Download Extinction

Extinction
“A meteorite of great size striking the Earth would produce a cataclysm that would wipe
out entire species and destroy..all the remnants of human history.”
Laplace 1797
• When Charles Lyell championed the cause of uniformitarianism he hoped to
> Explain the history of the Earth in terms of gradual, continuous change
> Refute catastrophism as a scientific theory
• He may have done his job a little too well…
• Lyell had to take a lot on faith, like his more Biblically-minded opponents
• He had to assume that the brief sliver of geological time he could observe first hand
was actually representative of the rest
• He had to explain the often abrupt transitions in the fossil record
• Some discontinuities seemed to show rapid and complete turnover of entire
ecosystems
• How could gradual and continuous processes explain that?
• Despite the fundamental problems, Lyell’s ideas became widely accepted
• Arguments over Earth’s history increasingly became a scientific debate, less of a
theological one
• Those who supported any version of catastrophe were roundly criticized, isolated from
mainstream academia
• One such loner was Immanuel Velikovsky
• Velikovsky was a psychiatrist, dabbled in literature, history, mythology
• Became convinced there was a common thread running through a variety of ancient
texts
• Many ancient stories from several continents hinted at unknown cosmic catastrophes
• Velikovsky concluded that the planet Venus had been torn from Jupiter ~ 1500 BC
• Gravity drew the new planet toward the sun, blazing like a giant comet
• Grazed Mars, nearly knocking it into the Earth ~ 700 BC
• Finally settled into orbit between Earth and Mercury
• Not before nearly striking the Earth
• Velikovsky attributed the Biblical plagues, among other events, to the passage of Earth
through the tail of the comet
• Wrote Worlds in Collision in 1950 to describe Earth’s close encounter with the giant
comet
• Certainly not the first person to imagine such a scenario
“Comets have occasionally struck the Earth, causing extinction by altering the
atmospheres and oceans.”
Pierre de Maupertuis, 1742
• Captures spirit of the earlier catastrophists, invoking cosmic causes and spectacular
events to explain the Earth’s history
• Based on loose translations, dubious correlation of ancient texts, events
• Not great science, but it sure makes for great reading (like The Da Vinci Code!)
• Extreme views like Velikovsky helped persuade academics to avoid even the most
remote association with catastrophism
• Catastrophism was reborn in the 1980’s
• The evidence was discovered entirely by accident…
• Luis and Walter Alvarez weren’t looking for evidence of a cosmic collision
• They were trying to determine the best way to date a layer of rock
• No way to tell the age of a layer of rock by its thickness alone
• Sediments accumulate gradually, but the rate of sedimentation varies from place to
place, time to time
• But one thing is constant – the rain of small particles of matter from outer space
• Blankets the Earth like a thin, steady layer of dust, sealed into the layers of rock
• Some elements, like iridium, are rare on Earth’s surface, but common in outer space
• Cosmic dust deposits them over the Earth’s surface at a known, steady rate
• If you could measure the amount of iridium in a layer of rock, you could determine
how long it took that rock to form
• Problem - no known methods precise enough to measure the trace amounts of iridium
in rocks (parts per billion)
• Luis Alvarez was a Nobel award winning physicist, devised a new method of
measuring trace elements in rock
• Went to Gubbio, Italy with his son Walter Alvarez
• Measured the iridium in exposed rocks spanning the boundary between the Cretaceous
and the Paleogene ( boundary)
• The Alvarez team was shocked to discover a significant spike in iridium levels right at
the boundary
• Found ~ 9 ppb iridium, should have been only 1-2 ppb iridium
• Double checked at a boundary site in Denmark, found even higher spike, 42 ppb!!
• What could have caused such a massive increase in cosmic iridium?
• Alvarez concluded it came from an asteroid impact
• Published their results in Science in 1980
• Very controversial – few geologists would accept it at first
• Many geologists still refused to believe that Meteor Crater in Arizona was due to an
impact – attributed it to volcanism!
• Alvarez predicted that similar iridium anomalies would be globally distributed
• Iridium anomalies were later found at over 100 sites around the world
• Now widely accepted as evidence of impact
• Only 10-20 ppb iridium at most sites
• But that is 1,000 times its normal abundance in surface rocks, and a very typical
concentration in meteors, asteroids
• Alvarez estimated from the amount of iridium spread over the world, meteor or
asteroid would have been ~ 10 km diameter
• So in our rush to the truth we may have “thrown the baby out with the bath water”
• In one fundamental sense, the catastrophists were right
• The history of life has been punctuated by a series of global catastrophes
• June 30, 1908, an asteroid exploded about five miles off the ground over the Siberian
tundra, near the Tunguska River
• Explosion was heard around the world
• Travelers on the Trans-Siberian railroad saw the fireball from 350 miles away
• Asteroid weighed about 100,000 tons
• Force of the explosion was about 2,000 times the power of the Hiroshima bomb
• The impact flattened forests for miles around, knocked down horses 400 miles away
• Still a bit of a mystery – no meteorites have been discovered to this day
• Explanations range from an alien spaceship exploding, to a mini-black hole, antimatter
explosion etc…
• Physical evidence favors asteroid or comet
• Some of these ancient events were so spectacular they may have come very close to
eliminating all life on Earth
• And as we’ll see, the next big one may be long overdue…
• Are most or all mass extinctions caused by cosmic impacts?
