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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The Spanish replacement

It's strange to think about, but there is a point in human history at which you can divide all of the inhabitants into two categories: those who left no living descendants at all, and those who are the ancestors of every single person alive today.

Anthropologists differ as to when that date is, but it's probably more recently than most of us would guess.  It certainly happened after the Toba bottleneck, a point about 74,000 years ago when there was a massive eruption of the Toba volcano (in the Indonesian archipelago) and a worldwide climate impact that may have reduced the entire population of ancestral humans to fewer than 7,000 total.  (Nota bene: scientists are still debating how big the bottleneck was, and whether it was the volcano that actually caused it; but I'm referring to the event by its most common nickname even so.)

What's cool is that with our current ability to do genetic analysis, we can narrow in on the answers to these sorts of questions.  Just last week, some research was published in Cell giving us an interesting lens into settlement patterns in Spain -- and that only 4,500 years ago, an influx of people from Eastern Europe and Russia resulted in the replacement of nearly all of the Y-chromosomal DNA that had been there for the previous forty thousand years.

This doesn't mean that the previous inhabitants left no descendants (although that could be true) -- all we can infer with certainty is that the men who had lived there prior to the invasion left very few patrilineal descendants.  As Y-chromosomal DNA is passed only father-to-son, any male descendants a man has through his daughters would share none of his Y-DNA.  (The same is true, but with the opposite genders, about mitochondrial DNA, which is only passed through the maternal line.)

In "Survival of Late Pleistocene Hunter-Gatherer Ancestry in the Iberian Peninsula," by Vanessa Villalba-Mouco, Marieke S. van de Loosdrecht. Cosimo Posth, Pilar Utrilla, Johannes Krause, and Wolfgang Haak, of the Max Planck Institute for the Science of Human History working with the University of Zaragoza, the authors write:

The Iberian Peninsula in southwestern Europe represents an important test case for the study of human population movements during prehistoric periods.  During the Last Glacial Maximum (LGM), the peninsula formed a periglacial refugium for hunter-gatherers (HGs) and thus served as a potential source for the re-peopling of northern latitudes...  Western and central Europe were dominated by ancestry associated with the ∼14,000-year-old individual from Villabruna, Italy, which had largely replaced earlier genetic ancestry, represented by 19,000–15,000-year-old individuals associated with the Magdalenian culture.  However, little is known about the genetic diversity in southern European refugia, the presence of distinct genetic clusters, and correspondence with geography.  Here, we report new genome-wide data from 11 HGs and Neolithic individuals that highlight the late survival of Paleolithic ancestry in Iberia, reported previously in Magdalenian-associated individuals.  We show that all Iberian HGs, including the oldest, a ∼19,000-year-old individual from El Mirón in Spain, carry dual ancestry from both Villabruna and the Magdalenian-related individuals.  Thus, our results suggest an early connection between two potential refugia, resulting in a genetic ancestry that survived in later Iberian HGs.  Our new genomic data from Iberian Early and Middle Neolithic individuals show that the dual Iberian HG genomic legacy pertains in the peninsula, suggesting that expanding farmers mixed with local HGs.

A different study, also published last week in the journal Science, added another piece to the puzzle.  "The Genomic History of the Iberian Peninsula Over the Past 8000 Years," by a team of scientists far too lengthy to list working at over a dozen research institutions, examined the DNA of 271 individuals and came to some fascinating conclusions about the settlement of Spain:

We assembled genome-wide data from 271 ancient Iberians, of whom 176 are from the largely unsampled period after 2000 BCE, thereby providing a high-resolution time transect of the Iberian Peninsula.  We document high genetic substructure between northwestern and southeastern hunter-gatherers before the spread of farming.  We reveal sporadic contacts between Iberia and North Africa by ~2500 BCE and, by ~2000 BCE, the replacement of 40% of Iberia’s ancestry and nearly 100% of its Y-chromosomes by people with Steppe ancestry...  Additionally, we document how, beginning at least in the Roman period, the ancestry of the peninsula was transformed by gene flow from North Africa and the eastern Mediterranean.

This influx pushed the Iberian farmers, who before had occupied the entire peninsula, into the mountainous northeastern parts of Spain -- and they are, apparently, the ancestors of today's Basque people, who are not only genetically distinct but who speak a language thought to be unrelated to any other existing language.  "The Basque country is a really difficult place to conquer; there are quotes from French rulers in medieval times saying that this is a nasty place to get in an army," said population geneticist Mattias Jakobsson of Uppsala University in Sweden, who was not part of either of the present studies.

Iñigo Olalde, a postdoc in the lab of population geneticist David Reich at Harvard Medical School in Boston, who is himself Basque, and who participated in the second study, agrees.  "The present-day Basques look like Iron Age people from Iberia," Olalde said.

What I find most fascinating about this is how we can use genetic analysis as a lens into a time period from which we have no written records at all, and make inferences about the movements of people who before had been entirely a mystery.  There's a lot we still don't know, of course, including how this genetic replacement took place.  "It would be a mistake to jump to the conclusion that Iberian men were killed or forcibly displaced," Olalde said, "as the archaeological record gives no clear evidence of a burst of violence in this period."

This opens up the potential for using this technique to study other time periods that are historical enigmas -- like the European "Dark Ages," between the Fall of Rome and consolidation of the Holy Roman Empire with the crowning of Charlemagne in 800 C.E.  Amazing that genetics, which tells us about who we are here and now, can also be seen as a history of where we came from -- a continuous record of information back to our earliest ancestors.

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This week's Skeptophilia book recommendation is a look at one of the most peculiar historical mysteries known: the unsolved puzzle of Kaspar Hauser.

In 1828, a sixteen-year-old boy walked into a military station in the city of Ansbach, Germany.  He was largely unable to communicate, but had a piece of paper that said he was being sent to join the cavalry -- and that if that wasn't possible, whoever was in charge should simply have him hanged.

The boy called himself Kaspar Hauser, and he was housed above the jail.  After months of coaxing and training, he became able to speak enough to tell a peculiar story.  He'd been kept captive, he said, in a small room where he was never allowed to see another human being.  He was fed by a man who sometimes talked to him through a slot in the door.  Sometimes, he said, the water he was given tasted bitter, and he would sleep soundly -- and wake up to find his hair and nails cut.

But locals began to question the story when it was found that Hauser was a pathological liar, and not to be trusted with anything.  No one was ever able to corroborate his story, and his death from a stab wound in 1833 in Ansbach was equally enigmatic -- he was found clutching a note that said he'd been killed so he couldn't identify his captor, who signed his name "M. L. O."  But from the angle of the wound, and the handwriting on the note, it seemed likely that both were the work of Hauser himself.

The mystery endures, and in the book Lost Prince: The Unsolved Mystery of Kaspar Hauser, author Jeffrey Moussaieff Masson looks at the various guesses that people have made to explain the boy's origins and bizarre death.  It makes for a fascinating read -- even if truthfully, we may never be certain of the actual explanation.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]