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Primate Evolution – chapter 18 29/03/11 7:45 PM 

Summary of Syllabus points:

-  relative size of cerebral cortex

-  olfactory/optical shift

-  gestation time and parental care

- 

mobility of the digits (opposability)

- 

teeth shape and dental arrangements

Primate Groups, Common Names and Examples

- Lower (less evolved) Primates

- Non – tarsier prosimians (lemurs, lorises, aye-aye)

- Tarsiers

- Tarsiers (Tarsiers)

- Higher primates

- New world monkeys (marmosets, spider monkeys, tamarins)

- Old World Monkeys (baboons, macaques, rhesus monkeys)

- Lesser apes (gibbons, siamangs)

- Great apes (orangutans, gorillas, bonobo, chimpanzees)

- Humans (extinct and modern humans)

Primate Characteristics

Body: Not specialized for any particular environmentLimbs: Generally unspecialized

Hands/Feet: Pentadactyl – five fingers/toes

Nails instead of claws

Grasping fingers and toes with friction ridges for gripping

First digit opposable (save humans – big toe)

Eyes: Forward facing for 3D/stereoscopic/binocular vision

Most are able to distinguish colour

Sense of Smell: Very poor

Teeth: Four incisors in both the upper and lower jaw

Brain: Large and complex

Cerebrum increases in size as primates become more evolved

Reproduction: Not restricted to a breeding season

Rhythmical sexual cycle

Usually only one offspring at a time

Long period of parental care for offspring

DIGITS

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-  Most primates have rods (vision in dim light) and cones (fine visual and colour

discrimination), however the tarsier, being nocturnal only has rods

-  Bony eye sockets have been developed to protect the eye

-  The area of the brain concerned with vision has increased, whilst the area involved with

our olfactory sense has decreased

CEREBRAL CORTEX

-  The cerebrum (responsible for complex functions – intelligence) has progressively

increased in size with primate evolution

-  The cerebral cortex, the outer region, has also significantly increased in size

-  It is responsible for ‘higher functions’: vision, memory, reasoning and manipulative ability.

-  The convolutions in the brain increase in umber and complexity. This results in the

increased SA of the brain which means an increase in the number of neurons and nerve

impulses between cells

-  This increase in the size of the cerebral cortex has allowed higher primates to move about

and find food and gain special skills (e.g. toolmaking)

-  There has been a greater increase in the relative brain size (brain size : body size) in apes

and humans compared to lower primates

-  Therefore apes and humans have rounder and larger skulls, to house their larger brains

GESTATION and PARENTAL CARE- 

not restricted to seasonal breeding season

-  however have a rhythmical sexual cycle

-  most primates only have one offspring at a time

-  compared to other mammals, primates have quite a large gestation period

-  among the primates, gestation is longest in apes and humans, however at birth, the

offspring are more immature and require a greater level of care and protection

-  length of parental care increases with primate evolution

- 

in lemurs, the offspring are weaned at 5 months

-  in apes, the offspring usually leave their mothers between the ages of 11 and 13

-  because of this increased parental care, there is a delay in maturation

-  sexual maturity is reached a lot later in apes in humans than it is in lemurs, lorises and

monkeys.

-  however, this lengthening in parental care means that their period of learning is also

greatly extended and they can learn more from their mother and other members in their group

- 

it also means there is a considerable amount of time and effort invested in their care and

survival

- 

this increase in parental care results in the offspring’s increased chance of survival

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Review Questions and Apply Your Knowledge 29/03/11 7:45 PM 

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Evolution of Humans - 19 29/03/11 7:45 PM 

EVOLUTIONARY TRENDS IN HOMININS

Erect Posture of a Bipedal

- Vertebral Column (spine)

•  S-shaped/double curve

• 

Less energy required (muscles are needed to keep the C-shaped spine of quadrapedals

from collapsing in on themselves)

•  5 parts – cervical (7 vertebrae), thoracic (12) and lumber (5) arches and the sacrum

and coccyx.

•  Reverse cervical and lumber arches bring vertebral column under centre of gravity of

the skull

- Foramen Magnum and Skull

•  Forward position of foramen magnum (hole where spinal column joins brain) allows

skull to sit on the top of the vertebral column – less energy required – large neck

muscles not needed to anchor skull to vertebral column

•  Humans are less reliant on sagittal crest for muscle attachment

•  Balance can be achieved with minimum muscle attachment – always comes back to

energy efficiency!

