Chapter 2: Origins of Human Brain and Behaviour
Cladogram – a graph that shows the relative time of origin of various closely related groups. Each branch
point distinguishes animals positioned before that time point from animals positioned after it by one or
more physical or behavioral traits.
In the past 5 million to 8 million years, Hominids – our humanlike ancestors – diverged from this
ancestral ape lineage by acquiring characteristics that distinguished them from other apes.
Dualists: distinction between human & non-human brain
Why Study Nonhuman Animals?
Psychologists who work with chimps and other apes assume that the things that they learn about our
closest animal cousin apply to the human brain and to human behavior. Others include: rats, cats, slugs
and fruit flies.
Bird brain are similar but they have no cerebral cortex (important in mammalian brain).
Rodents are evolutionarily the most closely related order to the primates. Yet the rodent brain
is distinguished by having large olfactory bulbs and a small cerebral cortex, whereas primates
have small olfactory bulbs and a large cerebral cortex.
Many whale species have neither olfactory bulbs nor some cortical structures associated with
olfactory bulbs. Yet dolphins and whales have good memories and engage in complex social
Questions Addressed by Studying Nonhuman Animals
Three primary lines of research drive neuropsychological investigations with animals:
1. understanding basic brain mechanisms
2. designing animal models of human neurological disorders
3. describing the phylogenetic (evolutionary) development of the brain.
1. Understanding Brain Mechanisms
The eye takes vastly different forms in different species (fruit flies and humans) eye evolved a number
of times. BUT the same genes are implicated in all species.
Claudia Hetzer-Egger: Pax – gene responsible for eye development in all seeing animals.
Homeobox Genes – dictate body segmentation in both fruit flies and humans.
Segmentation of the human nervous system into the spinal cord, brainstem, and forebrain is
produced by genes first discovered in fruit flies.
Mutations – slight alterations in the differences in the structure of the eye and the nervous system in
different animal species, in genes such as Pax and in the way in which the products of those genes
interact with the products of other genes.
2. Designing Animal Models
* Research animals substitute for humans because similar principles are assumed to underlie the
emergence and treatment of a disorder in humans and nonhumans.
Parkinson’s Disease – associated with aging in humans and can affect as many as 1% of the population
older than 65 years of age. The symptoms include rigidity that impedes voluntary movement, balance
problems, and tremors of the head, hands, and limbs.
The cause is unknown and there is NO cure.
3 goals in finding treatments: to PREVENT the disease, to SLOW its progression once it has
developed, and to TREAT symptoms as the disease progresses.
Mouse, rat, and monkey used L-DOPA (drug treatment)
3. Describing Evolutionary Adaptations
Comparative research on how the mammalian brain and behavior evolved progresses in 3 ways:
1. Experiments told us how the environment of mammals shaped its evolution, brain, and behavior.
2. Commonalities tell us what humans inherited in common with other mammals (primate lineage). 3. Differences in the brains and behaviors told us how species and individual differences arose.
Humans = Tools
Evidence also found in extinct predecessors.
BUT it’s also found in other species
o Eg. Birds surface area of the cerebellums of tool-using birds is larger
o Human cerebellum is associated with the development of tool use.
Use of a Quasi-evolutionary Sequence
William Hodos & C. B. G. Campbell: “Phylogenetic perspective” used by researchers Quasi-
Evolutionary Sequence – a hypothetical sequence of animals that represent consecutive stages in
Animal can be chosen because it is the living descendant of an extinct ancestor.
Assumption: Evolutionarily older present-day animals resemble a common ancestor closely
enough to stand for it
Eg. Tree shrews have striate cortex (primary visual cortex with a striped appearance) confers on shrews
an ability to see branches, heights, and insects (not in ground-dwelling hedgehog = earlier) = humans.
Eg. Bush baby has a large temporal lobe enabling a highly varied diet and excellent memory = humans.
Eg. Rhesus monkey’s large frontal lobes are due to its very complex group social life = humans.
Eg. Human’s large parietal lobe enables us to perform the skilled movements required in tool making.
Evolution of new brain features in living primates explains the evolution of the brain and
behavior of humans.