• Or are they due to normal geological forces, like volcanism and climate change?
• Extinction is the death of species
• Extinction is a normal and ongoing process, an inevitable consequence of natural
selection and environmental change
• Most of the organisms that have lived on this planet are long gone
• Only ~ 1/1,000 of all the species that ever existed are alive today
• Currently ~ 1.5 million named species, up to 30-40 million unknown
• May have been 5-50 billion species on Earth over its entire history!
• Where did they go?
• Why do species go extinct?
• If new species are adaptively superior, then new species should outlast ancestral
species
• But new species have the same extinction rate as ancestral species
• This implies that there is an upper limit to adaptation
• Adaptation is always trying to hit a moving target – the environment
• Environments constantly change, organisms must constantly adapt to these changes
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But if the environment changes too rapidly species can go extinct
Sooner or later every species fails to keep pace with local or global changes
Only a handful of real “living fossils” (Lingula)
Remember the Red Queen in Alice in Wonderland?
• Leigh Van Valen’s Red Queen Hypothesis provides one possible answer to the riddle
of extinction
• Species have a large amount of variation - but their genetic resources are finite
• Environment never stops changing
• Eventually even the most varied gene pool will be exhausted, species goes extinct
• Van Valen predicts that all species are ultimately doomed, including our own
• So is extinction due to bad genes – or bad luck (or both) ?
• Consider the problem of the gambler’s ruin…
• With 50/50 odds, sometimes you win, sometimes you lose
• If you make a $1 bet each time, your winnings will bounce up and down, eventually
will bottom out – busted!
• This is also known as a random walk
• Once you start, if each step is random there is no way to predict where you will be at
any given point
• Good gamblers beat the odds by knowing when to hold ‘em, when to fold ‘em, when
to walk away
• We like to think of evolution as a neat and orderly process, even knowing it is
ultimately ruled by trial and error, random genetic changes, good luck and bad luck
• But in the big scheme of things, “good species” should be rewarded with long life,
“bad species” punished with extinction
• Remember that founding populations for new species are usually small, isolated
• Small populations have a far greater risk of being wiped out by accidents, forces of
nature
• Like a gambler who starts with a small stake, more likely to go broke early on
• Only a handful of species and genera show long life in the fossil record
• Most disappear in a relatively short period of time
• Reflects the basic pattern of biodiversity
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Most genera have relatively few species
Most species have relatively few populations
Most species live in narrow geographic areas
Makes for a “low stake”, a perilous existence for most species…
• Daniel Simberloff suggests four ways in which smaller populations are vulnerable
> Demographic randomness – random accidents can have a much greater effect on
small popn.
> Genetic deterioration – less variability, highly inbred, can’t always adjust to rapid
environmental change
• Daniel Simberloff suggests some ways in which smaller populations are vulnerable
> Social dysfunction
– Harder for males to find females in small popns
– Social facilitation in colonial breeders
> External limiting factors – fires, floods, epidemics etc. are density-independent
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May be a minimum viable population, below which the population cannot sustain
itself
Concept comes from conservation biology
Much of the research on why modern species go extinct applies to past species
Important principle of conservation biology is the relationship between biodiversity
and geographic area
Species/area curve shows that larger areas can support more species
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Mass extinction events reduce the amount of habitable areas
Greatly reduces the overall number of species
Easy to see the effects of loss of habitat on modern animals – main cause of
ongoing extinctions
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Some species are much more vulnerable than others
> Small population
> Restricted range, habitat
> Specialized niche
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Most evolution is divergent evolution - divergence of new species from a common
ancestor
Species tend to become very specialized
> Live in restricted habitat - ex. water in air plants
> Specialize on a narrow diet - fruit, nectar, certain types of insects….
