Great Extinctions and Evolution

Great extinctions are times when the number of species that cease to exist per century gets a lot higher than the average. There are always extinctions going on, and the vast majority of species that ever existed are already extinct. It seems to be a common destiny of a species to begin, to flourish, to plateau, to diminish and then to become extinct. Lots of things make species go extinct, such as the food supply being cut off, some environmental change or catastrophe such as their only habitat being flooded, long-term or short-term. New predators can evolve. More efficient species can out-compete them for food supplies or nesting spaces. And so on.

Species counts are done by checking fossil records. It seems from fossils that there were multiple times when a large fraction of the existing land and/or sea animal species ceased to appear in later fossils, meaning they became extinct. These extinctions typically happen over a geologically short time, and geology being what it is, there is a minimum time that can be determined by fossil research, so no understanding of exactly how long the extinction lasted can be obtained, if it was shorter than the minimum measurable geological interval. Nonetheless, there are multiple periods in Earth’s history when a large number of animals became extinct, up to 96% as reckoned by fossil counts on the worst of these great extinctions.

The causes of such extinctions are obviously of great interest to geologists and many other scientific specialties. The earliest one is sometimes attributed to the oxygenation of the atmosphere, meaning that all sea creatures which could not tolerate the oxygenation of the atmosphere, and correspondingly, the seas, died off. No other atmospheric change seems to have caused other extinctions, but climate change has been called on as one possibility for some of them, although not the largest ones, unless you call a temporary clouding and the subsequent cooling of the Earth a climate change. Instead, some of these largest ones are attributed to either a large asteroid strike or a basalt flooding. A controversy arose over the last one, the End-Cretaceous extinction. Someone came up with a measurement that 76% of animal species became extinct during this one.

One of the earliest theories as to the cause of this extinction was the Decca Basalt Flooding, which was chronologically pinpointed to occur at the time of the extinction. A basalt flooding, when thousands of square kilometers become like a volcano, with exposed mantle material, produces such massive amounts of dust in the atmosphere that sunlight is blocked and photosynthesis stopped. The flooding lasts for many millennia, unlike a volcano whose principal eruption lasts only a matter of days or weeks. It certainly is reasonable that this could cause mass extinction.

Later, a large crater was discovered on the coast of the Yucatán peninsula, and it was timed to also be at the time of the End-Cretaceous extinction. The crater was named Chicxulub. Asteroidal material, specifically a higher than normal concentration of iridium, was discovered all over the world and dated in sediments to have been deposited by the Yucatán asteroid. The impact would have filled the atmosphere with dust as well, although for a much shorter period. If the End-Cretaceous extinction happened within a very short period, a few years which is the dust residence time from a single near-instantaneous event, this could also have been the cause of the extinction.

It is curious that the Deccan Traps, the current geological formation from the basalt flooding there, and the Chicxulub crater are almost on opposite sides of the planet. When a rapidly flying object hits a large obstacle, shock waves are generated which move from the impact location in the direction of the incoming object. These shock waves carry some of the momentum of the incoming object, and when they strike the opposite surface, they tend to spall off the outermost layer. Spallation of this type occurs when very fast projectiles strike armor plate, for example. On something of the size of a planet, it would mean that the crust at the arrival point of the center of the shock wave would heave upward, rupturing it and leaving it no longer intact. The Deccan volcanic activity may have been going on before the impact, but the shock wave may have been the cause of the large size and huge amount of atmospheric debris that was deposited.

The crater and the traps are not on exactly the opposite sides of the planet, now, but tectonic plate motions are of the right magnitude to make it more so 60 million years ago. Both India and South America have been moving north. Furthermore, if the impact was not perpendicular, there would have been some deviance of the shock wave direction from directly through the center of the planet so it did not go from a point on one side to the exact opposite.

The crater and the traps can be used to backwards track the asteroid, and doing this shows that if the impact was in the spring or fall, the asteroid would have been traveling in the sidereal plane, where all planets and almost all asteroids reside. Thus, the two events, asteroid impact and basalt flooding, may have been closely connected.

What is considered the most serious great extinction, the End-Permian one about 250 million years ago, is often attributed either to the Siberian basalt flooding, near the north pole, or to the Wilkes Land crater, in Antarctica near the south pole. The details of these two events are much, much less certain than those related to the End-Cretaceous one, but the possibility of a basalt flooding event triggered by an asteroid impact on the opposite side of the globe exists. For some other great extinctions, there are even less certain connections to basalt flooding and asteroid impact.

What happens after an extinction? When the atmosphere returns to transparency, there is much less life around. Most of the species have gone entirely extinct, and the remaining ones were greatly reduced in numbers. It has become a field day for evolution, because the constraints on new life are largely removed. Resources are available in abundance, and competition is less. Predation is almost absent. Thus, evolution goes much faster in creating new species that it could in a crowded environment where all ecological niches were already filled. Within some tens of millions of years, hordes of new species exist everywhere.

We humans arose from the chaos of new life that occurred after the End-Cretaceous extinction. Life was not extinct, and what life existed still kept many of the genetic tricks that had evolved up to that point. Thus, evolution took off again, but from a much higher starting point genetically. This is not to say that life could not have evolved intelligence without the extinctions, but it would have taken much longer. Maybe a factor of ten in speedup happened.

What exactly does that mean for a planet somewhere in the galaxy with eukaryotic, multicellular life already started. To get to this point might have taken 3 billion years of evolution. It will take another billion years of evolution, if the rate of evolution is the same as Earth’s, to get to intelligent life. However, Earth’s evolution rate was governed and greatly accelerated by all the mass extinctions our planet has known. If there were no extinctions, or only very few, it might take ten times longer to get to that point, or ten billion years. Almost no planets are that old.

So, one interesting thing to ponder is: Can an intelligent alien civilization come into extinction in a solar system where there are no or very little asteroid impact? Is the answer: Yes, but only billions of years from now. Thus we might have another peculiarity of our solar system which might be essential to the eventual capability for contemporaneous space travel: huge numbers of large asteroids.