Written in the genes

Two years ago, I wrote about a mysterious plunge in global average temperature that occurred 12,800 years ago.  It's nicknamed the "Younger Dryas event," after the tundra wildflower Dryas octopetala, which showed a population explosion over the following millennium (as judged by pollen in ice core samples).  This plant only flourishes when the winters are extremely cold, and the pollen spike, along with various other lines of evidence, supports a rapid drop in temperature averaging around six degrees Celsius worldwide.

[Image licensed under the Creative Commons xulescu_g, Dryas octopetala (41907904865), CC BY-SA 2.0]

The obvious question, of course, is what could cause such a rapid and catastrophic drop in temperature.  There are three reasonably plausible answers that have been suggested: 

1. an impact by a comet or meteorite causing an ejection of ash into the atmosphere, blocking sunlight
2. the collapse of an ice dam across what is now the St. Lawrence Seaway -- the temperature had been warming prior to the event -- allowing the emptying of an enormous freshwater lake into the North Atlantic, shutting off the thermohaline circulation and propelling the Northern Hemisphere back into an ice age
3. a nearby supernova in the constellation Vela frying the ozone layer, causing a collapse of ecosystems worldwide and an atmospheric chain reaction resulting in a global drop in temperature

The discussion amongst the scientists is ongoing, but the weight of evidence seems to favor the impact hypothesis.  (The link I posted above has more details, if you're curious.)

What's more certain is that the Younger Dryas event had a massive effect.  A number of large mammal groups -- including mastodons, North American camels, dire wolves, and gomphotheres (a bizarre-looking elephant relative) -- all went extinct shortly after the event itself, whatever it was, occurred.  Humans very nearly bit the dust, too; two of the dominant cultures of the time, the Natufian culture of the Middle East and the Clovis culture of North America, both collapsed right around the same time.

It's the latter that brings the topic up, because of some fascinating new research that came out last week, led by Paula Paz Sepúlveda of the Universidad Nacional de La Plata (Argentina), which looks at the effects this wild climate reversal had on the human genome.

What the researchers did was look at the makeup of the Q Y-DNA haplogroup.  You probably already know that two bits of our genome, the Y chromosome and the mitochondrial DNA, are frequently used for analyzing ethnic group affiliations because they don't recombine each generation -- they're passed down intact through (respectively) the paternal and maternal line.  So your mtDNA is the same as your mother's mother's mother's (etc.), and if you're male, your Y DNA is the same as your father's father's father's (etc.).  This means that the only differences in either one are due to mutations, making them invaluable as a measure of the degree of relatedness of different ethnic groups, not to mention providing a way to track patterns of human migration.

The Q haplogroup is ubiquitous in indigenous people of North and South America, so it was a good place to start looking for clues that the climate shift might have written into the human genome.  And they found them; coincident with the Younger Dryas event there was a marked drop in genetic diversity in the Q haplogroup.  It looks like the climate calamity caused a bottleneck -- a severe reduction in population, resulting in a loss of entire genetic lineages:

The YD impact hypothesis states that fragments of a large disintegrating asteroid/comet hit North America, South America, Europe, and Western Asia at 12,800 cal BP.  Multiple airbursts/impacts produced the YD boundary layer (YDB, Younger Dryas boundary), depositing peak concentrations of a wide variety of impact markers.  The proposed impact event caused major changes in continental drainage patterns, ocean circulation, in temperature and precipitation, large-scale biomass burning, abrupt climate change, abrupt anomalous distribution of plants and animals, extinction of megafauna, as well as, cultural changes and human population decline.  The diversity of the set of markers related to the cosmic impact is found mainly in the Northern hemisphere, including Venezuela, but they have also been recorded in the Southern hemisphere, in Chilean Patagonia, and Antarctica.

It's fascinating to think of our own genomes, and (of course) the genomes of other species, as being a kind of proxy record for climate; that not only gradual fluctuations, but sudden and unexpected events like impacts and volcanic eruptions, can leave their marks on our DNA.  It brings home once again how interlocked everything is.  Our old perception of humans as being some kind of independent entity, separate from everything else on Earth, is profoundly wrong.  We were molded into what we are today by the same forces that created the entire biosphere, and we can't separate ourselves from those forces any more than we could disconnect from our own heartbeats.  As Chief Seattle famously put it, "Man did not weave the web of life, he is merely a strand in it.  Whatever he does to the web, he does to himself."

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Analysis of a cold snap

Almost exactly 12,800 years ago -- and yes, we know it to that degree of accuracy -- there was a sudden plunge in the global temperature.

It's known as the "Younger Dryas" event, after the steppe wildflower Dryas octopetala, which only grows well when the conditions are very cold in winter.  The proxy records (bubbles in ice cores, patterns of glaciation, and types of pollen found in ice and sediments -- such as the aforementioned Dryas) are all in good agreement that in only ten or twenty years, the temperature in the Northern Hemisphere plunged, in some places by as much as 6 C.

