The Road to Bio-diversity: Evolution of Life on Earth

The Big History of our Planet

– from 5th to 6th threshold

HOW TO UNDERSTAND LIFE

• We would like to understand 1. Definitions: What is life? 2. Complexity: In what sense are living things

more complex? 3. Mechanisms: How does life adapt to the

surroundings? 4. History: What are the key stages as life on Earth

changes from simple bacteria to complex animals like us?

A Tardigrade (‘water bear’) in moss: animals up to 1 millimetre

in size, that can survive extreme environments, even, briefly, in

space: http://apod.nasa.gov/apod/ap130306.html

What is Life?

• What distinguishes

life from non-life?

• Some traditional answers:

1. Made from different stuff?

2. A ‘life-force’ or ‘soul’?

3. There’s no absolute

difference

Michelangelo: Sistine Chapel God grants Adam the gift of life

Refuting the idea of different “stuff” • A traditional “scientific” idea:

– Life “organic” chemicals (based mainly on Carbon)

– Non-life “inorganic” chemicals

• In 1828 a German chemist, Friedrich Wöhler, disproved this idea. How?

– He used inorganic chemicals to synthesize an organic chemical (urea) in a laboratory

• Significance? – Newton showed that the same physical laws worked in the heavens

and on Earth

– Wöhler showed that life and non-life follow the same chemical laws

Refuting the idea of a “life-force”

• Pasteur disproved that life cannot be spontaneously generated due to “life-force” in the air.

What makes life different? A modern perspective

• Life is a new form of complexity! – It has many diverse components

– Arranged in precise patterns under the right ‘Goldilocks’ conditions

What makes life different? A modern perspective

• Life is a new form of complexity! – It has many diverse components

– Arranged in precise patterns under the right ‘Goldilocks’ conditions

– That give rise to new ‘emergent’ properties

– That can survive only with sustained energy flows

• So living things die when the energy flow stops or the Goldilocks conditions vanish

Emergent Properties of Life

What are they? 1. Metabolism:

actively mobilizing energy from their surroundings (to constantly adjust to changing environments)

METABOLISM: extracting energy from the environment to support themselves

Dinner for an amoeba: Paramecium sandwich

Eating and breathing are our main ways of getting the energy we need to survive

Emergent Properties of Life

What are they? 1. Metabolism:

actively mobilizing energy from their surroundings (to constantly adjust to changing environments)

2. Reproduction: making and reproducing templates (i.e. transmitting information)

REPRODUCTION: making almost perfect copies of successful ways to extract energy

An amoeba dividing into two amoebas

The two will be more or less identical

Human offspring normally vary a bit more

Emergent Properties of Life

What are they? 1. Metabolism:

actively mobilizing energy from their surroundings (to constantly adjust to changing environments)

2. Reproduction: making and reproducing templates (i.e. transmitting information)

3. Adaptation: slowly modifying over many generations so as to adapt to changing surroundings

ADAPTATION: average qualities of a species can slowly change, to create new ways of surviving

Individuals can’t change genetically; species can Generation by generation, stonefish got uglier Camouflage improves their survival chances

Camouflage as Adaptation

A month-old chameleon

practicing turning pink

ADAPTATION

Juvenile Brookesia micra, the chameleon’s tiny cousin. In Madagascar, some chameleons have adapted by becoming extremely small

http://news.nationalgeographic.com/news/2012/02/pictures/120215-smallest-chameleons-new-species-madagascar-science/#/tiniest-chameleon-found-match_48801_600x450.jpg

Threshold 5, Life: Why life is different:

A new type of complexity:

active rather than passive …

Living things survive in unstable

environments

unlike stars, they must

•Detect changes (information enters

the story)

•Keep adjusting (purposefulness &

meaning enter the story because

getting it right matters!)

Life as a Form of Complexity

The fuzzy borderline between life and non-life: Viruses

Avian flu virus: computer-generated model

• No metabolism by itself.

• But it can highjack energy from other

cells to reproduce:

• A bag of genes in a protective

protein casing.

• Its genes can highjack the

metabolism of other organisms to

reproduce inside their cells, and …

The fuzzy borderline between life and non-life: Viruses

Avian flu virus: computer-generated model

• Eventually, to adapt.

• Becoming resistant to treatment

Sometimes “alive”, sometimes “dead”,

viruses straddle the border between life

and non-life.

How does life ‘Adapt’to its surroundings

Eyes are

exquisitely

‘adapted’ for

seeing; how did

they get that

way?

Left: simple eyes of a spider

Below left: compound eyes of a fruit fly

Below: a human eye

The Traditional Answer: Organisms are created ‘pre-adapted’

• Linnaeus: Founder of ‘taxonomy’

– Organisms don’t change or adapt

– God the ‘Engineer’ made creatures already ‘adapted’

– Since then they haven’t changed Carl Linnaeus

(1707-78)

• What’s wrong with this idea?

SPECIES DO CHANGE: The fossil evidence

Trilobites: 200 Mill. Ys. old.

