(This is an extended version of the talk I have at EA Global San Francisco 2017. Long-time readers will recognize it as an updated version of a post I wrote last year. It was wonderful meeting people there!)
This is a graph of extinction events over the history of animal life.
There are five canonical major extinction events that have occurred since the evolution of multicellular life. Biotic replacement has been hypothesized as the major mechanism for two of them: the late Devonian extinction and the Permian-Triassic extinction. There are three other major events – the Great Oxygenation Event, End Ediacaran extinction, and the Anthropocene / Quaternary extinction.
Let’s look at four of them. The first actually occurs right before this graph starts.
I decided not to discuss the Great Oxygenation Event in the talk itself, but it’s also an example – photosynthetic cyanobacteria evolved and started pumping oxygen into the atmosphere, which after filling up oxygen sinks in rocks, flooded into the air and poisoned many of the anaerobes, leading to the “oxygen die-off” and the “rusting of the earth.” I excluded it because A) it wasn’t about multicellular life, which, let’s face it, is much more relevant and interesting, and B) I believe it happened over such a long amount of time as to be not worth considering on the same scale as the others.
(I was going to jokingly call these “animal x-risks”, but figured that might confuse people about that the point of the talk was.)
The End-Ediacaran extinction
We don’t know much about Precambrian life, but it’s known as the “Garden of Ediacara” and seems to have been a peaceful time.
The Ediacaran sea floor was covered in a mat of algae and bacteria, and ‘critters’ – some were definitely animals, others we’re not sure – ate or lived on the mats. There were tunneling worms, the limpets, some polyps, and the sand-filled curiosities termed “vendozoans”. They may have been single enormous cells like today’s xenophylophores, with the sand giving them structural support. The fiercest animal is described as a “soft limpet” that eats microbes. They don’t seem to have had predators, and this period is sometimes known as the “Garden of Ediacara”. (1)
At 542 million years ago, something happens – the Cambrian explosion. In a very short 5 million years, a variety of animals evolve in a short window.
Molluscs, trilobites and other arthropods, a creative variety of worms eventually including the delightful Hallucigenia, and sponges exploded into the Cambrian. They’re faster and smarter than anything that’s ever existed. The peaceful Ediacaran critters are either outcompeted or gobbled up, and vanish from the fossil record. The first shelled animals indicate that predation had arrived, and that the gates of the Garden of Ediacara had closed forever.
The end-Devonian extinction
Jump forward a few million years – 50% of genuses go extinct. Marine species suffered the most in this event, probably due to anoxia.
There’s an unexpected possible culprit – plants around this time made a few evolutionary leaps that began the first forests. Suddenly a lot of trees pumping oxygen into the air lead to global cooling, and large amounts of soil lead to nutrient-rich runoff, which lead to widespread marine anoxia which decimates the ocean.
We do know that there were a series of extinction events, so forests were probably only a partial cause. The longer climate trend around the extinction was global warming, so the yo-yoing temperature (from general warming and cooling from plants) likely contributed to extinction. (2) It’s strange to think that the land before 375 million years ago didn’t have much in the way of soil – major root structures contributed to rock wearing away. Plus, once you have some soil, and once the first trees die and contribute their nutrients, you get more soil and more plants – a positive feedback loop.
The specific trifecta of evolutions that let forests take over land: significant root structures, complex vascular systems, and seeds. Plants prior to this were small, lichen-like, and had to reproduce in water. (3)
The Permian-Triassic extinction
96% of marine species go extinct. Most of this happens in a 20,000 year window, which is nothing in geologic time. This is the largest and most sudden prehistoric extinction known.
The cause of this one was confusing for a long time. We know the earth got warmer, or maybe cooler, and that volcanoes were going off, but the timing didn’t quite match up.
Volcanoes were going off for much longer than the extinction, and it looks like die-offs were happening faster than we’d expect from increasing volcanism, or standard climate change cycles. (4) One theory points out that die-offs line up with exponential or super-exponential growth, as in, from a replicating microbe. Remember high school biology?
One theory suggests Methanosarcina, an archaea that evolved the chemical process that turned organic carbon into methane around the same time. Remember those volcanoes? They were spewing enormous amounts of nickel – an important co-factor for that process.
(Methanosarcina appeared to have gotten the gene from a cellulose-digesting bacteria – definitely a neat trick. (5) )
The theory goes that Methanosarcina picked up its new pathway, and flooded the atmosphere with methane, which raised the surface temperature of the oceans to 45 degrees Celsius and killed most life. (2)
This report is a little recent, and it’s certainly unique, so I don’t want to claim that it’s definitely confirmed, or sure on the same level that, say, the Chicxulub impact theory is confirmed. That said, at the time of this writing, the cause of the Permian-Triassic extinction is unclear, and the methanogen theory doesn’t seem to have been majorly criticized or debunked.
