Thread by @nils_gilman

My own view is that gradualism followed by a final exclamation point is a very likely extinction scenario, both for dinosaurs — and for humans. Indulge me in long thread that ends with meditations on #TransformationOfTheHuman... 1/

https://twitter.com/nils_gilman/status/1033548612309987328

Today, we are clearly in the middle of what scientists refer to “The Sixth Great Extinction,” akin in many respects to the Fifth one, that killed the dinosaurs, and whose causes are debated in that @TheAtlantic article. So: when did the SGE begin, and how will it end? 2/

First, a small methodological points: geological eras are generally demarcated by either species die offs and emergences and/or some other geological market. This is the debate over how to date the start of the anthropocene, which I discussed here: smallprecautions.blogspot.com/2012/06/period… 3/

TL;DR: I argue the Anthropocene is coincident with the start of the Sixth Great Extinction. So when does that start? well, about 40k years ago, when the first humans arrive in the new world (Australia) and kill off a huge percentage of the megafauna on that continent. 4/

Same thing when humans arrive in the Americas: massive species die-off. More recently, industrialization has led to further & accelerating mass extinctions, from mechanized hunting & habitat destruction. (These days, a bird or mammal species is declared extinct almost weekly.) 5/

The latest twist is anthropogenic climate change, driven by humans burning most all of the fossilized sunlight they can get their hands on. This has only just begun but will certainly further accelerate the ongoing Sixth Great Extinction. 6/

One possible endgame: a 1000 years from now, the ice caps have melted, environmental destruction has killed virtually all the large non-monetozable beasts. Finally, the remaining humans, competing for the ecological scraps, screw up and precipitate a massive thermonuclear war. 7/

The result: humans go extinct, as well as numerous other species. In this scenario, looking at the Sixth Great Extinction start to finish, maybe 90% of the species that were present 50K BPE will have been killed. Oh well... evolution gets a pretty clean slate to start anew... 8/

Here’s where I would like to propose a thought experiment. Fast forward 66m years. Imagine some intelligent life form arrives (or re-evolves) on earth. It wants to know: what “caused” the sixth great extinction? What are they likely to conclude from the available evidence? 9/

Well, very likely, they will look at the geological evidence and say, “Well look, there’s clearly this layer of radioactive Uranium and Plutonium that is scattered all over the earth. Some terrible event must have happened suddenly that killed off all the megafauna! 10/

Some may speculate that it was a weird radioactive asteroid. Eventually maybe they’ll figure out that it was some effect of this self-destructive ancient civilization. But they’ll be pretty sure that “it was a sudden event.” 11/

But then there will be these dissenting scientists. They will say, hold on a minute, you nuclear catastrophists: if you look at this 66m year old rock, there’s this huge spike in atmospheric carbon right below the radioactive layer. What’s up with that? 12/

And they’ll say, if you look closely, the big die-off began quite a few tens of thousands of years BEFORE that nuclear coup de grâce. Maybe those crazy humans were already screwing things up... 13/

So what will be the truth of what “caused” the Sixth Great Extinction? Clearly it was anthropogenic. But was it humans eating all the megafauna? humans poisoning the atmosphere with carbon and throwing the climate off? Or humans with the radioactive weapons? 14/

The right answer of course is: all of the above. And the deep answer is that even the human capacity to do all that is arguably itself just an unintended result of the emergence of modern human cognitive capacities. So, what caused THAT? 15/

Well, here’s where it gets funny: no one really knows why “the behavioral revolution” happened. But one theory (usually dismissed as crank, for lack of solid evidence) is that an evolutionary bottleneck might have caused these traits to emerge. 16/

And funnily enough, shortly before the modern behavioral revolution takes place, around 50-60k BPE, there actually *was* a terrible population bottleneck. Genetic analysis suggests that around 70k BPE, the human population was reduced to as few as 5000 breeding pairs. 17/

And what caused that human population bottleneck? Well, funnily enough: the leading theory is that it was... yes, you guessed it, a massive volcanic eruption, the Toba Episode — the largest volcanic event in the last couple million years. 18/

And this makes a certain intuitive sense: massive 1000 year eruption causes huge ecological dislocation. Humans are pushed to the brink of extinction... but somehow, almost miraculously, a few thousand survive. 19/

Who are those few thousand that survive the terrible Darwinian pressure cooker? The answer, very likely, are the humans with the best planning, communication, and imaginative skills. 20/

Whether it is cultural innovation or an actual biological change, the humans that survive the Toba bottleneck emerge on the other side with what turn out to be wild new capabilities! In some crucial sense, there are altogether different sorts of human beings. 21/

These new humans begin by displacing some of their cousins: the Neanderthals, the Denisovans, the Florienses. They soon figure out that they can kill our outcompete virtually every beast on earth. 22/

This, then, is perhaps how the Sixth Great Extinction began: from a bizarre feedback effect of the Toba eruption — one that almost killed off humans but which made the surviving one subtly but crucially much stronger. 23/

Ever since then, the human population has, almost continuously, grown exponentially — eating its way through everything, appropriating resources in previously unimaginable ways, creating whole new elements that don’t appear anywhere else in the solar system 24/

But actually, despite humans’ great consciousness & technical capabilities, the so-called “anthropogenic” effects are actually just themselves the result of a feedback mechanism — a second order effect of a volcano that erupted 70K years ago in what’s now Indonesia. 25/

The Toba eruption, by spawning the #TransformationOfTheHuman known as behavioral modernity, lies at the root of the Sixth Great Extinction. Rather than a malign effect of human technical mastery, the #Anthropocene might be better conceived as a mere volcanic feedback effect. /26