History of Life

•	Life is affected by global change
•	Living organisms are themselves a constant source of global change
•	The origin and early evolution of life altered the structure of the Earth itself

•	The origin of life can be studied from the perspective of systems science
•	Modern system science helps us to understand complex non-equilibrium systems like living organisms

•	Matter and energy continually flow through living systems
•	Life is an open system (exchanges matter and energy with the environment)
•	The patterns of living systems are maintained by the constant flow of energy from the sun

•	The pattern of living systems are relatively stable, but only as long as the flow of energy is maintained
•	Analogous to a waterfall…
•	Waterfall retains its shape (pattern) only as long as the flow of water is uninterrupted

•	Life is a steady state - balance can only be maintained by a constant flow of materials through the system

•	Living systems consist of complex interlocked networks of patterns
•	The pattern of living systems is self organizing and self-replicating
•	Life is a highly ordered state, which seems to defy entropy

•	Universe is governed by a natural universal tendency towards disorder (Second Law of Thermodynamics) = entropy 
•	Ironically, entropy can actually result in spontaneous patterns and ordered states

•	Systems that are pushed far away from their equilibrium point act in strange ways
•	Excite atoms in a laser, spontaneously discharge their energy in a coherent blast
•	Heat a thin layer of liquid, can form hexagonal patterns, ordered flow that dissipates energy from the system

•	Ordered states (life) may form when chemical systems are far from equilibrium
•	Such systems may dissipate energy (satisfying the law of entropy)
•	May also evolve into more complex, ultimately self-reproducing systems

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Chemical reactions (patterns) in living systems are maintained far away from their thermal equilibrium point
•	For living systems, thermal equilibrium equals death

•	Systems can also have emergent properties - the whole is greater than the sum of its parts
•	Life is an emergent property of complex networks of chemical reactions
•	The mind may be an emergent property of complex patterns of neural connections in the brain

•	How far up the scale do such properties go?
•	We’ve talked about the Gaia hypothesis
•	Is the universe itself literally alive?

•	In a fundamental sense, we are the universe, becoming aware of itself…
•	Life may exist throughout the universe, due to spontaneous chemical reactions which form complex systems far from equilibrium
•	Life may also have been carried to Earth from other planets - panspermia

•	The origin of life is still a mystery…
•	By 3.5 billion years ago, life had already diversified into several lines of bacterial evolution
•	Most were anaerobic - may have existed far below the surface of the Earth, as the planet continued to cool

•	Best “big picture” is to divide life into three Domains
>	Archaea
>	Bacteria
>	Eukarya - everything else

•	Bacteria and Archaea are the most primitive cell type - called prokaryotes:
>	Unicellular
>	Lack a cell nucleus (no nuclear membrane around the chromosomes) 
>	Lack cellular organelles bound by membranes (no chloroplasts, no mitochondria, etc…)

•	Archaebacteria may be as distantly related to bacteria as bacteria are to man
•	All higher organisms (eukaryoytes) probably share a common ancestor with the Archaea 

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Archaea are exceedingly strange, poorly understood group
•	Completely different type of prokaryotic cell from bacteria
•	Many are called “extremophiles” because they seem to prefer living in unusual, extreme environments

•	Methanogens - very primitive, anaerobic
•	Can't use sugars, proteins, or carbohydrates, “eat” methanol, formate, and acetate!
•	Get energy converting CO2 and H2 into CH4

•	Methanogens produce ~ 2 billion tons of methane per year
•	30% of that methane is made in the bellies of cows, where methanogens aid in the digestion of cellulose
•	Surest sign of intelligent life on Earth?? Cattle flatulence !!

•	Other archaebacteria survive in harsh conditions
>	Halophilic bacteria - salt-loving, live in Great Salt Lake or the Dead Sea, can live in a salt solution up to 36%!
>	Thermophilic bacteria - heat-loving, live in boiling mud in hot springs, hot briny water, near volcanic vents under the sea

•	Among the “true” bacteria, cyanobacteria are especially important
•	Cyanobacteria are photosynthetic, once called blue-green algae

•	Cyanobacteria dominated ecosystems for 1,900 million years (2,500 mya - 600 mya)
•	Piled up to form stromatolites - thick mats, going back ~ 2.7 billion years

•	Stromatolites formed one of the first ecosystems on Earth

•	Cyanobacteria produce oxygen as a “waste product” of photosynthesis
•	Ancient cyanobacteria were the primary source of free atmospheric oxygen

