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
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
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
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!
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
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