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When things change, specialization can become deadly
If the diet or habitat you rely upon is gone, so are you (ex. plant / pollinator)
When everything changes at once, specialized species die in large numbers
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Normal pattern of extinction are reinforced during mass extinctions - ex. specialists
are even more vulnerable than usual
Tropical organisms tend to be very highly specialized
Mass extinction events hit tropical species the hardest
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• Reef communities are extremely hard hit during mass extinction events
• Many specialized niches, rely on constant shallow water depth
• Specialist or generalist, when the food runs out everyone starves…
• Adaptation has very little to do with survival during mass extinctions
• Chance plays an even greater role in survival than usual
• Mass extinction events have had a profound effect on the evolution of life
• Curious thing - pattern of who lives and dies may be very different from that of normal
background extinctions
• Characteristics that help survival
> Being very widespread - greater chance that someone will be spared
> High population densities - more numerous you are, higher chance some will make
it through
> Being lucky…blind chance
• David Raup has proposed three modes of extinction
> Field of bullets – Random survival
> Fair game – Darwinian, fittest usually survive
> Wanton extinction –Extinction is selective, but not tied to adaptation
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David Raup has proposed three modes of
> Field of bullets – Random survival
> Fair game – Darwinian, fittest usually survive
> Wanton extinction –Extinction is selective, but not tied to adaptation
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Wanton extinction may be best explanation of the patterns observed in mass
extinctions
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Webster’s defines wanton as “ having no just foundation or provocation: being
without check or limitation:
Consider the effects of a nearby supernova, flooding the Earth with deadly cosmic
rays
Burrowing animals, deep sea fish would survive, but only because they were hidden
Species whose habitat was at or near an impact event would be wiped out regardless
of their adaptive superiority
Natural selection is not a factor in wanton extinction
There is no hard and fast definition of a mass
It’s ultimately statistical…may be as few as 5 or 6, as - many as 20, depending on
where you draw the line
When does the usual become the unusual?
• There is a normal average rate of speciation
• There is also a normal rate of extinction
• Roughly 180-300 species go extinct every one million years
• There is a normal (average) rate of speciation, extinction
• Average lifetime of a species in the fossil record is ~ 4 million years
• The ongoing normal rate of natural extinction is called background extinction
• There is no hard and fast definition of a mass extinction
• It’s ultimately statistical…may be as few as 5 or 6, as - many as 20, depending on
where you draw the line
• When does the usual become the unusual?
• Every so often, however, it really hits the fan…
• We find a rapid turnover of fossil organisms, entire communities, abrupt changes in
the nature of the very rocks themselves
• Fingerprints of a mass extinction event
• Many possible causes of mass extinction - geologically normal causes
> Climate change
> Sea level change (regression)
> Extensive volcanism
> Predation, diseases, parasites
> Competition
• Many possible causes of mass extinction – cosmic causes:
> Cosmic collisions – asteroids, comets, meteors
> Cosmic radiation – nearby supernovae
> Invaders from Mars…
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Kill curve (plots the probability of percentage of species killed in a mass extinction
event over time)
• We can use a “kill curve” to predict:
> Likelihood of major extinction events
> Average waiting time between mass extinctions
• Common features of a mass extinction event:
> Many species go extinct in a short period of time (30% or more)
> Vanished species span all habitat types, sizes - marine and terrestrial, large and
microscopic
• If we restrict extinction rates to events with a probability of 5% or less, we are left
with the “Big Five” mass extinction events
> End Ordovician
> Late Devonian
> End Permian
> End Triassic
> End Cretaceous
• Many mass extinction events may have preceded those we see in the fossil record
> Bombardment era (3.8 – 4.6 bya) – heavy meteor showers periodically cleansed the
Earth
> Oxygen revolution (~ 2.5 bya) – rise in O2 wiped out most anaerobic bacteria
> Snowball Earth (~ 600-700 mya)
• Precambrian extinction may have wiped out the Ediacaran fauna, paved the way for
the Cambrian explosion
• At least one or two minor extinctions in the Cambrian
• Second Cambrian extinction took out early trilobites (Agnostus), first reef formers
• Widespread environmental stress might not be sufficient to cause mass extinction
• Most organisms could escape to refugia, migrate to a new habitat
• Most can migrate faster than climate can change, or shorelines rise or fall
• Large impact event would not be selective, no one could escape
• End Triassic extinction (~200 mya), for example wiped out 50% of all genera
• Most ammonites disappeared, 58 families of cephalopod mollusks, major groups of
brachiopods, bivalves, marine reptiles
• No known cause, but may have been an impact event
• Manicougan Crater in Canada is about the right age (~214 mya)
• Crater is about 200 km across!!
• Recently discovered Chicxulub Crater, off the coast of the Yucatan, is the smoking
gun of the extinction event
• What is the evidence for large impacts?
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Big craters (duh)
Anomalous levels of iridium and other elements
Shocked quartz or other minerals
Tektites in sediment
Soot particles
• Presence of large craters is a big clue…
• Three of the “big five” events now have candidate craters
• Several smaller extinction events also match the age of known craters (end of Eocene,
end of Carboniferous ex.)
