Extinction!
From my book: How do we know?
by
Kenny A. Chaffin
All Rights Reserved © 2013 Kenny A. Chaffin
Over 98% of known species are
dead – gone, kaput, vamoosed, disappeared, extinct! We will be too someday by
those odds, but for now let us enjoy our time under the sun. That 98% doesn’t
count the unknown species; those that we could have no idea about, like the
ones just getting going when Theia struck the nascent Earth and turned it once
more into a molten sea of lava. In the last half billion years there have been
at least five planet-spanning extinction events in which half of all animal
species on Earth have been destroyed. Could it happen again and if so is there
anything we can do about it?
The answer
of course is, certainly it could happen again and very likely will and odds are
there is very little we could do about it. The best we might be able to do
would be to get out of the way. The earliest extinction may have been from the formation
of our Moon when Theia, a Mars-sized protoplanet slammed into the Earth some
half-a-billion years after its initial formation. At that time the early Earth
would have likely had surface water, atmosphere and have been cool enough to
support primitive life or its origin whether from panspermia or abiogenesis.
Life literally could have been gaining a foothold on Earth when the massive
impact by Theia would have rebooted life to begin again.
Once life
returned it was still no easy pickings. Early simple single celled organisms
and their predecessors would live in a kind of stasis for billions of years as
best we can tell. The geological and fossil records are virtually non-existent.
The earliest indicators of life we have are geological layers and stromatolites
produced by cyanobacteria. It would be another 2.5 – 3 billion years before
these single cells began cooperating and forming multicellular aggregates and
another billion years – approximately 4 billion years since the Earth’s
formation before the first simple animals appeared. There would be what has
been called the Cambrian explosion 540 million years ago right around Earth’s
four billionth birthday when there was a major expansion in the diversity of
life. It is this abundance of life and the traces it left in the geological
record which allows us to gain insight into extinctions. By excavating and
analyzing layer upon layer of sediment left on the sea floor and subsequently
pushed to the surface or through direct core sampling we can count, test, and examine
layers in relation to one another and determine reasonably precisely when life
was abundant, when certain species died out or when all life seems to struggle
or disappear.
Our first glimpse of the great
Cambrian explosion came from the Burgess shale a 505 million year old fossil
bed in Canada which contains some of the best preserved fossils of all time.
This plethora of Cambrian life has been validated in other sites around the
world as well. There may have been a greater diversity of life at that time
than at any other in Earth’s history. The Ordovician-Silurian extinction which
followed would definitely put a damper of that explosion.
The O-S
event was actually two closely spaced extinction events in the 450-440 mya
timeframe. It killed off 70% of all species and is ranked second only to The
Great Dying of 251 mya. At 450 mya the cephalopods may have been the dominant
life-form, but many others existed. There were sea urchins and jawed eels and
jawless vertebrate fishes. This event killed off two-thirds of all those species.
Of course what most people want to
know is what caused it? Was it an asteroid like killed the dinosaurs, was it an
ice age or global warming? The fact is that we cannot always associate a
specific climatic change or other cause with an extinction event.
There have been five ‘major’
extinctions and dozens of smaller extinctions which we see in the fossil
record. There is even a hypothesis that extinctions occur every 26 million
years and are somehow linked to a dark companion star (named Nemesis of course)
that is orbiting around the sun and perturbs the Oort cloud and/or asteroid
belt. There is of course no evidence supporting this nor does it match
precisely with the timing of extinction events. Other associated and/or more
relevant possibilities are flood basalt flows of magma from the Earth’s
interior. This would be events such as the Deccan and Siberian trap events
which released massive flows of lava as well as volcanic gasses into the
atmosphere. Sea-level falls appear to be associated with all of the major
extinction events, but it is unclear if this is a cause or a result of other
events.
And of course there is the one we
all fear – an impact event such as the Chicxulub impact which killed the
dinosaurs 65 million years ago. An ocean impact can have an even more
devastating effect than a land impact. The energy – particularly heat generated
by an ocean impact can release massive amounts of CO2 gas which
would kill any air-breathing organisms. Since ocean impacts do not always leave
an obvious crater it is difficult to attribute as the cause of a mass
extinction. There is always the possibility for climate to shift out of control
producing either an ice age or global warming that can be disastrous to all
life. Again like the sea-level falls these conditions are thought to be more a
result of others than the cause themselves. There are other less obvious but
possible events which either contribute to or trigger extinction events such as
near-space supernova explosions, anoxia conditions in the ocean, super volcano events
such as the Toba eruption, unusual plate tectonics movement and others. (See the
links in the resource section if you really want to dig deeper)
It is
interesting to note that immediately following the Ordovician-Silurian extinction
(450 mya) we see the first plant life move onto land from the seas. This may be
an example of how an extinction event can ‘accelerate’ evolution. Not that
evolution can be actually accelerated, but by clearing out the environment or
opening up niches for new life to flow into there can be a greater rate of
expression of mutations that take advantage of those previously unexploitable
environmental niches. We see this happen following the extinction of the
dinosaurs with the rise of mammals to take advantage of the vacant
environmental areas. Following the O-S extinction sharks became the dominant
predator in the ocean. Crawling insects arose on land and would soon be flying.
