From beginning (485 million years ago) to the end (444 million years ago) it experienced two of the biggest global climatic swings the Earth has ever seen. In the early Ordovician the climate was a sauna -- an intense greenhouse effect caused the highest temperatures the Paleozoic Era would see, and glacial ice all but vanished. Life was abundant in the shallow seas. One of the dominant groups were the conodonts:
Those of you who know your fish might guess that conodonts like Panderodus were related to modern lampreys, and you're right. But it took a really long time to figure that out. Their soft bodies didn't fossilize well, so about all that we had were the cone-shaped teeth that gave them their name. In fact, those teeth are the most common fossils in Ordovician sedimentary rocks, so we knew whatever grew them must have been abundant -- but it took a while to determine what kind of animal they came from.
So things were warm, humid, with tropical conditions virtually pole to pole. Then... something happened. We're still not entirely sure what. Part of it was undoubtedly simple plate tectonics; the supercontinent of Gondwana was gradually moving toward the South Pole. There's some evidence of a large meteorite strike, or possibly more than one. But whatever the cause, by the end of the Ordovician, glaciers covered much of what is now Africa and South America, resulting in a drastic drop in sea level and a massive extinction that wiped out an estimated sixty percent of life on Earth.
At this point, life was confined to the oceans. The first terrestrial plants and fungi wouldn't evolve until something like twenty million years after the beginning of the next period, the Silurian, and land animals only followed after that. As the Ordovician progressed, and more and more ocean water became locked up in the form of glacial ice, much of what had been shallow, temperate seas dried up to form cold, barren deserts. And that was all there was on land -- thousands of square kilometers of rock, sand, and ice, without a single living thing larger than bacteria to be found anywhere.
Then, the climate reversed again. The seas flooded back in, and the warmer, sulfur-rich, oxygen-poor water upwelling from the bottom knocked out about twenty percent of the cold-adapted survivors. By the time the period ended, the Earth had a seriously impoverished biosphere, with something like fifteen percent of the original biota making it through the double-whammy.
But what survived this pair of climate swings was to shape Earth's biological history forever. Because it included primitive vertebrates with paired jaws -- the gnathostomes -- which became the ancestors of 99% of modern vertebrate animals, including ourselves.
But what survived this pair of climate swings was to shape Earth's biological history forever. Because it included primitive vertebrates with paired jaws -- the gnathostomes -- which became the ancestors of 99% of modern vertebrate animals, including ourselves.
The reason this comes up is some new research out of the Okinawa Institute of Science and Technology that analyzed thousands of fossils from species that made it through the Late Ordovician bottleneck -- and an equal number of those that didn't. And they found two interesting patterns. First, the survivors were mostly species that found their way into refugia -- small, isolated pockets of ecosystems with (slightly) more hospitable conditions that allowed them to squeak their way through the worst times. Second, each of the major extinction pulses was followed by dramatic diversification, as the surviving populations expanded into niches vacated by the ones that weren't so fortunate.
As Ian Malcolm so accurately put it, "Life, uh, finds a way."
A couple of other things strike me about this research, though.
The first is how contingent our existence here is. Evolutionary biologist Stephen Jay Gould wrote a provocative piece about "replaying the tape of life," coming to the conclusion if you were to start over from the beginning, so much of the path of evolution has rested on chance occurrences that the chances of it turning out exactly the same way is nearly zero. In a situation like the Late Ordovician Mass Extinction, which assortment of species made it into the few hospitable refugia must have had as much to do with luck as with being well-adapted; had a different set of populations survived, life today almost certainly would look very different.
The second is the fact that both of the Ordovician climate swings were far slower than what we're currently doing to the environment. Like, hundreds of times slower. The second one, in fact -- the warm-up and subsequent melting of polar ice -- was almost certainly a very gradual rebound toward the greenhouse conditions that were to pertain by the mid-Silurian. We're talking about something on the order of ten million years to go from cool to warm.
What we're doing now has taken only a couple of hundred.
What happened in the Late Ordovician should be a wake-up call for us. Yet somehow, we arrogant humans think we're immune to the effects of our out-of-control fossil fuel burning. We have a striking fossil record documenting the terrible effects of rapid climate change in prehistory; at the moment, mostly what we seem to be doing is saying, "Yeah, but it won't happen to us, 'cuz we're special."
So that's our cautionary tale for today. The climate change deniers are fond of saying, "Earth's climate has changed many times before now," and almost never add, "... and when it did, enormous numbers of species went extinct." And the difference, too, is that the natural fluctuations (such as those caused by plate movement, asteroid strikes, and changes in insolation) aren't something we could control even if we wanted to, but what we're doing now is entirely voluntary.
And until the people in charge realize that addressing climate change is in all of our best interest, I'm afraid our path forward is not likely to change.
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