A light on bias

A woman walks into the kitchen to find her husband on all fours, crawling around peering at the floor.

"What are you doing?" she asks.

"Looking for my contact lens."

"Oh, I'll help."  So the woman gets down on the floor, too, and they spend the next fifteen minutes fruitlessly searching for the missing lens.  Finally, she says, "I just don't see it.  Are you sure you dropped it in here?"

The husband responds, "Oh, no, I dropped it in the living room."

"Then why the hell are you looking for it in the kitchen?" she yells at him.

"Because the lighting is better in here."

While this is an old and much-retold joke, there's an object lesson here for scientists -- which was highlighted by a paper this week out of George Washington University that appeared in Nature Ecology & Evolution.  In it, paleobiologists Andrew Barr and Bernard Wood considered a systematic sampling bias in our study of fossils of ancestral hominid species -- and by extension, every other group of fossils out there.

A large share of what we know of our own early family tree comes from just three sites in Africa, most notably the East African Rift Valley and adjacent regions in Ethiopia, Kenya, and Tanzania.  Clearly that's not the only place early hominids lived; it's just the place that (1) has late Cenozoic-age fossil-bearing strata exposed near the surface, and (2) isn't underneath a city or airport or swamp or rain forest or something.  In fact, the Rift Valley makes up only one percent of Africa's surface area, so searching only there is significantly biasing what we might find.

[Image licensed under the Creative Commons Michal Huniewicz, Great Rift Valley - panoramio, CC BY 3.0]

"Because the evidence of early human evolution comes from a small range of sites, it's important to acknowledge that we don't have a complete picture of what happened across the entire continent," said study co-author Andrew Barr.  "If we can point to the ways in which the fossil record is systematically biased and not a perfect representation of everything, then we can adjust our interpretations by taking this into account."

You can only base your understanding on what evidence you actually have in your hands, of course; besides the areas that might bear fossils but are inaccessible to study for one reason or another, there are parts of Africa where the strata are from a different geological era, or simply don't contain fossils at all (for example, igneous rock).  But you still need to maintain an awareness that what you're seeing is an incomplete picture.

"We must avoid falling into the trap of coming up with what looks like a comprehensive reconstruction of the human story, when we know we don't have all of the relevant evidence," said study co-author Bernard Wood.  "Imagine trying to capture the social and economic complexity of Washington D.C. if you only had access to information from one neighborhood.  It helps if you can get a sense of how much information is missing."

Now, don't misunderstand me (or them); no one is saying what we have to date is likely to be all wrong.  I absolutely hate when some new fossil is discovered, and the headlines say, "New Find Rewrites Everything We Knew" or "The Textbooks Are Wrong Again" or, worst of all, "Scientists Are Forced Back To The Drawing Board."  For one thing, our models are now solid enough that it's unlikely that anything will force a complete undoing of the known science.  I suppose something like that could occur in newer fields like cosmology and quantum physics, but even there we have tons of evidence and excellent predictive models -- so while there might well be additions or revisions, a complete overturning is almost certainly not gonna happen.  

Second, as astrophysicist Neil deGrasse Tyson put it, "As scientists, we're always at the drawing board.  If you're not at the drawing board, you're not doing science."  We are always exploring what he calls "the perimeter of our ignorance," testing and probing into the realms we have yet to explain fully.  What Barr and Wood are doing for the field of human paleobiology is to define that perimeter more clearly -- to identify where our inevitable sampling biases are, so that we can determine what direction to look next.  Not, like our hapless contact-lens-searchers, to continue to look in the same place just because the lighting happens to be better there.

Biases are unavoidable; everyone's got 'em.  The important thing is to be aware of them; they can't bite you on the ass if you keep your eye on them.  In science -- well, in everything, really -- it's good to remember the iconic line from physicist Richard Feynman: "The first principle is that you must not fool yourself; and you are the easiest person to fool."

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A beautiful hellscape

One of the strangest places on Earth is in eastern Ethiopia.  Not that many people have even heard of it, for the very good reason that if you go there, there are about a million and one ways you could die.

It's called Dallol, which comes from a word in the Afar language meaning "disintegration."  The name comes from the fact that this is what would happen to you if you went for a swim there.  It lies in the Danakil Depression, and is a maze of hot springs, filled with water that gets up to 95 C and can have a pH of less than 1.  It's surrounded by evaporite plains covered with layers of magnesium, calcium, and iron oxide, crystalline salt, and elemental sulfur.

The place doesn't even look real:

[Image licensed under the Creative Commons Kotopoulou Electra, The hydrothermal system of Dallol, CC BY-SA 4.0]

The highest elevation in Dallol is 48 meters below sea level.  The region gets ridiculously hot -- think Death Valley in midsummer -- so tourism, even if you were so inclined, is pretty much out of the question.

Where it gets even more interesting is why, if the place is entirely below sea level, it's not under water.  And this has to do with the geology of the region, and how it was created in the first place.

Dallol and the Danakil Depression are part of the East African Rift System, which formed in the Miocene Epoch on the order of fourteen million years ago.  Basaltic magma upwelling from the mantle created a crack in the Earth's crust and began to fracture the African Plate.  This generated a long rift valley running more-or-less northeast to southwest, from the shore of the Red Sea in Ethiopia, under Lake Victoria, then southward through Tanzania and all the way to Malawi.  The entire thing is seismically active, but the north end especially so, experiencing nearly constant earthquakes and volcanic eruptions -- not to mention a huge amount of hydrothermal activity, such as you see at Dallol.

