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."

****************************************

Insect rebound

I vividly recall my first visit to the American Museum of Natural History in Washington, DC, perhaps fifteen years ago.  Having a fascination for evolutionary biology and paleontology, I was thrilled to take a walk down the hallway with exhibits of each biological taxon, in phylogenetic order -- put simply, all the groups of living things in the order they come on the family tree of life.

So I'm walking up the hall, and things are progressing the way I'd expect -- bacteria to protozoans to plants to primitive animals, and within Kingdom Animalia, jellyfish to flatworms to roundworms to more complex invertebrates, and then on to fish, amphibians, reptiles, birds, and mammals.

But that wasn't the end of the hall.  The usual approach to the "Great Tree of Life" -- with, of course, mammals at the top of the heap and humans at the top of the mammals, as befits the pinnacle of evolution -- wasn't applied here.  If you progress past mammals, you're into Phylum Arthropoda, those animals with jointed legs and an exoskeleton, which include arachnids, crustaceans, centipedes, millipedes, and the most successful creatures on Earth...

... insects.

Being that it's the end of summer in upstate New York, I can verify that insects are highly successful life forms, given that there are millions of mosquitoes in my back yard alone, every single one of which divebombs my wife whenever she goes outside.  Something about Carol just attracts biting insects.  In fact, she claims that I bring her along to tropical destinations just to draw the mosquitoes away from me.

Which is not true.  Honestly.

In all seriousness, there is incredible diversity amongst insects, and many taxonomists believe that the number of insect species outnumbers all other kinds of animals put together.  Just beetles by themselves -- Order Coleoptera -- represents over 400,000 species, or about 25% of the total animal biodiversity on Earth.

This is the origin of the famous story about biologist J. B. S. Haldane, who was not only a vocal proponent of evolution but was an outspoken atheist.  Haldane frequently had hecklers show up at his talks, and one such asked him at the end, "So, Professor Haldane, what has your study of biology told you about the nature of God?"

Without missing a beat, Haldane replied, "All I can say is that he must have an inordinate fondness for beetles."

Metallic Shield Bug (Scutiphora pedicellata) from Australia [Image licensed under the Creative Commons Benjamint444, Metallic shield bug444, CC BY-SA 3.0]

It's curious that such a diverse and ubiquitous group still has a great many questions unresolved about its origins.  It's known that the big jump in insect diversity came after the Permian-Triassic Extinction of 252 million years ago, the "Great Dying" that wiped out (by some estimates) 95% of life on Earth.  There's a common pattern that a sudden burst of species formation always follows a mass extinction, but in this case, because of a poor fossil record following the event, it's been hard to connect later biodiversity to speciation amongst the survivors.

We just got a huge boost in what we know about insect evolution because of the discovery of a fossil deposit in China dating from 237 million years ago, or only ("only!") fifteen million years after the extinction itself.  The site had eight hundred fossils representing 28 different insect families that had survived the bottleneck, including the ancestors of modern beetles, flies, and cockroaches.

The study, done jointly by Zheng Daran and Wang Bo of the State Key Laboratory of Paleobiology and Stratigraphy in Nanjing, China and Chang Su-Chin of the University of Hong Kong, is only a preliminary analysis of the fossils at the site, and has already helped to connect the dots between pre-Permian-Triassic insects and more modern ones.  As Elizabeth Pennisi, senior correspondent for Science magazine, writes:

The sites underscore that this burst of evolution took place much earlier than researchers had thought, particularly for water-loving insects.  Among the remains are fossil dragonflies, caddisflies, water boatmen, and aquatic beetles.  Until now, paleontologists had thought such aquatic insects didn’t diversify until 130 million years ago.  These insects—which include both predators and plant eaters—helped make freshwater communities more complex and more productive... moving them toward the ecosystems we see today.

It's always fascinating when we add something to our knowledge of past life, and even more impressive when it's about one of the most diverse groups that has ever existed.  Seeing how life rebounded after the Permian-Triassic Extinction should also give us hope -- that even after a cataclysm, the survivors can still come back and rebuild Earth's biodiversity.

Or, as Ian Malcolm put it in Jurassic Park, "Life finds a way."

*********************************

This week's Skeptophilia book recommendation is part hard science, part the very human pursuit of truth.  In The Particle at the End of the Universe, physicist Sean Carroll writes about the studies and theoretical work that led to the discovery of the Higgs boson -- the particle Leon Lederman nicknamed "the God Particle" (which he later had cause to regret, causing him to quip that he should have named it "the goddamned particle").  The discovery required the teamwork of dozens of the best minds on Earth, and was finally vindicated when six years ago, a particle of exactly the characteristics Peter Higgs had described almost fifty years earlier was identified from data produced by the Large Hadron Collider.

