Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Saturday, September 2, 2023

The bottleneck

When I was young, I was very much attracted to stories where things worked out because they were fated to happen that way.

It explains why so many of my favorite books and movies back then were Hero's Journey stories -- The Lord of the Rings, The Chronicles of Prydain, A Wrinkle in Time, Star Wars.  The idea that there's a reason things happen -- that life isn't just chaotic -- is seductive.  (And, of course, it's a major theme in most religions; so many of them have some version of "God has a plan.")

Appealing as this is, my view now is more like the conclusion Brother Juniper comes to by the end of Thornton Wilder's brilliant and devastating novel The Bridge of San Luis Rey -- that either God's plan is so subtle the human mind can't fathom it, or else there is no plan.  In my sixty-two years on this planet, most of what I've seen is much less like some orderly pattern than it is like a giant pinball game.

This seems to be true not only in the realm of human affairs, but in the natural world as well.  There are overall guiding principles (such as evolution by natural selection), but much of what happens isn't destined, it's contingent.  Even such basic things as our bilaterally symmetric body plans with paired organs, and our having five digits on each appendage, seem to be the result of what amount to evolutionary accidents.  (Which is why, if we're ever lucky enough to contact alien life, it is extremely unlikely to be humanoid.)

Another chaotic factor is introduced by random geological and astronomical occurrences -- the eruption of the Siberian Traps, for example, that kicked off the cataclysmic Permian-Triassic Extinction, and the Chicxulub Meteorite collision that took out (amongst many other groups) the non-avian dinosaurs.  Each of those events radically altered the trajectory of life on Earth; what things would look like now, had either or both of these not occurred, can only be vaguely guessed at.

It's a little humbling to think of all of the different ways things could have happened.  Most of which, it must be said, would result in Homo sapiens never evolving.  And researchers have just identified one more near miss on nonexistence our species had -- a colossal genetic bottleneck around nine hundred thousand years ago, during which our entire ancestral population appears to have dwindled to around thirteen hundred breeding individuals.

[Image licensed under the Creative Commons Jerónimo Roure Pérez, Homo heidelbergensis. Museo de Prehistoria de Valencia, CC BY-SA 4.0]

Species like ourselves, that are slow to reach maturity, which have few offspring at a time and require lots of parental care -- ones that, in the parlance of ecological science, are called K-selected -- tend not to recover from events like this.  The precariousness of the situation is highlighted by evidence that the population didn't really bounce back for over a hundred thousand years.

We were teetering on the edge of oblivion for a long time.

Evidence for this bottleneck comes from two sources -- a drastic decrease in human remains in the fossil record, and strong genetic evidence that all modern humans today descend from an extremely restricted gene pool, a little less than a million years ago.  This event coincided with the onset of a period of glaciation, during which sea level dropped, ice coverage expanded from the polar regions, and there were widespread droughts.  These conditions destroyed all but a tiny remnant of the human population -- and those few survivors are the ancestors of all seven billion of us modern humans.

Populations this tiny are extremely vulnerable, and that they survived long enough to recover is downright astonishing.  "It’s an extraordinary length of time," said Chris Stringer, of the Natural History Museum of London, who was not involved in the study.  "It’s remarkable that we did get through at all.  For a population of that size, you just need one bad climate event, an epidemic, a volcanic eruption and you’re gone."

We made it through, though.  Somehow.  And I guess near-catastrophes like this don't really settle the issue of whether it was all Meant To Be.  You can just as well interpret our winding path from the origins of life four billion years ago, with all of the close calls and almost-wipeouts we survived, as coming from our being part of some Master Plan.  But to me, it seems more like the vagaries of a chaotic universe -- one where all of us, humans and non-human species alike, are walking a tightrope.  If you went back sixty-seven million years and looked around, you'd have seen no reason to believe that the dinosaurs would ever be anything but the dominant group on Earth, but in the blink of the eye geologically, they would all be gone.  It's a cautionary tale about our own fragility -- something we should take to heart, as we're the only species on Earth that has evolved the intelligence to see the long-term consequences of our own actions, and potentially, to forestall our own being toppled from our position of dominance.


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