Tracking the Course of Evolution
EXTINCTION: FOREST FIRES
THIS IS the outline of a talk presented in March 2000. As background material, see also an essay on mass extinction in general and a separate essay devoted to the Cretaceous-Tertiary extinction.
AMONG THE TREES OF LIFE
I will also post, as additional background, a few pages on the terrestrial planets from Chapter 1 of my book, since Kevin Padian was kind enough to speak well of them.
At the Geology Department at the University of California, Davis, I try to maintain other Web pages of interest:
The Gaia Hypothesis
Most of us are familiar with that quintessential 1970s idea that Mother Earth is a nurturing, caring (sounds feminine!) entity that we can conveniently call Gaia, the Earth goddess. Gaia encompasses all living things, and there are serious implications that the biological world is balanced to the point that it provides feedback against environmental perturbations, always ensuring that the Earth remains habitable.
We should take a hard look at that. It is a fact that since at least 3500 million years ago, life has evolved continuously on Earth. It is a fact that there have been environmental fluctuations and mass extinctions, yet life has continued. And there are feedback loops. But we are here: we are the survivors describing our history. The Gaia story is a description of the survival of life during environmental extremes. It's not a hypothesis of any sort.
If Gaia is a hypothesis, it predicts that life will always maintain its equilibrium. And that's not a testable hypothesis, so it's not science.
I would argue that if anything, the Earth has tried repeatedly to kill off all life on it, and the survival of life is a tribute to its tenacity and resilience rather than to any nurturing capacity of Earth itself. I think we'd do a better job of managing our future if we stopped regarding our future as automatically assured.
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Ways to Exterminate Life on Earth
What I shall do at first is to describe the ways in which the Earth has tried to kill off life, then fit those processes to known extinctions, and finally, if I have time, to show how resilient some forms of life have been, compared with others.
COLD: SNOWBALL EARTH
How Snowball Earth would operate (in general):
I don't necessarily believe in either Snowball, but many people love it. (Most of them are geophysicists and geochemists, not biologists!).
The early Proterozoic Snowball As Earth's atmosphere begins to have free oxygen (as cyanobacteria proliferate and photosynthesis comes to be a very significant biological process), its methane content has to drop (free oxygen breaks down methane). This drops Earth into a Snowball situation, according to Joe Kirschvink of Caltech and/or James Kasting of Michigan. This occurred about 2200 to 2400 Ma, depending who you believe.
The late Proterozoic Snowball Earth This Snowball Earth is triggered by the drawdown of carbon dioxide as planktonic algae proliferate, and as carbon is buried efficiently.
COLD: ICE AGES
The last Ice Age didn't do much to global biotas, probably because it snuck up on the world. There was a gradual cooling of the world over the past 55 million years. Some events in that cooling were rather large steps, but there was, on the whole, evolutionary time for organisms to adjust. The Antarctic became isolated from other continents and began to cool at the end of the Eocene, around 55 Ma. Soon after that, cold water began to sink around Antarctica to turn the bottom water of the oceans cold and form the cryosphere. When the deep-water Drake Passage opened, the entire world began to cool, around 32 Ma, in the Oligocene. Further cooling in Miocene times and then again toward the end of the Pliocene finally dropped the world into what we call The Ice Age.
There seem to have been no major extinctions associated with the Late Paleozoic glaciations that were centered on Gondwanaland.
But a sudden extinction at the end of the Ordovician has been linked with a severe but very short-lived episode of glaciation. Whether this was a genuine "ice age" or a short, aborted "Snowball" is not clear.
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Clathrates are methane hydrates, and they can build up in seafloor sediments, or in permafrost, through the action of methanogens on buried organic sediment.
Clathrates are metastable, and have dangerous potential for rapid release, flooding the atmosphere with a greenhouse gas directly, and indirectly with breakdown products that are themselves greenhouse gases (carbon dioxide and water vapor). That release could be triggered by climate warming; by volcanic eruptions into massive clathrate deposits; and by sealevel change. The number of potential scenarios is large. Examples that have been suggested, in decreasing order of probability:
End of Paleocene (the Late Paleocene Thermal Maximum)
Contribution to sudden Pleistocene climate fluctuations
The Late Permian extinction, triggered by the Siberian Trap eruptions flooding permafrost areas
The Cretaceous-Tertiary extinction (perhaps the stupidest of the formally published hypotheses).
HEAT: CARBON DIOXIDE CATASTROPHES
For this, we need a release of carbon dioxide that would overpower normal feedback mechanisms such as photosynthesis, so it has to be sudden. There is a huge reservoir of carbon dioxide in ocean water, much larger than that of the atmosphere. (That's why planting trees won't help to solve global warming.) The trick is to keep the ocean and atmosphere reservoirs separate, build up carbon dioxide in the latter, than release it quickly into the atmosphere.
To do that, shut off the normal circulation of the oceans that involves surface water sinking and deep water upwelling. That way, the deep ocean can build up huge amounts of carbon dioxide, and carbon. Then find a way to release it quickly.
There is a modern-day analog: the Black Sea. It is anoxic below the surface, which is less saline than regular ocean water because of the huge rivers that pour into it (Danube, Dnepr, Don). The surface water cannot sink, as regular ocean water does off Antarctica and in the North Atlantic; and deep water cannot upwell, as it does, for example, in the Southern Ocean. If the ocean as a whole were to lack vertical circulation, over 90% of the water on Earth would be anoxic.
Panthalassa, the giant world ocean, could well have gone anoxic after the Late Paleozoic glaciations had gone. There would be no supercold water at or near the poles to sink, and the lack of isolated ocean basins would not provide much supersaline water to sink (as water from the Mediterranean and Persian Gulf does today).