Carroll's book is a wonderful look at how science is done, and how we have developed the ability to peer into the deepest secrets of the universe.

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

All in the family

Racists have cast about for years for some sort of scientific basis for their horrible worldview.  Evidence that their race is the superior one in intelligence, physical strength, or vigor, or simply support for their contention that interracial marriages are bad in a biological sense.

Of course, the problem for people who turn to science is that science often provides answers whether you end up liking them or not.  And inquiries into a biological basis for race have shown that any real genetic variations between different ethnic groups are tenuous at best.  Luigi Luca Cavalli-Sforza, one of the leading specialists in human population genetics, says:

Human races are still extremely unstable entities in the bands of modern taxonomists…  As one goes down the scale of the taxonomic hierarchy toward the lower and lower partitions, the boundaries between clusters become even less clear.  There is great genetic variation in all populations, even in small ones. 

From a scientific point of view, the concept of race has failed to obtain any consensus… the major stereotypes, all based on skin color, hair color and form, and facial traits, reflect superficial differences that are not confirmed by deeper analysis with more reliable genetic traits and whose origin dates from recent evolution mostly under the effect of climate and perhaps sexual selection.

Now, let me make it clear that this doesn't mean that there are no differences between racial groups.  It's just that those differences are primarily social and cultural, not biological, which neatly kicks the legs out from underneath some of the racists' primary arguments.

[image courtesy of the Wikimedia Commons]

And it's been known for years that lumping together all dark-skinned Africans as "black" is ignoring the fact that there's more genetic variability on the African continent than there is in the entire rest of the world put together.  The Zulu and the !Kung people of southern Africa, for example, are more distantly related to each other than a typical white American is...

... to a person from Japan.

And just last month, Iain Mathieson of Harvard University punched another hole in racist genetics when he released his research team's findings that the genes for white skin are only about 8,000 years old.

According to Mathieson et al.:

(M)odern humans who came out of Africa to originally settle Europe about 40,000 years are presumed to have had dark skin, which is advantageous in sunny latitudes.  And the new data confirm that about 8500 years ago, early hunter-gatherers in Spain, Luxembourg, and Hungary also had darker skin:  They lacked versions of two genes—SLC24A5 and SLC45A2—that lead to depigmentation and, therefore, pale skin in Europeans today... 

Then, the first farmers from the Near East arrived in Europe; they carried both genes for light skin. As they interbred with the indigenous hunter-gatherers, one of their light-skin genes swept through Europe, so that central and southern Europeans also began to have lighter skin.  The other gene variant, SLC45A2, was at low levels until about 5800 years ago when it swept up to high frequency.

The reason the two light-skin genes took hold in northern latitudes is thought to be vitamin D synthesis -- while having dark skin is an advantage in equatorial regions, from the standpoint of protection from ultraviolet skin damage, dark skin inhibits endogenous vitamin D production in areas with low incident sunlight.  So once the mutations occurred, they spread rapidly, but only in regions at high latitude.  This explains why even distantly-related equatorial groups have dark skin (such as the Bantu and the Australian Aborigines), and even distantly-related high-latitude group have light skin (such as the Swedes and the Inuit).

And apparently the gene for blue eyes is of equally recent vintage.  The earliest genetic evidence for the gene HERC2/OCA2, which causes blue eyes, is in southern Sweden from about 7,700 years ago.  The gene's provenance might date back to 10,000 years ago, but certainly not much before that.

So all of us descend from dark-skinned, brown-eyed people.  Sorry, white supremacists.

Of course, given that there is good evidence that around 70,000 years ago, an eruption of the Toba Volcano in Indonesia caused climate shifts that killed nearly all of our ancestors -- best estimates are that there were only 10,000 humans left on Earth after the bottleneck occurred -- we're all cousins anyway.  After that event, those 10,000-odd survivors can be put into two groups; the ones who left no descendants at all, and the ones who are the ancestors of everyone on Earth.

It'd be nice if we could count on people using science to inform their behavior, but we don't have a very good track record in that regard, do we?  I mean, think about it; we're still pushing the fossil fuel industry as the world warms up and the climate destabilizes around us.  So unfortunately, even when we have direct and incontrovertible evidence that what we're doing isn't reasonable, we usually continue doing it.

And I guess the argument that the genes for white skin are 8,000 years old is going to gain no traction whatsoever with the people who believe that the Earth is 6,000 years old.

But still, it'd be nice, wouldn't it?  Just as the first photographs of the Earth taken from the Moon changed a lot of folks' perspective on our place in the universe, it'd be wonderful if research like this could alter us from "those people... they're not like us" to "we're all one family, and we're all in this together."