- The Jaw

•  Much flatter facial profile – decreased prognathism/protrusion

•  Parabolic jaw

- Pelvis/Pelvic Girdle•  Stronger, wider and shorter than that of a quadrapedal

•  Wide, shallow like bowl to support abdominal organs (and developing foetus)

•  Tipped forward at 45° - ensures legs are under centre of gravity (sacrum)

•  Widened and flattened pelvic bones provide good attachments points for large leg

muscle groups (gluteus maxim) which help keep body upright

- Carrying Angle

•  The angle at which the femur converges toward the knee

•  The orientation and shape of the pelvis places the hip joint directly under the head and

trunk – the weight of the body is transferred from the pelvis to the legs

•  Weight distribution remains close to the central axis of the body

•  Allows for:

- Greater stability

- the body to be able to rotate around the lower leg and foot

- each footstep to follow a straight line

•  Gorillas/Chimpanzees have swaying gaits

- The foot

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•  longitudinal (front back) and transverse (side to side) arches convert the foot into a

complex spring

•  It absorbs the stress/pressure on the foot as the foot hits the ground and it propels the

body forward as the foot leaves the ground

• 

The ankle also allows flexion and extension in only one plane

•  The foot is a highly specialized locomotion organ and has therefore lost its requirement

for opposability and prehensibility.

- The knee

•  Two hinge points, side by side, separated by ligaments

•  Keeps flexion and extension in one plane

•  Centre of gravity line passes in front of the knee

•  The resulting forces try to bend our knee backwards (prevented by ligaments)

• 

Needs no energy to support the body

- Prognathism and Dentition

•  Canine teeth don’t project beyond the level of the other teeth (look like incisors)

•  Dental arcade (shape of tooth row) has evolved into a parabolic shape instead of the

U pattern of apes

•  With evolution of humans, teeth become less specialised due to generalized diet

(molars became smaller, canines became smaller)

•  Modern humans appear to be gradually losing their wisdom teeth

• 

Extent of prognathism decreases with hominin evolution•  Modern humans have prominent chin, yet flat face

The effect of the Environment of Hominin Evolution

- Forests began to disappear, leaving increasingly large gaps between trees (between 5 and 6

million years ago)

- Primates had to walk from tree grouping to tree grouping

- Natural selection favoured apes that were better at bipedal walking

- These apes evolved into early hominins

- The advantages of erect stance:

•  an increased range of vision for detecting prey and predators at a greater distance

•  increased size – deterring predators

•  hands free for carrying food, infants and wielding tools

•  higher reach for picking fruit from trees

•  improved cooling of the body (thermoregulation)

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Ch 19 Questions 29/03/11 7:45 PM 

REVIEW QUESTIONS

1a. A tribe is the name given to a relatively new level of classification

between subfamily and genus. Humans are in the tribe Hominini.

b. Hominini – Humans

Panini – chimpanzees

Gorillini – gorillas

2a. S-shaped spine, forward facing foramen magnum, wider and shorter

pelvis – carrying angle – knee, foot is a weight bearer

b. quadrupedal organisms have a c-shaped spine, quite a far back forman

magnum, longer and thinner pelvis, less of a carrying angle and their feet

are for opposability not weight bearing

ci. Bipedalism is energy efficient, allows one to see further, spot

predators, pick food from trees. Also provides a smaller SA for weather(UV rays) to affect – body cooling, better stability – centre of gravity

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Evidence for Evolution 29/03/11 7:45 PM 

Fossil: any preserved trace left by an organism that lived long ago e.g.

bones, laetoli footprints

Artefact: any preserved item made by a past living organism for a

particular reason e.g. tools, paintwork, hearths etc.

