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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A refuge from the cold

I've always wondered how our distant ancestors survived during the various ice ages.

After all, we're mostly-hairless primates evolved on the warm, comfy African savanna, and it's hard to imagine how we coped with conditions like you often see depicted in books on early humans:

Le Moustier Neanderthals by Charles Knight (1920) [Image is in the Public Domain]

Despite the bear pelts around their nether regions, I've always wondered how they didn't all freeze to death.  When the weather's nice, bare skin is fine; I only wear a shirt during the summer under duress, and can't remember the last time I wore swim trunks when I went swimming in my pond.  But when the weather's cold -- which, here in upstate New York, is more often than not -- I'm usually wearing layers, and that's even indoors with our nice modern heating system.  Okay, admittedly I'm a wuss about the cold, but the fact remains that we're evolved to dwell in temperate regions.  Which, for a significant part of the Pleistocene Epoch, most of the world was not.

In particular, during the Last Glacial Maximum, between twenty-six and twenty thousand years ago, much of the Northern Hemisphere was experiencing a climate that the word "unpleasant" doesn't even begin to describe.  The average temperature was 6 C (11 F) colder than it is today, which was enough to cause ice sheets to spread across much of North America and northern Europe (where I currently sit, in fact, was underneath about thirty meters of ice).  Much of the non-glaciated land experienced not only dreadful cold, but long periods of drought.  The combined result is that the sea level was an estimated 130 meters lower than it is today, and broad dry valleys lay across what are now the bottoms of the Bering Sea, the North Sea and English Channel, and the Gulf of Carpentaria.

These conditions opened up passageways for some people, and closed off living space for others.  This was the time that the various pulses of immigrants crossed from Siberia through Beringia and into North America, where they became the ancestors of today's Indigenous Peoples of North and South America.  (If you want to read a brilliant account of how this happened, and some of the science behind how we know, you must read Jennifer Raff's wonderful book Origin: A Genetic History of the Americas.)  The same sort of thing happened from southeast Asia into what is now Australia.

In Europe, though, things got dicey to the point that it's a wonder anyone survived at all.  In fact, what brings this up is a study that appeared in Nature last week by a humongous team led by paleogeneticist Cosimo Posth of the Max Planck Institute of Evolutionary Anthropology.  The team did a complete genomic analysis of 356 individuals whose remains range from thirty-five thousand to five thousand years of age -- so right across that awful Last Glacial Maximum period -- to try to figure out how groups moved when the ice started coming in, and afterwards, once it retreated.

What they found was that only one part of Europe showed a consistent human genetic signature throughout the time period: the Iberian Peninsula.  What this indicates is that modern Spain and Portugal were a "climate refugium" during the worst of the glaciation, where people came to stay when the climate turned very cold, and pretty much stayed put.  Other areas that you might think were possible candidates for comparatively warm hideouts, such as what are now Italy and Greece, show a significant genomic shift across the Last Glacial Maximum, indicating that the people there before the cold set in either migrated or else died out, and were replaced by immigrants who moved in after things warmed and the area once again became more hospitable for humans.

"At that time, the climate warmed up quickly and considerably and forests spread across the European continent," said Johannes Krause, senior author of the study, in an interview with Science Daily.  "This may have prompted people from the south to expand their habitat.  The previous inhabitants may have migrated to the north as their habitat, the 'mammoth' steppe, dwindled,.  It is possible that the migration of early farmers into Europe triggered the retreat of hunter-gatherer populations to the northern edge of Europe.  At the same time, these two groups started mixing with each other, and continued to do so for around three thousand years."

Me, I'm curious what happened to these people afterward.  As a linguist, not to mention a white guy of western European descent, I've wondered if we're talking about my forebears, here -- and what languages they spoke.  My suspicion is that we're looking at the ancestors of today's Basques, who still live in northern Spain; they speak a non-Indo-European language that is usually considered a relic of the earliest languages spoken in Europe.  The Indo-European-speaking peoples (therefore the ancestors of the majority of today's Europeans) didn't reach Europe until about four thousand years ago, so long after the heyday of the people who were the subjects of the Posth et al. paper.

So you have to wonder who the descendants of these very early Europeans are.  "Not me" is my general assessment, considering my general cold-hardiness.  Drop me into an ice age where I had to live in a cave, hike on glaciers, hunt mammoth, and fend off cave bears, and I'd last maybe three days, tops.  I'm highly impressed by the ability of these ancient humans to survive, but given a choice I'll stick with my warm house, indoor plumbing, electric stove, and coffee maker.