The first tetrapods wriggled and climbed their way onto land to become the
ancestors of all four-limbed vertebrate land animals including humans. At this
time the most recent supercontinent Pangaea was being formed from the
convergence of Euramerica and Gondwana.
While those
tetrapod descendants were likely affected by the next mass extinction it would
have its predominant effect on marine species. Overall some 70% of species on
the planet were destroyed by the Late Devonian Extinction which lasted an
incredible 15-20 million years from 375-360 mya. There is some evidence for
multiple ‘pulses’ of extinction during this period. Even given the long duration
there is no definitive evidence as to the cause. We know ocean anoxia existed but
that was likely a result rather than the cause.
Every
extinction event we’ve seen required 5-10 million years for species to
re-adapt, re-evolve, and re-enter the environmental niches opened by the
extinction. In the case of extreme
extinction events such as the Permian-Triassic Extinction it was more like 30
million years. This was known as The Great Dying and over 90% of all species
were destroyed, life itself was close to being wiped from the planet. This took
place 251 million years ago. Whereas the Devonian extinction affected primarily
the oceans, the P-T extinction was devastating to everything. It ended the
dominance of mammal-like reptiles on land. It destroyed 96% of all ocean
species and 70% of all land species down to the insects which is unique to this
particular extinction. There are numerous speculations as to the cause. The
Siberian Trap event is the prime candidate which lasted for a million year
period from 251-250 mya. A secondary possibility is an asteroid impact, perhaps
in the ocean. Beyond those, the fossil evidence shows a sea level change,
anoxia, drought-like conditions and apparent shifts in ocean circulation. Given
the near total extinction of life we see significant evidence of this event in
most any and all areas of the environment – in the geological layers, ocean
core samples as well as Arctic ice cores in all regions of the world.
Life would
recover from this the worst devastation ever during the next 50 million years
until the Triassic-Jurassic extinction event 201 mya just before Pangaea began
to break up. Compared to others it was short-lived at only 10,000 years yet it
still destroyed 70% of all living species at the time. Many of the land animals
- archosaurs, therapsids, and large amphibians were killed off which left open
an environment the dinosaurs would rule for the next 140 million years. There
was an evident and gradual climate change associated with this extinction but
that alone would not have induced it. A proposed meteor impact could have, but
no evidence has been found. There was a large basalt flood, of the Central
Atlantic Magmatic Province (CAMP) in the area of what would become the
mid-Atlantic ridge as Pangaea was beginning to break up. Many think this is the
most likely source for the Triassic-Jurassic extinction which made way for the
thunder lizards.
And now the
one you’ve been waiting for, the Cretaceous-Paleogene Extinction (K-T) of 66
mya which killed those very same dinosaurs and made way for us via a mammal
domination of the Earth which continues to this day. Of the five major
extinctions this is the one that is tied to an asteroid impact. The theory is
supported by a geological layer of iridium found world-wide and whose thickness
varies by location. Iridium is associated with asteroids and the distribution
of this layer – its thickness and position matches with the location of the Chicxulub
crater in the Gulf of Mexico off of the Yucatán peninsula. The layer is thicker
closer to the impact point and in predominant wind directions and thinner in
remote regions – e.g. the opposite side of the planet. The fact that this layer
is world-wide tells us that it sent massive amounts of dust and debris into the
atmosphere. The impact brought on a global and extended winter due to dust
blanketing the atmosphere. The dust and debris in the atmosphere was so thick
that it blocked most sunlight preventing photosynthesis and many plants died leaving
a massive hole at the bottom of the food chain which propagated upward and
resulted in the death of the dinosaurs at the top of that food chain. Also
implicated in this extinction is the Deccan Trap Event -- a basaltic flow in
northern India which spanned the same time period. Some 75% of all species were
destroyed, but the environment was opened to the small surviving mammals that
would thrive, expand and diversify into new creatures such as horses, whales,
bats, primates and more leading eventually to ourselves.
What of
future extinctions? There is certainly the possibility of an asteroid strike as
was seen dramatically over Russia recently and with the recent close-passes of
largish asteroids. It is thought that the Yellowstone caldera is well past due
for a massive explosion or basaltic flow. It could all happen again and there
would be very little we could do. We are in no position to evacuate the planet
in large or in small. We would simply die…or depending on the extent of the event
perhaps survive in some small and or primitive manner to potentially rebuild
civilization again.
References/Resources/Links
Mass Extinction Event:
Cambrian Explosion:
Timeline of Life on Earth:
Burgess Shale:
Tetrapods:
Extinction Events Graph:
Nemesis:
Chicxulub Crater:
Deccan Traps:
Siberian Traps:
Pangaea Map 201 mya:
Russian Meteor Event February 2013