Integrated different disciplines to study human evolution
• need to use multiple approaches
• anatomy
• molecular genetics
• study of fossils-taphonomy
• past environments
• comparisons with modern & fossil primates
• examination of behaviors among living people
Anatomy
•study of modern skeletons•effects of activity•growth•disease•age•diet•environmental effects•compare with fossils
Molecular Evolution
• can help us understand divergence in new ways
• timing
• geographical spread
• traits that are linked in evolution
Fossils & Taphonomy•modification of remains •animal damage•weathering•crushing & distortion through burial•chemical modification
Past Environments
plant remains
invertebrate remains large fauna
geologic setting
Living & Fossil Primates
Proconsul africanus Kenya,14-23 mya
tarsier
baboons
Living Humans •dietary decisions•sociality•sexual strategies•growth & development•population dynamics
What were the ancestors like?What does it mean to be human?
Homo habilisKNM ER-1813 Koobi
Fora, Kenya1.9 mya
Primates have grasping hands for locomotion
and feeding
pygmy marmoset
baboon
tarsier
Primates have large, forward facing eyes for stereo vision
tamarin
ring tail lemur
slender loris chimpanzee male
tarsier, Tarsius sp., small prosimian, Indonesia, vertical clinger & leaper
LemursMadagascar
•terrestrial locomotion•arboreal clingers & leapers•climbers•social group living
Vertical leaping•crouched and clinging to a limb
•thigh muscles provide the force to produce a leap
•arms used mostly for balance stabilization
•land feet first
Drawings by Luci Betti
sifaka, lemur
Baboons
terrestrial locomotion
New World Monkeys
capuchin monkey
•arboreal •suspensory•above limb walking•mostly group living
GibbonsAsia
•brachiation•underbranch suspension
OrangutanSumatra &
Borneo
•slow •solitary •suspensory locomotion
GorillaCentral Africa
•group living •terrestrial knuckle-walker
chimpanzee, Pan troglodytes
bonobo, Pan paniscus
Chimpanzees & Bonobos
Central Africa
•group living •highly social•terrestrial & arboreal adaptations•last great ape line that the hominin lineage diverged from ~10-7 mya
chimpanzee, Pan troglodytes, west and central Africa, forest knuckle walker-terrestrial
locomotion and climbing
Adaptive Radiations of Primates
•Paleocene~60 mya-earliest possible primates•Eocene mya ~56-35 mya•Late Oligocene ~30-22 •Miocene ~22-11 mya
tree shrews
resemble primitive primates
•different ear bones•have claws•non-grasping hands•eye orbits not closed
Ida” 47 mya adapiform, near Frankfurt, Germany
lemur-like prosimian ancestor
Aegyptopithecus zeuxisEgyptian Fayum, 29 mya, Miocene
Proconsul africanusRusinga Island, Kenya,14-23 mya,
Miocene
Sivapithecus indicus Potwar Plateau, Pakistan,
8.5-12.5 mya, Late Miocene, orangutan ancestor
Sivapithecines = ancestors of the Orangutan lineage
Sahelanthropus tchadensisChad, 6-7 mya
Sociality
Bra
in s
ize
% fruit in dietB
rain
siz
eGroup size
Findings from the “ecological” and “social” hypotheses
Points reflect individual species: both are upheld
Mountain gorilla
Look at relationship of “social part of brain”-neocortex- with respect to group size
Data tend to support “social” hypothesis, since this relationship doesn’t hold for ecological factors
Neocortex and group size
Human social organization
Human social organization is a mosaic of traits that have deep roots. Different traits arose at different times in different circumstances
Grooming
Male Aggression
primate sexual dimorphism associated with distinct feeding, mating, and
sociality
gorilla femalegorilla male
Tamarin reproductive biology
Insights into patterns of male care-giving and male fitness.
Tamarin groups 1-3 adult males and 1-3 adult females.
After mating with all group males, the alpha female gives birth to dizygotic twins, other females often don’t reproduce.
Each offspring weighs 8% of mom’s weight and grows rapidly (in humans, the equivalent to giving birth to two 9lbs infants)
The alpha female can give birth to two sets of twins in a year
Number of adult males/females
Ave
. nu
mb
er o
f su
rviv
ing
off
spri
ng
0 1 2 3 4 5 6
2.5
2
1.5
1
.5
0
males
females
Offspring survival depends on male care in tamarins
In one study group, males did 73% of all infant carrying.
Adult males will also provision infants with food.
Males will provision infants/juveniles for 50% of their growth period (long, relative to other primates)
Tamarin reproductive biology
Main point: male investment in offspring is a fact of tamarin male behavior; it is a function of local ecology, life history patterns, and reproductive options.
Tamarin males, like human males, have high potential reproductive rates, but the evolved tamarin mating system does not let males actualize their reproductive potential.
Instead, they share matings/reproduction with other males.
Brain
•not just an increase in size•differences in brain organization•cognition•thought•emotions•brain is a costly tissue•affects dietary needs•growth & development•birth
Primates: Unusually large brain relative to body size.Monkeys and apes: Very large brain when compared to other mammalsHumans: Carry this pattern to greater extreme
Large brains are expensive to maintain: Brains ~2% of body weight, consume ~20% metabolic energy
We wouldn’t expect natural selection to maintain this costly feature unless it was adaptive.
Neocortex
very thick in humans and non-human primates
•30-40% brain volume in non-primate mammals
•50% brain volume in prosimians
•80% brain volume in humans
Interspecific comparisons
•EQ the ratio of brain size to body mass based on interspecific comparisons. “Encephalization” refers to the degree of “excess” brain mass relative to body size.
•EQ’s take into account body size, since-obviously, an elephant’s brain will weigh more than a mouse’s brain on an absolute scale.• •However, taking the ratio of brain size to body mass allows us to compare brains in diverse taxa.
Encephalization Quotient (EQ): an to compare brain size across sepcies
EQ in humans
•the line shows expected brain size for a give body size
•points above the line indicates a larger than expected brain size for a given body size
larger than expected
Encephalization Quotient (EQ): ratio of brain size to body mass based on a linear regression of interspecific data for primates.
EQs for various catarrhine taxa:
baboon 1.1gorilla (male) 2.1chimpanzee 2.3australopithecines 2.5H. habilis 3.1H. erectus 3.5H. sapiens 7.5
Hypothesis that primate intelligence evolved in order to solve social challenges
Machiavellian intelligence hypothesis:The ability to use other individuals as tools; manipulating the social environment in order to meet preconceived goals:
•deception (groom as a means to steal food)•alliance formation (grooming predicts future support)