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Not fair

Being fascinated with population genetics and human evolution, I was pretty excited to see that scientists have sequenced the DNA of "Cheddar Man," a 9,000-odd-year-old fossil human skeleton found in Cheddar Gorge, Somerset, England.  Cheddar Man, as the earliest human fossil from Britain, has been studied extensively, and it's been found that he was probably suffering from an infected wound on his head at the time he died -- but succumbed to a second injury before the infection could kill him.

Just last week, Ian Barnes of the British Natural History Museum released the results of the DNA analysis of Cheddar Man, and amongst the findings of his team were the fascinating results that he had genes coding for dark skin, curly hair, and blue eyes.

A reconstruction of Cheddar Man [image courtesy of the Wikimedia Commons]

The blue eyes were kind of a surprise to me, as blue eyes are a relatively new innovation.  Research has shown that all blue-eyed individuals descend from a single common ancestor in whom the mutation occurred, somewhere between six and ten thousand years ago.  This is why blue eyes are pretty well limited to people of European descent -- and brown-eyed people are a great deal more common, pretty much everywhere.

Despite the fact that we're talking about England, I wasn't surprised by the dark skin, as fair skin is also thought to be a fairly recent development.  It's connected with the presence of two mutated genes, SLC24A5 and SLC45A2, both of which first occurred around eight millennia ago, and which are presumed to have spread in northern latitudes because light skin confers an advantage with regards to vitamin D synthesis.  So it's not to be wondered at that an early human from 9,000 years ago would have dark skin.

But when I read this, I couldn't suppress a wince, and said, "Torrent of racist remarks in 3... 2... 1..."

I wish I could say I was wrong, but there was something of an explosion on social media when Barnes et al.'s results were announced.  I give you below a list as a sampler.  Grammar and spelling is as written, so I don't have to write "sic" five hundred times.

• Bullshit political agenda, if you believe this, you'll believe anything.
• Hello, What a load of bloody bollocks.  Next 'they' will be telling us that Jesus was also 'not white'.  Bloody vegans, feminists and gender neutralists at it again! trying to appease the ethnic minority!
• Yeah its attempt to undermine our identity.  He doesn't look African, if that's what they are implying -- its a European skull shape.  If they would release the DNA data people would be able to make their own conclusions on his skin tone.
• BS.  Just more blackwashing of our history.  My grandad was white and his grandad was white. And that's science fact.  Nuff said.
• It's okay, we don't believe the rubbish scientists' sprout.
• What a load of shit.  Serves a purpose though under current scheme of mass immigration.  Typical mainstream media lies.

All of which made me want to weep softly and pound my head on the desk.

Sure!  Let's not listen to the "rubbish scientists' sprout!"  Let's give these nimrods the actual DNA data so they can "come to their own conclusions," because people who use "my grandad [sic] was white" as proof of a scientific claim are clearly capable of doing a detailed genetic analysis!  Otherwise people will listen to the vegans, and we'll start thinking Jesus was from the Middle East or something!

When I see stuff like this, my first reaction is, "How did we get here?  Aren't we better than this?"  Of course, I know that the great likelihood is that racism overall is far less now than it was even fifty years ago, but the idea that people could freak out to this extent over the fact that one of their ancestors has been shown to have dark skin just appalls me.  It bears mention that the concept of race has little genetic meaning; it's primarily a cultural, not a biological, phenomenon.  (Consider, for example, that a Khoisan and a Bantu, living right next door to each other in South Africa, are more distantly related to each other than a typical Japanese is to a typical Caucasian American -- even though most people would put the Khoisan and the Bantu in the same race -- "Black" -- and consider the Japanese and the Caucasian to be in separate races.)

But people who are committed to the whole concept of racial superiority (whichever race they've decided is superior) are going to have their fragile and counter-factual version of reality shaken by the fact that all humans go back to the same, fairly small, group of people who came out of Africa 60,000 years ago or so.  And they almost certainly had dark skin, brown eyes, and black hair.

If that bothers you, well, tough.  Science is under no compulsion either to comfort you or to reinforce your biases.  Or, as my grandma used to put it:  "You can wish all you like, but wishin' don't make it so."