•	It took millions of years for oxygen (O2) to start accumulating in the atmosphere
•	Most initial oxygen combined with elements in the soil and water
•	Huge buffer of ferrous iron, sulphide ions, that quickly absorbed any free O2

•	Banded iron deposits offer mute testament to this early phase of evolution
•	Bands result from reduced iron suspended in water
•	Reduced iron combines with oxygen
•	Iron oxides settled out of solution into layered deposits on the ocean floor

•	Oldest banded iron deposits are ~ 3.8 bya (origins of photosynthesis), youngest are ~1.9 bya
•	~ 2 bya, terrestrial sediments formed red beds, deposits of fully oxidized iron
•	Our planetary soils ~ 2 bya were probably rust-colored, like Martian soils today

•	When the initial oxidation phase was over, free oxygen started to accumulate in the atmosphere
•	Oxygen is toxic to most anaerobic bacteria
•	Anaerobic organisms retreated into low or no oxygen environments (deep underground, in bottom sediments etc.)

•	Why didn’t O2 build up faster?
•	Much of the first free O2 would have been immediately used up by organisms
>	Living organisms used it up in metabolism
>	Dead organisms also used it up (indirectly) - bacteria oxidized the organic molecules of decomposing organisms

•	The oxygen revolution was a major transition in evolution
•	Large, multicellular plants and animals could not exist at low levels of oxygen

•	Atmospheric oxygen also forms ozone (O3)
•	Formation of the ozone layer allowed life to exist on the land with minimal protection from UV radiation

•	Free oxygen changed the biosphere forever
•	Primitive bacteria relied on anaerobic respiration, break chemical bonds for energy without using oxygen
•	Process called fermentation, relies on carbon dioxide, alcohol is waste product (yeast - bread, wine, and beer)

•	As the oxygen level in the atmosphere increased, a more efficient type of cell evolved
•	Eukaryotic cell could use oxygen in respiration, obtain a much higher energetic yield, a more efficient form of metabolism

•	Eukaryotic cell was a giant leap forward in the early history of life
•	All higher organisms are eukaryotes
>	Usually multicellular, can be unicellular
>	Nucleus enclosed by a nuclear membrane
>	Cellular organelles enclosed by membranes

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Eukaryotic cells are much more complex, took another 2 billion years to evolve
•	May have evolved from endosymbiosis - devoured cell that became part of it - mitochondria, chloroplasts
•	Every cell in our body may be a miniature colonial organism

•	Eukaryotic cells quickly became the dominant form of life on Earth
•	All plants and animals evolved from primitive eukaryotic protozoa and algae

•	Once eukaryotic cells were commonplace, pace of evolution seems to have increased very rapidly
•	Most of our modern animal body plans evolved in the blink of an eye during the early Cambrian

•	First bacteria appear ~ 3.5 bya
•	First photosynthetic bacteria ~ 2 bya
•	First eukaryotic cells appear ~ 1.4 bya in banded iron deposits
•	Multicellular animals ~ 700 mya, just below the Cambrian/Precambrian border

•	Evolution of eukaryotic protists could not occur until oxygen levels reached ~1-2% atmospheric O2
•	Diffusion is a very slow process, but works OK for single cells at low oxygen tensions

•	Oxygen tension measures the partial pressure of oxygen (pressure exerted by oxygen as part of a mixture of gases)
•	Larger body sizes (multicellularity) required higher O2 tension, in order for oxygen to reach internal cells by diffusion
•	Would need ~10% O2 minimum for larger multicellular organisms to exist

•	May explain why before ~ 600 mya, there are no definite traces of multicellular life in the fossil record
>	No large fossils of any kind
>	No evidence of passage of organisms through stromatolites - no trails of grazers, no tunnels of burrowers

•	In the 1940’s, R.C. Spriggs made a momentous discovery
•	Working in South Australia’s Ediacara Hills, he found an ancient Precambrian community of organisms

•	Ediacaran fauna dates to ~ 585-550 mya (Cambrian border is 543 mya)
•	Consisted of a wide variety of curious creatures
•	Are they plants or animals?

•	We have no real clue as to how these animals fed or lived
•	Most likely filter feeders, sessile or sedentary (relatively inactive)
•	No known descendants in Cambrian fauna (controversial topic!)