This loaded the gun, and all it would take would be a trigger that would turn over the oceans and release their load of carbon dioxide quickly. There would be a carbon dioxide catastrophe, followed by a prolonged period of global warming. It looks as if this happened at the P-Tr extinction, probably triggered by the Siberian Traps eruption and/or a clathrate release.
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This is clearly a permanent threat for Earth.
Early Sterilizations before 3500 Ma? Life may have begun more than once, but the earliest life may have been sterilized by large asteroid impacts. (There is no evidence for this, however).
The K-T extinction certainly coincides with an asteroid impact. We have a massive crater right at the boundary, buried deep below the Yucatan Peninsula. We have tektites (molten glass beads) and shocked quartz crystals, evidence of giant tsunami, and a series of biological disasters that can easily be interpreted as following an impact. Yet if you look at all the evidence, it is clear that there was more going on than simply a gigantic impact.
The end-Triassic extinction is increasingly being linked with an impact, but the evidence is not yet clear enough to call this case solved.
The end-Devonian extinction Although there are claims of impact, it is increasingly clear that impact was not the, or perhaps, a, contributory factor.
GIGANTIC VOLCANIC ERUPTIONS
Ordinary volcanic eruptions occur all the time, without permanent effects. Krakatoa is completely recovered from the cataclysmic eruption of 1883, for example. Obviously it would take gigantic eruptions to have global effects. Only flood basalt eruptions are large enough, and they are rare events geologically. The Columbia Plateau eruptions of 15 m.y. ago did not have any noticeable effects. Perhaps only two flood basalt eruptions have had anything to do with major extinctions:
The Siberian Traps eruptions, the largest of which we have a record, coincide precisely with the P-Tr extinction.
The Deccan Traps eruptions, best seen in modern India, coincide precisely withe the K-T extinction.
An end-Triassic flood basalt event associated with the opening of the Atlantic remains to be investigated properly for a link with an end-Triassic eruption.
The Great Mass Extinctions
So what do we have? There are seven generally recognized mass extinctions over the past 500 Ma.
End-Devonian: do not know
End Permian: Enormous flood basalt eruptions, possibly resulting in a clathrate and/or carbon dioxide marine catastrophe, and a global warming that lasted for several million years.
End Triassic: don't know. Perhaps flood basalts, perhaps impact
K-T: Asteroid impact PLUS flood basalts. Long-term effect on climate.
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Climate Change as the Linking Factor
How do these physical events set off extinctions? It has to be something larger and/or longer-term than the physical disaster itself (especially, for example, for eruptions and impacts), and the obvious something is likely to have been climate change. That is documented in the aftermath of the P-Tr and K-T events. And it's ironic that climate change is increasingly recognized as the potential trigger for human evolution in the first place, and the most threatening challenge to our future.
Life does recover after major extinctions. But two factors are now clear. The process is slow by ecological standards, because entire ecosystems have been destroyed beyond recognition, as many or even most of their species have become extinct. The process is extremely fast by evolutionary standards, showing that exceptional conditions are in effect, promoting extraordinarily rapid evolution. And the link between these two factors is that ecosystems are reconstituted anew after mass extinctions.
The destruction of the flourishing reef ecosystems of the Late Permian meant that corals, for example, practically disappeared from the fossil record of the early Triassic. The reef ecosystems of the Mesozoic are not fully evolved until the Late Triassic, and at that time the ecological roles in the reef are played by creatures that are analogous yet quite different from their Permian predecessors.
In the same way, the dominant land vertebrates of the Late Cretaceous (the dinosaurs) are not replaced for 5 to 10 million years. During that time there are no large herbivores, and few predators of any size at all. Yet by the end of the Paleocene there are several different lineages of 4- to 5-ton herbivorous mammals, which are of different ancestry on the separate continents; and there are large carnivorous birds. No mosasaurs, ichthyosaurs, or plesiosaurs survive the K-T extinction, but by Eocene times there are very large mammals eating fish in the oceans (whales).
There is a major conservative effect in evolutionary ecology: the incumbency effect. It is difficult to remove an incumbent Congressman in US politics, and in much the same way it is difficult for a species to evolve to displace a species which is already well adapted to its niche. (Typically, it is invaders that can displace incumbents, rather than species evolving in the same ecosystem.)
Obviously, the force of incumbency is much diminished if an ecosystem is drastically affected in a mass extinction. Little wonder that we have subdivided the fossil record on this basis, with the P-Tr and the K-T extinctions marking the ends of eras. Evolution would not have stopped in the absence of mass extinctions: the ancestors of dinosaurs replaced the ancestors of mammals as dominant terrestrial animals without the aid of a mass extinction.
So mass extinctions indirectly bring about major renewals in the history of life, by bringing about major catastrophes. This is not a political statement: but it is a statement of evolutionary reality. In particular, the processes of renewal after mass extinctions are overdue for studies as detailed as those that have been devoted to the extinctions. That's likely to be a major item on the evolutionary agenda over the next twenty years.
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The Special Case: The Evolution of Brains
(I am indebted to David Simon of UC Davis for this idea)
Beginning with Homo erectus in Africa 1.5 m.y. ago, humans have been damaging ecosystems at an increasing rate. Of course, this is not deliberate. It simply reflects a competition for space (e.g., agriculture), competition for food (exterminating competing carnivores and herbivores), and multiplying numbers.
Any creatures would do this if they could. But we (Homo) are the first ones to do it globally, and that's because we have the brains to invent the technology to do it.
Probably that's why we have not had any visitors from outer space: the Drake equation that considers the chances of intelligent civilizations visiting us from elsewhere does not include the biological factor that an intelligent species will probably wreck its planet before it learns how to take care of it.
RC, March 2000.