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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Analysis of a triple-whammy

I always find it wryly amusing when I hear someone talk about the Strong Anthropic Principle -- the idea that the universe was fine-tuned by a creator to be hospitable to life, particularly human life -- because it doesn't take much of a close look to become aware of exactly how inhospitable the universe actually is.

Even here on Earth, the situation is kind of dicey.  Our planet's history is really one cataclysm after another; that our own particular lineage survived is probably more a matter of luck than anything else.  In fact, only 74,000 years ago, climate havoc caused by the eruption of the Indonesian volcano Toba is thought by some scientists to have nearly wiped out our species -- they estimate the "Toba bottleneck event" to have reduced the entire population of Homo sapiens to under ten thousand individuals.  (For reference, that means that the human population of the whole planet was about as many people as currently live in Sitka, Alaska.)  Besides major volcanic eruptions, add in earthquakes and tsunamis, global marine anoxia events, and meteor strikes, and you can see that where we live isn't nearly as hospitable as it might look at first.

So the truth lies closer to the Weak Anthropic Principle -- of course the universe, and the Earth in particular, have the conditions necessary for life to exist, because if they didn't, we wouldn't be here to consider the question.

The reason all this catastrophic stuff comes up is a paper that came out a couple of weeks ago in Nature Communications Biology.  Entitled, "Widespread Loss of Mammalian Lineage and Dietary Diversity in the Early Oligocene of Afro-Arabia," by Dorien de Vries (of the University of Salford), Steven Heritage (of Stony Brook University), and Matthew Borths, Hesham Sallam, and Erik Seiffert (of the Duke University Museum of Natural History), it looks at a little-known extinction event that was apparently the result of a one-two-three punch that nearly halted the mammalian species in Africa in their tracks.

Which, of course, also includes our own ancestors.

Some of the Eocene animal groups affected by the extinction -- the left two are primates, the upper right a hyaenodont, and the lower right a prehistoric rodent.  [Photo by Matthew Borths]

The researchers had their first clues about the magnitude of the event, which occurred on the order of thirty-three million years ago, from looking at fossilized mammal teeth from East Africa, and found that right on the boundary of the Eocene and Oligocene, the number of mammal species dropped drastically.

"In our anthropoid ancestors, diversity bottoms out to almost nothing around thirty million years ago, leaving them with a single tooth type,” said Erik Seiffert, senior co-author of the paper, in an interview with EurekAlert.  "That ancestral tooth shape determined what was possible in terms of later dietary diversification...  There's an interesting story about the role of that bottleneck in our own early evolutionary history.  We came pretty close to never existing, if our monkey-like ancestors had gone extinct thirty million years ago.  Luckily they didn’t."

What seems to have driven the extinction was a triple-whammy -- rapid cooling of the climate after the equally catastrophic Paleocene-Eocene Thermal Maximum (during which the global average temperature was eight degrees warmer than it is now), followed by the huge Chesapeake Bay meteorite collision and a near-simultaneous massive eruption of flood basalts in Ethiopia.

Diversity plummeted.  More interestingly, the extinction preferentially spared generalists -- mammals which, from their tooth morphology, apparently could eat a variety of different kinds of food.  Specialists got hit the hardest, once again reinforcing the general evolutionary concept that if the ecosystem is stable, specialization is a good thing, but during periods of rapid change, it's the generalists who come out on top.

This -- as Seiffert says, luckily for us -- left our own distant ancestors to continue loping about on the African savanna, and also knocked out a lot of the competition.  The shape and structure of our own teeth reflect that event.  "We lost a lot of diversity at the Eocene-Oligocene boundary," said study co-author Matthew Borths.  "But the species that survived apparently had enough of a toolkit to persist through this fluctuating climate."

This should be yet another cautionary note about the fragility of the Earth's suitability for humanity, not to mention our current heedless messing-about with the planetary climate and ecosystems.  It's unlikely to, of course.  We don't have a very good record of listening to scientists, even when our own survival is at stake.  Part of this is that in recent memory we haven't had to deal with planet-wide natural disasters -- it's made us cocky.

"We've always been fine before," is the general attitude.

All it takes is reading the de Vries et al. paper to realize that "always" is singularly inapt.

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My master's degree is in historical linguistics, with a focus on Scandinavia and Great Britain (and the interactions between them) -- so it was with great interest that I read Cat Jarman's book River Kings: A New History of Vikings from Scandinavia to the Silk Road.

Jarman, who is an archaeologist working for the University of Bristol and the Scandinavian Museum of Cultural History of the University of Oslo, is one of the world's experts on the Viking Age.  She does a great job of de-mythologizing these wide-traveling raiders, explorers, and merchants, taking them out of the caricature depictions of guys with blond braids and horned helmets into the reality of a complex, dynamic culture that impacted lands and people from Labrador to China.

River Kings is a brilliantly-written analysis of an often-misunderstood group -- beginning with the fact that "Viking" isn't an ethnic designation, but an occupation -- and tracing artifacts they left behind traveling between their homeland in Sweden, Norway, and Denmark to Iceland, the Hebrides, Normandy, the Silk Road, and Russia.  (In fact, the Rus -- the people who founded, and gave their name to, Russia -- were Scandinavian explorers who settled in what is now the Ukraine and western Russia, intermarrying with the Slavic population there and eventually forming a unique melded culture.)

If you are interested in the Vikings or in European history in general, you should put Jarman's book in your to-read list.  It goes a long way toward replacing the legendary status of these fierce, sea-going people with a historically-accurate reality that is just as fascinating.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]