FOSSILS

Formation

•  Parts of organisms may become fossilised when they are buried

by drifting sands, sediments, mud deposited by rivers, volcanic

ash or (in more recent human ancestors) by other members of

the species

•  To be fossilised, the remains must be buried rapidly, therefore

the activity of decay organisms and decomposition may beslowed or prevented

•  In wet acid soils the minerals in the bone are dissolved and no

fossilisation occurs

•  If the soil contains no oxygen, in the case of peat, complete

fossilisation of the tissues as well as the bones may occur

•  In alkaline soils the best fossils are produced as minerals in the

bones are not dissolved

•

 

New minerals, often lime or iron oxide are often deposited in thepores of the bone, replacing the organic matter, therefore

petrifying the bone (turning it into rock)

DATING TECHNIQUES

Absolute date: the actual age of the specimen in years

Relative date: the age of the specimen in relation to other specimens –

older than or younger than

Absolute Dating

Potassium-Argon dating

•  Based on the decay of radioactive potassium to form calcium and

argon

•  Concerned only about K40 as it is the radioactive isotope of

potassium

•  K40 ! Ca40 ! Ar40 (K40 is unstable and therefore decays to form

Ca and Ar)

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•  K-Ar dating is successful in dating rocks as the rock traps Ar40 as

it forms, the ratio of K40 to Ar40 can then give the age

•  The rate a which K decays is constant, extremely slow and

unaffected by weather, temperature, pressure or UV radiation

• 

This method is reliable as long as the rock is at least 100 000 –

200 000 years old 

•  To determine fossil age, there must be suitable rocks of the

same age around the fossil e.g. if the fossil was buried by

volcanic ash, then rocks would have been formed by the ash

around it

•  Half life: the time it takes for half of any sample of K40 (or any

other element) to decay

• 

K40’s half life is 1300 million years!! Told you it was slow :p

100% 50% 25% 12.5%

•  From 100% to 50% it took 1300my, from 50% to 25% it took

1300my, from 25% to 12.5% it also took 1300my and so on..

•  K40 decays on an exponential curve

Carbon – 14 (radiocarbon) Dating

•  C14 is radioactive, C12 is inert

•  C14 can exist as a result of the breakdown of nitrogen in the

upper atmosphere in the presence of cosmic rays

•  Photosynthesis traps c14 

o  CO2 + H2O ! C6H12O6 + O2 

o  The radioactive carbon is trapped in the CO2 and converted

to and trapped in glucose

•  Animals eat the stored energy (glucose) in the plant and hence

consume the C14 trapped inside it.

•  This C14 is then passed onto humans as we eat the animals and

the plants

•  C14 constantly decays back to nitrogen (and returns to the

atmosphere), however as we keep consuming plants and

animals, the C14 within us stays fairly level

•  When we die, we stop consuming C14 and therefore it all decays

and returns to the atmosphere

K 40 Ar 40 

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•  For every single carbon-14 atom there are 1012 stable carbon

atoms

•  By measuring the amount of radiation from a single sample, the

ratio of C12 to C14 can be estimated and the age of the specimen

can be calculated

•  Carbon-14 has a half life of 5730 40 years 

•  Carbon 14 is very effective on dating fossils up to 70 000 years 

old (approximately 12 half lives, therefore only 0.02% C14

remaining)

•  The specimen must be organic e.g. bones, soft tissue, plants,

hair, wood, blood etc.

•  Usually requires at least 3 grams of the sample to test

• 

Accelerator Mass spectrometry (AMS) radiocarbon dating onlyrequires approximately 100 micrograms of the sample, as it is

broken down into constituent atoms (e.g. when dating cave

paintings)

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Tree Ring Dating – Dendrochronology

•  Counting the concentric rings of the surface of a cut tree trunk or

specimen of wood

•  Each ring represents 1 year of growth

• 

The difference in width between each ring indicates how

favourable the growing season was – bigger gaps = better

conditions

•  Starting with a living tree, it is possible to correlate marker rings

with timber from ancient human structures (e.g. shafts of

spears) and gradually work back to timber thousands of years

old.

•  Limits: timber is rarely preserved more than a few thousand

years•  Good for timber younger than 9000 years old 

•  Has to be used in correlation with carbon – 14 or potassium-

argon dating

Relative Dating

Stratigraphy

•  The study of layers of strata

•

 

If you have three different layers (maybe different sediments),and found a specimen in each layer, the specimen in the bottom

layer is older than the specimens in the layers above it

•  The principle of Superposition: assumes that the layers at the

top of sedimentary rock are younger than the layers below them

•  Correlation of Rock Strata: matching layers of rock from different

areas

•  Index fossils: fossils that are of great importance as they were

widely distributed and present for a short period of time –

making relative dating of strata more precise e.g. fossilised

pollen grains or trilobites

Fluorine Relative Dating

•  When a bone is fossilised, F- present in the soil and water

replaces some of the ions in the bone

•  The older the bone, the more fluoride it will contain (as it has

absorbed more)