• Large impacts are relatively rare
• An impact on the scale of the Manicougan Crater would occur only once every 100
million years
• So a dinosaur killer (10 km asteroid, 50 km crater) would come along roughly once
every 100 million years
• Estimated power of a 10 km rock colliding with the Earth ~ 100 million megatons of
TNT!!!
• Since the discovery by Alvarez, others have searched for similar iridium anomalies
• Only CP and late Devonian extinctions among big five clearly show an iridium spike
• Some anomalies do correlate well with lesser extinctions
• Biggest problem with this research is that it is very expensive
• Usually only use it looking for “smoking guns”, so no systematic coverage of strata
• Additional evidence for large impacts comes in the form of shocked minerals
• Atoms in crystals are arranged in neat, ordered rows
• In the stress of collision, these rows of atoms can become dislocated
• Shocked minerals show characteristic pattern of displaced atoms
• Multiple intersecting shock planes are characteristic of meteoroid impacts
• Very intense shocks can even alter the basic crystalline structure itself
• Quartz crystals change from tetrahedrons to a more densely packed octahedron
structure, called stishovite
• Presence of stishovite indicates a colossal impact shock
• Shocked minerals have been discovered at the CP boundary layer
• Further support for the impact hypothesis
• Tektites are also evidence of impact events
• Small glassy spheres of rock, liquefied by the heat and pressure
• Thrown off as liquid splashes, harden into tektites
• Soot particles are another source of evidence for large impacts
• Thin layer of soot at the CP boundary suggests global wildfires started by the impact
• There are some problems with the impact extinction theory
• Not all impact sites correspond to known extinction events, nor do all extinction
events have a corresponding crater
• Dating of craters and times of extinction events are both pretty rough, only accurate
within ~ 10-20 my or so
• Some of the biggest craters have no matching mass extinction event
• Tookoonooka Crater, in Queensland, Australia, is 55 km wide, 128 mya
• Montagnais Crater, Nova Scotia continental shelf, is 45 km wide, 51 mya
• No corresponding extinction events
• Imagine the giant rock that killed the dinosaurs, 10 km wide, hurtling into the Gulf of
Mexico at cosmic speeds
• The impact and explosion would dwarf the energy of the worlds’ entire nuclear arsenal
• From the shallow marine sediments a plume of steam and debris shoots upward,
achieving escape velocity before settling into low orbit
• Through the winds of the upper atmosphere the dust shroud spreads
• Dust quickly envelops the Northern Hemisphere, obscuring the sun
• Many animals are killed outright, pulverized by the explosion or swept away by a
monstrous tidal wave over 100 feet tall
• Tsunami deposits have been found all over the Caribbean, including evidence in Cuba
of a 100 foot tidal wave
• Darkness settles over the Earth at high noon
• Photosynthesis stops, and plankton and terrestrial plants die of starvation
• Foraging in the dark for ever fewer scraps of living vegetation, the herbivores
gradually succumb
• Herbivores are followed shortly by the carnivores that preyed on them
• For a brief period, detritivores and scavengers rule the world
• Kinetic energy released as heat from the impacting asteroid sparks a global wildfire
that devastates terrestrial floras
• Adds even more smoke and dust to the thick blanket of death that girdles the globe
• Wildfires leave behind a thin layer of soot still visible in boundary layer sediments
• In the sea, a major long-term ecological disruption is underway
• The increasingly acid ocean waters kill the calcareous plankton, and dissolves
calcareous sediments on the ocean floor
• This releases immense amounts of carbon dioxide into the atmosphere, adding to the
outgassing of numerous volcanoes and combustion gases from the raging forest fires
• Adding to the carnage and chaos, the Deccan Traps pour a river of lava over much of
what is now India
• Additional dust, carbon dioxide, add another knockout punch
• A feedback loop between ocean and atmosphere creates a greenhouse effect, further
stressing the organisms that have managed to survive the initial blow
• Massive pulse of organic carbon from dead terrestrial organisms floods into the sea,
causing anoxia, stagnation
• The Cretaceous skies were full of diverse species of winged pterosaurs
• The oceans teaming with plesiosaurs, ichthyosaurs, elasmosaurs…
• All gone, in an instant of geological time
• Along with them went 50% of the large invertebrates, most of the biomass of land
plants, and most of the marine plankton
• The dinosaurs have vanished, and the wealth of ecological niches they formerly
occupied are opened up for the rapidly radiating mammals
• Finally, when the dust settles, the planet is transformed
• Tropical conditions have given way to a temperate climate
• For a while, ferns dominate the land, “fern spike” at CP boundary reminiscent of spike
seen after Mt. St. Helen’s eruption
• Ferns slowly give way to pines and other gymnosperms and finally to the recovering
flowering plants.
• The rules, however, have changed forever…