•	Ediacaran organisms turned out to be common, globally distributed
•	Fossils have since been found in numerous locations, including Africa, England, Sweden, and the USA (California)

•	Oldest known specimens are from Canada’s Mackenzie Mountains, Northwest Territory (~ 600 mya)
•	Canadian fauna includes soft cup-shaped animals, oldest known fossils of a multicellular animal

•	The extreme position is that these animals left no descendants
•	Ediacarans seem to thin out and disappear well before the start of the Cambrian
>	Early mass extinction event??
>	Imperfections in the fossil record??

•	First really “modern” organisms appear ~ 543 mya, near the start of the Cambrian
•	Suddenly find most major phyla in place - annelids, molluscs, arthropods, ancestors of all modern animals

•	This spectacular divergence of body plans only took 60-70 million years or less
•	We call it the Cambrian Explosion….
•	Not one major new phylum has evolved since then, though several have gone extinct

•	No one can explain the Cambrian explosion
•	Some evidence suggests that the change in global atmosphere may be responsible
•	Atmospheric oxygen rises sharply at about 580-590 mya, just before the Cambrian explosion

•	Most early animals were soft-bodied, no hard parts to preserve as fossils (preservation bias in fossil record)
•	How do we know so much about early evolution of animals??

•	In 1909, Charles Doolittle Walcott discovered an incredibly well preserved bed of Cambrian fossils, 505 my old

•	Thick layer of rock high atop the Canadian Rockies, near the eastern border of British Columbia, a bed called the Burgess Shale

•	Two remarkable things about the Burgess Shale fossil beds
>	Soft parts of organisms incredibly well preserved
>	Many creatures unlike anything seen on Earth before or since, many seem unrelated to any living creature!

•	Animals lived in mud banks at the foot of a steep primitive reef, formed by thick mats of algae
•	Mud banks were unstable, mudslides were probably common

•	Animals suddenly trapped in mudslide, little or no oxygen or scavengers, fine-grained sediments
•	Perfect conditions for preservation, can even dissect the fossils!

•	Sanctacaris is the first known chelicerate, ancestor of horseshoe crabs, spiders, ticks and mites, scorpions

•	Many are obvious relatives of modern groups, like arthropods

•	Canadaspis is a typical crustacean - early member of a diverse modern taxa that includes crabs, shrimp, lobsters, and crawfish

•	But even those that belong to familiar groups are often structurally and functionally unique - like Odaraia

•	Many, like tiny Marella, are so bizarre, so structurally different from any living organism, they may have been dropped into Earth’s oceans by a passing space ship

•	Consider Opabinia, so weird that different parts of it were classified as four different animals - jellyfish, echinoderm, bivalve mollusc, arthropod - before we discovered one intact !!

•	One of the most unusual critters in the Burgess Shale is Anomalocaris, the “terror of the trilobites”
•	Mighty 3 feet long, the world’s first ferocious predator, great white shark of the Cambrian oceans

•	When the first predators start to appear in the fossil record, the rules of survival are forever changed
•	Wiwaxia is an ordinary polychaete worm, clothed in an impressive suit of armor

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Sometimes the bizzare can become the familiar…
•	Consider what at first glance seems to be a profoundly alien, the aptly names Hallucigenia…

•	Subsequent specimens revealed that our first reconstructions were simply upside down
•	Hallucigenia turned out to be a velvet worm with a sharp set of protective spines

•	Dozens of marvelous experiments in how to construct an animal, only a handful have survived to this day
•	Was the decimation of the Burgess Shale a turning point in the history of life?

•	Gould’s original thesis in his classic book on the Burgess Shale “Wonderful Life” - our ancestors survived by blind luck alone
•	Since then, we have discovered that the Burgess Shale is not unique

•	Now have about 30 sites around the world, including Greenland, mainland China with same bizarre fossils
•	These were the normal organisms of the Cambrian oceans

•	Chinese fauna even older - 570 mya
•	All these diverse body plans evolved in a mere 30 million years!
•	Survivors of the Cambrian seas are the ancestors of all life on Earth

•	What happened to all these marvelous experiments in evolution?
•	Why did some survive while others went extinct?

•	Among the wonders of the Burgess Shale is a humble little creature named Pikaia
•	Barely 2 inches long, Pikaia  is the world’s first chordate

•	Suppose Pikaia had been a victim of the Cambrian mass extinction?