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

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ize

% fruit in dietB

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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

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mb

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f su

rviv

ing

off

spri

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0 1 2 3 4 5 6

2.5

2

1.5

1

.5

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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)