Michael Oldham & Daniel Geschwind: Using comparative genetic analysis of this quasi-evolutionary
sequence of primates to investigate the neural basis for the origins of language and other behaviors.
Darwin’s publication of On the Origin of Species by Means of Natural Selection 1859. In The Descent of
Man he speculated that humans descended from an ancestral “hairy, tailed quadruped, probably arboreal
in its habits.”
The evolution of humans from an ape ancestor to Homo sapiens is NOT linear. The hominid family tree is
a bush: for most of its history, many family members were alive at the same time. Our species is the only
surviving member on the last living hominid branch.
Sediments where bones of different hominids are found = lineage of hominid + time of origin.
Skull casts = brain structure.
Examination of the habitat and tools = behavior.
Morphological Reconstruction – skeletal remains used to approximate the appearance of a hominid
body to reveal similarities and differences between hominids and us.
Neanderthal – a hominid species related to modern humans who lived in Europe but disappeared about
40,000 years ago. Similar physically and behaviorally: tools, family, music, and religion.
Biochemical and Genetic Research
Evidence for rapid hominid speciation is supported by biochemical research. The amino acid sequence of
a cellular protein provides a molecular clock that can be used to compare the ages of different species.
Eg. Old World and New World monkeys diverged from each other 30 million years ago. Their 24
differences in albumin amino acids suggest a rate of one amino acid change every 1.25 million years. If we
apply this rate of change to apes, we can conclude that chimpanzees and humans diverged from each
other between 5 million and 8 million years ago.
DNA – the genetic material in the nucleus of the cell; Used to determine relatedness of different species
Genes – segments of DNA that specify what proteins a cell should make. o Each gene is a long chain of four kinds of nucleotide bases. Through mutations, the
bases can change and still leave a functional gene compared to other species.
Even fossils of long-extinct animals’ DNA can be identified.
Genome – the full set of genes of a species
o Human evolution description would include information on genetic modifications.
Eg. Modern humans and chimpanzees have 99% of their genes in common and the closest living relatives.
Jane Goodall: behavioral studies of chimpanzees; very similar to humans. Gallup’s mirror test
Occupy large territories that the males defend with war and kill to expand their territories.
Great travelers, omnivores, hunt cooperatively, rich communication, and advanced tool use.
Live in complex social groups within which family relations are important both for the
individual chimpanzee and for the group structure.
Stages of Human Evolution
A. Discontinuous evolution of humans because exact relationships are unknown.
B. The behavioral changes, including tool making, were associated with the increases in brain size.
Steps from Chimpanzee-like
to modern-like human:
1. An UPRIGHT
POSTURE in which
the hands were free
2. Extensive TOOL USE
3. A TRAVELING life
4. An elaborate
Australopithecus: Upright Posture
Raymond Dart: The ANCESTOR OF ALL HOMINIDS was an animal somewhat like Australopithecus
(Australo, meaning “southern” and pithecus, meaning “ape”). These animals lived in eastern Africa and
possessed a distinctly human characteristic: they walked upright. fossils of their footprint features
Yves Coppens: “East Side Theory”: about 8 million years ago, a tectonic crisis produced the Great Rift
Valley splitting Africa in two, leaving a wet jungle climate to the west and a much drier climate to the east,
where the apes had to evolve rapidly to survive in the mixture of trees and grass that formed their new
brushwood habitat. Changes: reduction in the size of the incisors, flattening of the molars, varied diet,
longer legs suited to over-ground locomotion.
Down-From-Trees hypothesis: proposes that the trees being farther apart required apes to adopt
bipedal locomotion posture reduced the body’s exposure to the sun allowing the loss of body hair.
Water-Baby hypothesis: proposed by Alister Hardy; a hypothetical naked ape swimming and foraging
on ocean beaches and later forced to abandon its semiaquatic habitat when the ocean receded.
Bipedalism and lack of body hair advantageous in swimming retains these features.
All stories ape continued to climb trees but changed to an upright posture and adopted varied diet.
brain size did not change much though (brain enlargement due to other factors).
Homo habilis: Tool Use
Homo – the genus to which modern humans belong
Louis Leakey: found the oldest Homo fossils in the Olduvai Gorge in Tanzania in 1964, dated at about
1.75 million years old. Similar to Australopithecus, but