Download A history of life on earth – Chapter 5

A history of life on earth – Chapter 5
Important patterns in the history of life
 Climates and land masses have changed over time
 Taxonomic composition has changed
 There are periods of mass extinction
 High diversification, esp. after mass extinctions
 Diversification includes increases in number of species and variety of forms
Important patterns in the history of life
 Extinct taxa replaced by similar but unrelated forms
 Ancestral members of a taxa more alike than their descendants
 Only a few members of higher taxa have persisted
 Geographic distribution has changed greatly
The Big Bang Theory
 12-15 billion years ago all matter and energy were concentrated into a
(relatively) small space the size of our sun
 About ten billion years ago, the Universe began in a gigantic explosion
 Matter then condensed to form atoms, elements, and eventually galaxies and
stars
Geologic time units
 Eon – a unit of time equal to 1 billion years
 Era – a unit of time marked by a new or distinct order of things
 Period – an unit of time of any length
 Epoch – a period that began by some significant change or event
Geologic Time Line
Geological time
Formation of Earth
 Earth was formed 4.6 billion years ago by the collision and aggregation of
many smaller bodies
 As Earth cooled, crust formed and water and other gases were released
Earth is “just right” for life
 If Earth were smaller in diameter, gravity would not be great enough to hold on
to atmosphere
 If Earth were closer to sun, water would have evaporated
 If Earth were further from sun, water would have been ice
What is life?
 Self-replication
Differential reproduction, with variation = genotype
 Metabolism
Intake, processing, outflow of materials = phenotype
 Individuality
Coherence as integrated system, subject to selection
All life is related
 All share similar features
Use only L-optical isomers of amino acids
Universality of genetic code
Archeon Eon
 3.8 – 2.5 bya
Chemical and molecular evolution
First cells (anaerobic bacteria)
How did life originate? The problem…
Simple chemicals DNA, RNA,
proteins (enzymes!),
membranes
metabolism
integrated system!
The setting…
 Energy: lightning, volcanoes, sunlight, heat
 Materials: CO2, CO, N2, H2O, CH4? NH3? But NO oxygen!
 Environment: rocks, water
PRIMORDIAL SOUP
The Oparin-Haldane model
Stanley Miller made his own “soup”:
 CH4 + NH3 + H2 + ENERGY = amino acids, sugars
Some controversy now over whether this adequately represented earth’s
atmosphere
Simple organic chemicals can be made by abiotic chemical reactions
Origin of agents of metabolism
 Spontaneous formation of porphyrin rings from formaldehyde
 Components of chlorophylls and cytochromes
The RNA World
 Probably some sort of simple, self-replicating polymers arose; later, RNAbased system spread
 RNA has catalytic properties (ribozymes can cut, splice, elongate other
oligonucleotides)
 Experimental evolution of RNA has been shown
The RNA World
 So far, though, no demonstration that RNA can completely copy itself; only
short sequences can be copied by ribozymes
 If RNAs associate into ‘individuals’, copying by ribozymes is more efficient
 The evolution of protein enzymes may be biggest remaining question - a very
active area of research!
Going Cellular
 Compartmentalization into ‘individuals’ makes replication more efficient
 Lipid membranes spontaneously self-assemble
Possible sequence leading to self-replicating systems
The first cells
 Stromatolites from 3.5 bya
 Were prokaryotic heterotrophs
 Secured energy through anaerobic pathways
No oxygen present
Relied on glycolysis and fermentation
Origin of photosynthesis
 Origin of photosynthetic eubacteria
Noncyclic pathway first (3.5 mya)
Cyclic pathway next (2.5 bya)
 Oxygen accumulates in atmosphere and increases greatly 2.2 bya
 Origin of aerobic respiration
The first cells were prokaryotes
 Reminder: prokaryotes are single celled organisms lacking a nuclear
envelope
 Two of the three domains (Bacteria, Archaea) are prokaryotes
 Prokaryotes were only life forms for 2 billion years
Prokaryotic cells
 Archaebacteria
Live in extreme habitats
 Salt, extreme temperatures, anaerobic
 Methane-producing bacteria
 Bacteria
Live everywhere - aerobic conditions
Live on your skin, digestive systems
Eukarya
 Distinct nucleus with membrane, distinct chromosomes; majority with
organelles, many multicellular
Protistans
 Some are single cell/others are multicellular
 Larger than bacteria
 Usually live in aquatic environments
 Amoeba, paramecium, algae
Fungi
Plants
Animals
Emergence of eukarya
 Fossils from 1.5 bya
 Chemical evidence of emergence 2.7 bya
Where did organelles come from?
 Theory of Endosymbiosis
Free-living prokaryotes were engulfed by early eukaryotes and became
permanent internal symbionts
Theory of endosymbiosis
 Mitochondria and chloroplasts resemble simple bacteria
 Sequencing of plastids from organelles suggests bacterial symbiont origin
 Membrane number suggests origin type
Theory of endosymbiosis
 Some extant eucarya have no mitochondria
 Rely on endosymbiotic bacteria for respiration
Theory of endosymbiosis
 Corals and algae
 Deep sea fish ‘headlights’
Multicellularity
 Origin of life, 3500 mya
 1500 mya eukaryotes
 640 mya large multicellular animals
 WHY THE WAIT?
Multicellularity
 Multicellular organisms need more oxygen than single-celled organisms
Advantages of multicellularity
 Replacement of cells = longer life
 Larger organisms have greater physiological stability
 Cell differentiation into tissues means different sorts of body architectures (can
exploit new niches)
 Ingest small organisms, produce more offspring
Disadvantages of multicellularity
 Longer development means later reproduction (lower r)
 Larger size means problems with respiration, circulation (volume increases
more rapidly than surface area)
Paleozoic Era (570-240 mya)
Cambrian period 542-489 mya
 Continents were flooded by shallow seas
 The super continent of Gondwana just formed
The Cambrian explosion
 At beginning of Cambrian Period, modern animal phyla present, little in fossil
record
 By 530 Ma, almost all modern animal phyla present in fossil record!
Brachiopods and trilobites
Burgess Shale of British Columbia
The Cambrian Explosion
 Earliest known vertebrate ancestors
 Teeth of cellular bone, lacked vertebrae but had notochord: conodonts
Animal body plans evolved during Cambrian
What caused the Cambrian Explosion?
 Environmental changes: increased O2, calcium carbonate (for shells!)
 Ecological setting: lots of unfilled niches, available as new modes of
locomotion evolved
 Key innovations: multicellularity, important new developmental pathways
Why no replay?
 Similar availability of niches after end-Permian extinction!
 Developmental processes malleable in early metazoans, but more canalized
later: can’t evolve truly unique forms
End of Cambrian
 Extinction marked the end of the Cambrian period, perhaps caused by cooling
of the seas
 Trilobites greatly reduced
 Several classes of echinoderms extinct