Quaternary and Anthropocene extinctions
Finally, I’m going to combine the Quaternary and Anthropocene events. They don’t show up on this chart because the data’s still coming in, but you know the story – maybe you’re an ice-age megafauna, or rainforest amphibian, and you are having a perfectly fine time, until these pretentious monkeys just walk out of the Rift Valley, and turn you into a steak or a corn farm.
Because of humans, since 1900, extinctions have been happening at about a thousand times the background rate.
(Looking at the original chart, you might notice that the “background” number of extinctions appears to be declining over time – what’s with that? Probably nothing cosmic – more recent species are just more likely to survive to the present day.)
Impacts from evolutionary innovation
You can probably see a common thread by now. These extinctions were caused – at least in part – by natural selection stumbling upon an unusually successful strategy. Changing external conditions, like nickel from volcanoes or other climate change, might contribute by giving an edge to a new adaptation.
- In some cases, something evolved that directly competed the others – biotic replacement
- In others, something evolved that changed the atmosphere.
- I’m going to throw in one more – that any time a species goes extinct due to a new disease, that’s also an evolutionary innovation. Now, as far as we can tell, this is extremely rare in nature, but possible. (7)
Are humans at risk from this?
From natural risk? It seems unlikely. These events are rare and can take on the order of thousands of years or more to unfold, at which point we’d likely be able to do something about it.
That is, as far as we know – the fossil record is spotty. As far as I can tell, we were able to pin the worst of the Permian-Triassic extinction down to 20,000 years only because that’s how narrow the resolution on the fossil band formed at the time was. It might have actually been quicker.
Even determining if an extinction has happened or not, or if the rock just happened to become less good at holding fossils, is a struggle. I liked this paper not really for the details of extinction events (I don’t think the “mass extinctions are periodic” idea is used these days), but for the nitty gritty details of how to pull detailed data out of rocks.
That said, for calibrating your understanding, it seems possible that extinctions from evolutionary innovation are more common than mass extinctions involving asteroids (only one mass extinction has been solidly attributed to an asteroid: the Chicxulub impact that ended the reign of dinosaurs.) That’s not to say large asteroid impacts (bolides) don’t cause smaller extinctions – but one source estimated the bolide:extinction ratio to be 175:1. (2)
Plus, having a brain matters, and I think I can say it’s really unlikely that a better predator (or a new kind of plant) is going to evolve without us noticing. There are some parallels here with, say, artificial intelligence risk, but I think the connection is tenuous enough that it might not be useful.
If we learn that such an event is happening, it’s not clear what we’d do – it depends on specifics.
But consider synthetic biology – the thing where we design new organisms and see what happens. As capabilities expand, should we worry about lab escapes on an existential scale? I mean, it has happened in nature.
Evolution has spent billions of years trying to design better and better replicators. And yet, evolutionary innovation catastrophes are still pretty rare.
That said, people have a couple of advantages:
- We can do things on purpose. (I mean, a human working on this might not be trying to make a catastrophic geoweapon – but they might still be trying to make a really good replicator.)
- We can come up with entirely new things. When natural selection innovates, every incremental step on the way to the final result has to an improvement on what came before. It’s like if you tried to build a footbridge, but at every single step of building it, it had to support more weight than before. We don’t have those constraints – we can just design a bridge and then build it and then have people walk across it. We can design biological systems that nobody has seen before.
This question of if we can design organisms more effective than evolution is still open, and crucial for telling us how concerned we should be about synthetic organisms in the environment.
People are concerned about synthetic biology and the risk of organisms “escaping” from a lab, industrial setting, or medical setting into the environment, and perhaps persisting or causing local damage. They just don’t seem to be worried on an existential level. I’m not sure if they should be, but it seems like the possibility is worth considering.
For instance, a company once almost released large quantities of an engineered bacteria that turned out to produce soil ethanol in large enough quantities to kill all plants in a lab microcosm. It appears that we don’t have reason to think it would have outcompeted other soil biota and actually caused an existential or even a local catastrophe, but it was caught at the last minute and the implications are clearly troubling. (9)
- Ediacaran biota: The dawn of animal life in the shadow of giant protists
- On the causes of mass extinctions
- Terrestrial-Marine Teleconnections in the Devonian: Links between the Evolution of Land Plants, Weathering Processes, and Marine Anoxic Events
- The Permo-Triassic extinction
- Methanogenic burst in the End-Permian carbon cycle
- Natural Die-offs of Large Mammals: Implications for Conservation I’m pretty sure I’ve seen at least a couple other sources mention this, but can’t find them right now. I had Chytridiomycosis in mind as well. This seems like an important research project and obviously has some implications for, say, biology existential risk.
- Rather sensationalized description from Cracked.Com