[Image licensed under the Creative Commons Bjoertvedt, Dryas octopetala IMG 5641 reinrose reinsdyrflya, CC BY-SA 3.0]

That may not seem like very much, but six degrees is huge.  In fact, the word that comes to mind is "catastrophic."  The glaciers that had been receding -- this is, or at least was, an interglacial period -- suddenly began to extend their reach.  The cold period didn't abate for over a thousand years, with enormous impact on the humans around at the time.  The Younger Dryas correlates with the collapse of two of the dominant cultures, the Clovis civilization of North America and the Natufian culture of the Middle East.

But what could cause such a sudden and calamitous change in the temperature?

For years, the culprit was thought to be Lake Agassiz, a colossal freshwater lake that encompassed all five of the Great Lakes (and a lot more square milage as well), which was held back by an ice dam across what is now the Saint Lawrence Seaway.  As the temperature warmed -- remember, interglacial period -- the dam became unstable and finally collapsed, causing a humongous (I'm running out of words for "big," here) outflow of cold fresh water into the North Atlantic.  The result was a drastic slowing of the North Atlantic Meridional Turnover, which powers the Gulf Stream and keeps the Northeastern United States, Great Britain, Iceland, and Scandinavia at least reasonably warm.  The Turnover is caused by saline water (which is denser) sinking south of Iceland, and when the ice dam collapsed and the lake drained, the entire North Atlantic was covered by a sheet of water that was too fresh to sink.  The result: a slowdown of the circulation, and a return of glacial conditions.

Another, more far-fetched possibility is that the Earth got blasted by the shock wave of a supernova in the constellation Vela.  There is good evidence that the Vela supernova was coincident with the beginning of the Younger Dryas -- but connecting this to the drop in temperature is a bit of a stretch for most climatologists.

Recently, a third option has been gaining strength, and that's the fallout from the impact of a comet or meteor.  Here, the idea is that the debris thrown skyward by the impact blocked sunlight and caused a drop in temperature.  The impact hypothesis just got a boost last week with a paper in Scientific Reports, about a microanalysis of sediments from a place called Abu Hureyra that show good evidence of being flash-fried 12,800 years ago.

The sediments were collected decades ago, because the site itself was drowned when the Taqba Dam was put in place in 1970.  Archaeologists figured they better get what they needed from Abu Hureyra before the waters rose, and that included samples of everything they could get their hands on.  And an analysis by a team led by Andrew Moore of the Rochester Institute of Technology found that the bits of rock and other debris from the site dating to -- you guessed it, 12,800 years ago -- were coated with melted glass, indicative of a temperature of at least 2,200 C.

"To help with perspective," said James Kennett of University of California-Santa Barbara, who co-authored the paper, "such high temperatures would completely melt an automobile in less than a minute."

So the impact hypothesis is sounding more and more plausible.  What this kind of research always brings home for me, though, is how fragile the Earth's climate balance is.  Climate change deniers like to point out that there have been climatic ups and downs in the past, and the Earth has recovered; what they seldom add is that those ups and downs often resulted in mass extinctions.  So sure, the temperature rebounded after the Abu Hureyra collision.

A fat lot of good that did for the Pleistocene megafauna, such as mastodons, dire wolves, North American camels, and gomphotheres -- a bizarre North American elephant relative.  The humans didn't do much better; the ones who didn't get cooked and/or flattened by the impact very likely starved to death because of the mass die-off of plants and animals in the years following the collision.  How many made it through the bottleneck, and became our direct ancestors, is unknown, but it probably wasn't many.

So that's our cheery scientific discovery of the day.  A meteor impact triggering temperatures hot enough to melt glass, followed by a shower of debris and a drastic drop in global temperature.  I'd like to think this would be a cautionary tale, showing the effect one event can have on the climate, but at this point I know better.  We've pretended that everything is hunky-dory and ignored the scientists so far, so we'll be fine, right?

Of course right.

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This week's Skeptophilia book-of-the-week is brand new: Brian Greene's wonderful Until the End of Time.

Greene is that wonderful combination, a brilliant scientist and a lucid, gifted writer for the scientifically-inclined layperson.  He'd already knocked my socks off with his awesome The Elegant Universe and The Fabric of the Cosmos (the latter was made into an equally good four-part miniseries).

Greene doesn't shy away from difficult topics, tackling such subjects as relativity, quantum mechanics, and the nature of time.  Here, Greene takes on the biggest questions of all -- where the universe came from, how it has evolved and is evolving, and how it's going to end.

He begins with an observation that as a species, we're obsessed with the ideas of mortality and eternity, and -- likely unique amongst known animals -- spend a good part of our mental energy outside of "the now," pondering the arrow of time and what its implications are.  Greene takes a lens to this obsession from the standpoint of physics, looking at what we know and what we've inferred about the universe from its beginnings in the Big Bang to its ultimate silent demise in the "Heat Death" some billions or trillions of years in the future.

It's definitely a book that takes a wide focus, very likely the widest focus an author could take.  And in Greene's deft hands, it's a voyage through time you don't want to miss.

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