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•  Not absolute as levels of F- in soils and water fluctuate over time

and from place to place

•  Dating technique is relatively easy, just measuring fluorine levels

•  No age limits on specimen

Geological Time Scale

COMPARATIVE STUDIES IN BIOCHEMISTRY

DNA

•  All living organisms have DNA ! come from common ancestor

Speciation

•  Two or more species evolving from one common ancestor

•  The new species will have similar (not exact) DNA

• 

Distant relations – more differences

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•  Closer relations – less differences

Genome

•  The complete set of an organisms DNA

•  Chimpanzees share 98% of the same DNA sequencing as

humans

•  Chimpanzees have 24 pairs of chromosomes, humans have 23

Endogenous Retroviruses (ERV’s)

•  Are a way to compare SNA by looking at the introns

•  An ERV is a viral sequence in an organisms genome

•  It gets into an organisms by reverse transcribing its RNA into the

organisms DNA

•  It then acts as a marker and doesn’t actually do anything

• 

For a retrovirus to become endogenous it must be inserted intothe germline – the gametes, therefore it can be passed on

through generation to generation

•  Apes and humans have 3 of the same ERV’s

•  Monkeys, great apes, gibbons and humans all have ERV 1

•  ERV’s make up 8% of a humans genome

•  Comparing shared ERV’s between organisms determines their

degree of relation and indicates they have evolved from a

common ancestor

Mitochondrial DNA (mtDNA)

•  mtDNA has a different structure to nuclear DNA

•  It forms small circular molecules

•  5-10 molecules in each mitochondrion, each molecule contains

37 genes

•  13 genes are concerned with providing instructions to make

enzymes for cellular respiration, 24 are involved with making

tRNA

mtDNA DNA

Structure Circular Double helix

Genes 37 (13 for cellular

respiration enzymes,

24 for tRNA)

thousands

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•  it is possible to track parentage and lineage through

mitochondrial DNA (maternal line – mtDNA is passed on only

from the mother)

•  mtDNA is plentiful – 500-1000 copies per cell

• 

has a higher rate of mutation than nuclear DNA

•  mtDNA is moving away from our female ancestor, the amount of

mutations is roughly proportional to the time passed

•  it has been estimated that Neanderthals and Homo sapiens

diverged approximately 600 000ya

Proteins

•  Ubiquitous proteins appear in all species 

• 

They have the same function no matter where they appear ! common ancestor

•  E.g. cytochrome – C, alpha and beta chains of haemoglobin

•  Cytochrome-C has appeared in protein sequences for organisms

right back to 2billion years ago, all have cytochrome – C,

however there are more differences between less closely related

species (humans and tuna) then more closely related species

(humans and chimpanzees)! common ancestor

•

 

The amino acid sequence for proteins are identical for memberswithin a species, for different species these sequences may be

slightly different or rearranged

•  Just like DNA analysis, the degree of difference between proteins

enables an estimate of the amount of evolution that has taken

place since two species developed from a common ancestor

COMPARATIVE STUDIES IN ANATOMY

Embryology

•  Comparing the very early stages of the development of

organisms

•  Embryos of all different species look very similar

•  Provides evidence for evolutionary change over time

•  All vertebrate embryos have gill pouches/arches, a well

developed brain, a two-chambered heart and similar brain

development

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Homologous Structures

•  Organs that are similar in structure but are used in different ways

•  Classic example: forelimb of vertebrates

•  homologous structures possess a similar structure – therefore they are likely to have a

common ancestor, the more similarities, or the closer the similarities are, relates to the

extent of their relation! more similar – closer related

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Vestigial Organs

•  organs that may once have been important but have lost or

changes their functions

• 

structures of reduced size that have no apparent function

•  vestigial organs common in species suggests common ancestor

•  humans have as many as 90 vestigial organs

•  e.g. appendix, coccyx, nipples on males, third molar – wisdom

tooth, hair on body, nictitating membrane (in eye), muscles to

move ears

Geographical distribution

• 

Isolated land areas and island groups have frequently evolvedtheir own distinctive plants and animal species

•  The only egg-laying mammals (the echidna and platypus) are

found in Australia

•  Finches in South America and the Galapagos islands have

evolved into 13 different species due to environmental pressures

and natural selection

o  They al have differences in beak shapes and sizes as they

specialise in different diets