Since a sixth mass extinction appears to be under way...
From my book: How do we know?
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.
Mass Extinction Event:
Timeline of Life on Earth:
Extinction Events Graph:
Pangaea Map 201 mya:
Russian Meteor Event February 2013