Midge fly fossilized in amber

Darwin found fossilized

armadillo ancestors

Dinosaur footprints

Animal breeders know that living organisms are still changing now

• Modern breeds of dogs are all descended from wolves

• Differences have been created by dog breeders, mostly in

the last few centuries

Lamarck’s attempt (early 19th century)

• Species adapt because individuals try to adapt

• e.g. giraffes

– Stretch to reach leaves high in the trees

– The offspring of those that stretch most have longest necks

A female giraffe feeding its young

What’s wrong with Lamarck’s idea?

What’s wrong with Lamarck’s idea? The modern answer

Any animal breeder understands what’s wrong:

• Lamarck (and Darwin, too, occasionally!) confused:

– ‘Acquired characteristics’ (e.g. getting fit in the gym, or stretching your neck to eat leaves), with

– ‘Inherited characteristics’ (e.g. blue eyes)

• Acquired characteristics cannot be inherited – [to be fair, we now know there are some mechanisms by which this

can happen, but they are rare]

• For species to adapt, new characteristics must be passed on by inheritance

Darwin’s Theory

• The Key Move: Thinking Statistically

• Distinguish between

– Individuals (you and I cannot change our basic biology)

And

– Populations or species (i.e. large numbers of individuals; their average qualities can change)

• It is species or whole populations that evolve, not individuals

Darwin’s voyage of discovery: 1831-36

And its journey

Galapagos Islands

H.M.S. Beagle

The Galapagos Islands

Species on the Galapagos Islands:

Galapagos Marine Iguana

Galapagos Giant Tortoise

WHAT DARWIN SAW:

many strange, new species that were closely

related, but differed from island to island. Why?

The Galapagos Finches

There were 14 nearly identical species, but their

beaks and heads were all slightly different

Why were the beaks and heads slightly different?

Was it a form of adaptation?

Natural Selection: A statistical process 1. Species: A collection of individuals similar enough

to breed with each other; species adapt, not individuals

2. Variation: Within species, individuals differ (look around)

3. Fitness/Stability: By chance, some individuals are better adapted (‘fitter’) for a particular environment

Natural Selection: A statistical process 4. Differential Reproduction: The ‘fittest’ get more

food, and so can have more chance to reproduce

5. Heredity: Variations are likely to be inherited by an individual’s offspring. So later generations will look more like the ‘fittest’ as the offspring inherit the ‘adaptive’ qualities from the ‘fittest’

6. Endless Change: the environment continually changes, so evolution never stops

From Simple to Complex Organisms

• 8 main stages in the evolution of life on Earth

– 4 concern single-celled organisms and take over 3 Bys

– 4 concern multi-celled organisms and take 600 Mys

• A Human-centred account: We focus on those stages leading to us

– We ignore other evolutionary pathways

– E.g. why do female preying mantis kill the males with whom they mate?

STAGE 1. First Organisms

Similar to Prokaryotes, maybe?

From about 3.8 billion years ago?

• Probably like modern bacteria

• Anaerobic (did not use oxygen); Chemoautotrophs (got energy from chemicals)

• Tiny: ~ 5 microns across

• No nucleus DNA unprotected many mutations rapid evolution

DNA but no

nucleus

Some modern prokaryotes

Andrew Syred, using a Scanning Electron Microscope

STAGE 2. Photosynthesis: the first energy revolution

• A ‘battery’ for life: the Sun – Prokaryotes near the surface of

the seas – Began to use sunlight directly for

energy

• PHOTOSYNTHESIS – All plants can do photosynthesis – Most complex life (including us)

is supported by photosynthesis

The first photosynthesizers?

The oldest fossil bacteria are

about 3.5 billion years old

Possibly photosynthesizers,

like ‘cyanobacteria’ today

A modern

cyanobacterium

The First Clear Evidence of Life

Modern Stromatolites

Cyanobacteria created coral-

like ‘Stromalites’ near the

surface of early seas, similar

to these modern ones.

STAGE 3. Eukaryotes and the ‘oxygen revolution’

• Oxygen started to accumulate in the atmosphere due to photosynthesis

• Oxygen was poisonous for most species – Some cells evolved to use the exceptional chemical

energy of oxygen

• The first ‘eukaryotes’: ~ 2 billion years ago

• 3 key features 1. Could use energy from oxygen

2. Genes protected inside the nucleus

3. Larger and more complex than prokaryotic cells

Eukaryotes (100s to 1,000s times as large as prokaryotes) Some can be seen with the naked eye

1) Mitochondria can

generate energy from

oxygen

2) The nucleus protects

the cell’s DNA

3) Larger &

more complex,

with many

‘organelles’

STAGE 4. Sexual Reproduction • Simple Reproduction: Most prokaryotes split in two,

producing ‘clones’

– Result? • Parents and children identical: like identical twins

• Limited genetic variation between individuals

– (Though bacteria, unlike eukaryotes, can share genetic material in other ways)

• Sexual Reproduction: Almost all eukaryotes swap genes before reproducing

– Result? • Greater variety in their offspring

• Greater variety accelerated evolution

STAGE 5. First multi-celled organisms

• From ~ 1.2 billion years ago:

– Some eukaryotic cells gathered together in ‘societies’

– Cells became specialized, and more dependent on their neighbours

– They learnt to communicate with each other

– Some became so dependent they turned into large, single organisms

Some “organisms” today are still really groups of organisms

The Portuguese man of

war: a partnership of

different types of cells

Lichen: a partnership

between algae and fungi

The Cambrian explosion

• From about 540 million years ago: – Sudden proliferation of fossils

– New multi-celled organisms with hard parts

• Most of the ‘body plans’ we find today

• Why? We don’t really know: hypotheses – High oxygen levels for the first time more energy?

