as well as the specific demands and subsequent adaptations that evolved
to deal with these challenges.
Discovery of evolutionary theory credited to both Charles Darwin andAlfred Russel Wallace
(two of them came up with the theories independently)
The contributions of others aided Darwin & Wallace
• The classification of organisms based on structure by Carolus Limnaeus.
• Lyell & Smith and their discovery that the earth is much older than thought.
• Malthus – relationship between food & population – survival of the fittest.
Historical Theory of Evolution
• Historical theory of evolution can be summarized by 3 terms: variation, inheritance, and
differential reproduction. In other words, all individuals vary, which results in differences
in morphology. These differences can be passed from one generation to the next which then
results in variations in success in the environment in terms of survival & reproduction.
• Example:All vary in height (variation) – but height is somewhat inherited from parents
(inheritance)– environment favors tall people so only those survive – thereby producing
greater number of tall people (differential reproduction).
• 1) Natural Selection
• Is the mechanism underliying these changes
• Requires that all individuals are unique and that those characteristics with a reproductive
advantage will result in the maginification of these traits in the population.
• Competition among individuals for survival to reproduce that determines which genes
remain in the gene pool and which genes disappear from the face of the planet.
• When a trait results in a reproductive advantage and is selected for.
• It must be inherited from one generation to the next.
• But natural selection is not the only explanation for evolution...
• 2) Sexual Selection
• Competition among individuals for reproduction that determine a gene`s fate.
• 2 types
• Intersexual selection
• 1 sex chooses a mate from among members of the other sex on the basis of
• E.g. Physical attractiveness, muscularity, beauty
• Intrasexual selection
• Members of the same sex compete for partners of the opposite sex. Females
compete with each other and males compete with each other for access.
• Historical theory of evolution: original theory of evolution by Darwin and Wallace.
• Modern Synthesis is the modern theory.
• Combines info from diverse areas such as molecular biology and paleontology. • Includes knowledge about genes, DNA, and chromosomes.
• Genotype vs. Phenotype
• Genotype gets passed on/inherited while phenotype does not
• Genes do not make traits or diseases, they make proteins
• Alleles: different forms of a given gene
• Inheritance patterns of genes follow very simple rules of expression.
• Composition of genes (DNA) was not known at the time
• Gregor Mendel – Mendelian genetics
• Agene can either be dominant or recessive – dominant genes are always expressed
whenever they are present and recessive genes are expressed only when there are no
dominant genes present.
• When both alleles are the same, individual is homozygous for that trait and when two
alleles are different, individual is heterozygous for that trait.
• This view of genes is way too simplistic though – multiples genes can affect a trait too, and
when this happens the trait is polygenic.
• For instance, eye colour is affected by at least 3 different genes on 2 different
• Polygenic model of eye color inheritance is more complex
• Many polygenic traits associate & behave like best friends – like eye color and hair
color. The genes for hair & eye color are associated & tend to pass from 1 generation to
the next together.
• Many polygenic traits are additive & don't follow the simple dominant/recessive pattern
– each allele only has a small effect in determining final phenotype.
• In any population, there is genetic variation which occur from random mutation and/or
recombination of DNA.
• Mutations are harmful to the organism.
• This variation, whatever the cause, (mutation or recombination) results in differing
ability to adapt to the env't. Evolution works by selecting individuals who are better able
to survive & reproduce, thus passing their genes on to their offspring.
• Summary of Modern Synthesis
• 1) Central tenet: Certain environment select certain phenotypes, & phenotypes are an
expression of the genotype interacting with the env't.Although environmental factors
may affect rates of mutation, they do not provide a direction for the mutation to occur.
Therefore, env't does not produce adaptations. Rather, mutations result in characteristics
that in a specific env't are either adaptive, neutral or deleterious.
• 2)Although genes occur at the level of the individual, evolutionary change occurs at the
level of populations. Populations evolve by changes in gene frequency that are brought
about by natural selection, random genetic drift, and gene flow. Natural selection can
produce evolutionary change if a gene or genes provides a slight fitness, or
reproductive, advantage. Genetic drift is the tendency for isolated populations to depart
from the original genetic composition of the population, also can produce evolutionary
change. Drift is the result of extensive inbreeding and adaptation to the isolated env't.
Gene flow is the movement of genes through a population that results from mating.
• 3) Species represent different gene pools, rather than fundamentally unique
• There are 3 differences btwn modern synthetic theory of evolution and historical
• 1) For Modern Theory, traits are the result of genes that are inherited from one's parents
and interact with the environment.
• 2) Recognition that there are mechanisms other than natural selection that can affect
evolutionary change. • 3) What we call species are only differences in gene pools of a population, not totally
distinct or new organism that arrived fully formed from nowhere.
Evolution of the Nervous System
Researchers examine fossilized skulls to obtain clues regarding the brain of our ancestors
We have found that there is in fact an increase in skull size and brain size.
Cautious about directly implicating brain size with increased intelligence of “humanness”.
We tend to have one of the larger brains for the size of our body.
We do not however, have the largest brain in proportion to our body – the tree shrew
What's important in humans is how much of the brain is cortex – most of it is cortex for us –
almost 3.2 times the amount of cortex than other species.
• Another way to examine the skull of animals is to make endocast
• It is a mold of the inside of the skull that looks much like the brain when it is covered with
the meninges and that allows the skulls of mammals to be examined.
• From studying endocasts, we can see that the brain has undergone significant changes in
orgranization – human brains are now assymetrical
• Frontal Lobe
• Intriguing – development of frontal lobe area associated with language in humans
• For the frontal language areas, no significant differences btwn endocasts obtained from
Neanderthals suggesting that they could have used language too.
• Researchers suggest that the study of endocasts, although flawed, is the best way that we
currently have to study the evolution of the brain.
Changes in parietal lobe
• The largest changes in the modern hu