– More complex DNA more genetic information

– Rapid warming after ‘snowball Earth’

STAGE 6. 1st vertebrates (chordates): internal skeletons

from ~ 500 million years ago

The first vertebrates probably

looked like ‘lancelets’.

Extremely simple fish.

No heart and no brain!

But, beginnings of a nervous

system along the spine

Pikaia, ~ 505 Mys old, the oldest known chordate fossil, from the Burgess shale, Canada

Artist’s reconstruction

•c. 5 cm long

•No skull, but

•Down its back, a semi-rigid rod of

tissue known as the ‘notochord’,

forerunner of the backbone

STAGE 7. To land! from ~ 475 million years ago

• Most organisms lived in water

• Surviving out of water was tough (like living in space)

• You needed support systems: – Portable supplies of water

– Tough skins to prevent drying out

– Protection for eggs and offspring

– A bit like having a space suit

Plants and Insects were probably first on land

Some of the earliest plants may have

been giant ferns. This is a fossilized fern

from the ‘Carboniferous Period’ Some of the earliest land insects may have been

giant dragonflies. Here, a dragonfly perches on

a lotus flower in NY botanical gardens.

Ancestral Amphibia: First vertebrates to flourish on land

Ichthyostega: ~ 370 million years ago.

Lived in water and on the land.

Like all amphibia, it returned to water to lay its eggs

Reptiles: better adapted to the land

• 1st reptiles: ~ 350 million years ago

– Dry skin seals in moisture

– Eggs can be laid on land

Green sea turtle: The oldest

reptiles may have looked like

turtles

Or perhaps

like crocs?

A Permian Extinction • ~ 250 Million years ago, a mass

extinction destroyed

– ~ 95% of all marine life

– ~ 70% of all land life

• Causes? Uncertain

– Asteroid impact?

– Massive volcanic eruptions?

• Impact?

– After major extinctions, evolution accelerates as new species fill empty niches

Trilobites

vanished forever!

Major Extinction Events

“Permian”

“Cretaceous” Others

Soon afterwards Dinosaurs Ruled!

STAGE 8. Mammals: from ~ 250 million years ago

• Warm-blooded

• Fed their young with milk

• Fur (even you and me!)

• Large brains

A common shrew

Eomaia Scansoria the oldest known

placental mammal

Remains discovered near Beijing

Lived about 125 million years ago

Closest to modern tree shrews

Monotremes: Reptiles or Mammals?

• Transitional species

• furry and warm-blooded

• but they lay eggs!

• Only 2 species survive: Echidna, and Duck-billed platypus

Echidna & echidna babies Platypus

Mammals have large brains

so they can learn during their

lifetimes

MRI (Magnetic

Resonance Imaging)

scan of arteries in the

brain of a 27-year-old

woman

65 Million Years ago a meteorite landed off the Mexican coast

The impact of a nuclear war

Most species of dinosaurs went extinct

One more mass extinction

If the Asteroid had missed?

Dale Russell’s model of how a brainy reptile might have evolved

A Mammalian Radiation

• Within 25 mys mammals were filling the niches for large animals

• The surviving dinosaurs (aka birds) mostly remain small today

The extinct Paraceratherium, which lived 30

million years ago, weighed some 15 tonnes

With dinosaurs out of the way,

Mammals flourished & diversified

Becoming the main large species on earth

The ‘Order’ of Primates:

Our Ancestors

Primates include

lemurs and monkeys

Oxygen

increasing

1) Earliest life-on earth: prokaryotes

3) 1st Eukaryotic organisms

5) Cambrian ‘explosion’

Sudden increase in fossils of

Multi-cell organisms

4) 1st Sexual reproduction

2) Photosynthesis: cyanobacteria

Creation of Earth Today 6) Vertebrates

7) To Land 8) Mammals

1st multicelled

organisms?

Summary • A brief history of life on earth

– Pt. 1: up to 600 million years ago

• Stage 1: First organisms

• Stage 2: Photosynthesis

• Stage 3: First Eukaryotes: ‘Oxygen revolution’

• Stage 4: Sexual reproduction

– Pt. 2: from the ‘Cambrian’ period to now

• Stage 5: First Multi-cellular organisms

• Stage 6: First Vertebrates

• Stage 7: First living organisms on Land

• Stage 8: Evolution of Mammals