The water of the Red Sea is currently being held back by the barrier of the Erta Ale Range, which blocks the East African Rift Valley on its northeastern end.  Eventually, though, the barrier will be breached as the rift continues to open up, and the water will come pouring in.  At that point, all of Dallol and the Danakil Depression -- and a large part of the rest of the valley -- will be an inlet of the Indian Ocean.

That won't stop the rift from continuing to spread, though.  The entire "Horn of Africa" will separate from the rest of the continent and go sliding off to the east.  As I've pointed out before, it's only our short life spans that make us think the current configuration of continents is permanent.

For now, though, the Erta Ale Range is holding the ocean back, allowing us to take a look at one of the most inhospitable places on Earth.  What I find most curious is that a part of this same system of rifts -- farther south, in Kenya and Tanzania -- is thought to be the cradle of humanity.  Much of the history of our earliest ancestors, species like Paranthropus and Australopithecus and Ardipithecus, took place here.  Somehow they dealt with the heat and drought and seismic activity (as well as the predators), surviving long enough to evolve into Homo sapiens, who then pretty much rushed out and took over the whole planet.

Odd to think that a beautiful hellscape was where humanity first got its start.

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What doesn't kill you

In evolutionary biology, it's always a little risky to attribute a feature to a specific selective pressure.

Why, for example, do humans have upright posture, unique amongst primates?  Three suggestions are:

• a more upright posture allowed for longer sight distance, both for seeing predators and potential prey
• standing upright freed our hands to manipulate tools
• our ancestors mostly lived by the shores of lakes, and an ability to wade while walking upright gave us access to the food-rich shallows along the edge

So which is it?  Possibly all three, and other reasons as well.  Evolution rarely is pushed in a particular pressure by just one factor.  What's interesting in this case is that upright posture is a classic example of an evolutionary trade-off; whatever advantage it gave us, it also destabilized our lumbar spines, giving humans the most lower back problems of any mammal (with the possible exceptions of dachshunds and basset hounds, who hardly got their low-slung stature through natural selection).

Sometimes, though, there's a confluence of seeming cause and effect that is so suggestive it's hard to pass up as an explanation.  Consider, for example, the rationale outlined in the paper that appeared this week in Science Advances, called "Increased Ecological Resource Variability During a Critical Transition in Hominin Evolution," by a team led by Richard Potts, director of the Human Origins Program of the Smithsonian Institution.

What the paper looks at is an oddly abrupt leap in the technology used by our distant ancestors that occurred about four hundred thousand years ago.  Using artifacts collected at the famous archaeological site Olorgesailie (in Kenya), the researchers saw that after a stable period lasting seven hundred thousand years, during which the main weapons tech -- stone hand axes -- barely changed at all, our African forebears suddenly jumped ahead to smaller, more sophisticated weapons and tools.  Additionally, they began to engage in trade with groups in other areas, and the evidence is that this travel, interaction, and trade enriched the culture of hominin groups all over East Africa.  (If you have twenty minutes, check out the wonderful TED Talk by Matt Ridley called "When Ideas Have Sex" -- it's about the cross-fertilizing effects of trade on cultures, and is absolutely brilliant.)

Olorgesailie, Kenya, where our distant ancestors lived [Image licensed under the Creative Commons Rossignol Benoît, OlorgesailieLandscape1993, CC BY-SA 3.0]

So what caused this prehistoric Great Leap Forward?  The Potts et al. team found that it coincides exactly with a period of natural destabilization in the area -- a change in climate that caused what was a wet, fertile, humid subtropical forest to change into savanna, a rapid overturning of the mammalian megafauna in the region (undoubtedly because of the climate change), and a sudden increase in tectonic activity along the East African Rift Zone, a divergent fault underneath the eastern part of Africa that ultimately is going to rip the continent in two.

The result was a drastic decrease in resources such as food and fresh water, and a landscape where survival was a great deal more uncertain than it had been.  The researchers suggest -- and the evidence seems strong -- that the ecological shifts led directly to our ancestors' innovations and behavioral changes.  Put simply, to survive, we had to get more clever about it.

The authors write:

Although climate change is considered to have been a large-scale driver of African human evolution, landscape-scale shifts in ecological resources that may have shaped novel hominin adaptations are rarely investigated.  We use well-dated, high-resolution, drill-core datasets to understand ecological dynamics associated with a major adaptive transition in the archeological record ~24 km from the coring site.  Outcrops preserve evidence of the replacement of Acheulean by Middle Stone Age (MSA) technological, cognitive, and social innovations between 500 and 300 thousand years (ka) ago, contemporaneous with large-scale taxonomic and adaptive turnover in mammal herbivores.  Beginning ~400 ka ago, tectonic, hydrological, and ecological changes combined to disrupt a relatively stable resource base, prompting fluctuations of increasing magnitude in freshwater availability, grassland communities, and woody plant cover.  Interaction of these factors offers a resource-oriented hypothesis for the evolutionary success of MSA adaptations, which likely contributed to the ecological flexibility typical of Homo sapiens foragers.

So what didn't kill us did indeed make us stronger.  Or at least smarter.

Like I said, it's always thin ice to attribute an adaptation to a specific cause, but here, the climatic and tectonic shifts occurring at almost exactly the same time as the cultural ones seems far much to attribute to coincidence. 

And of course, what it makes me wonder is how the drastic climatic shifts we're forcing today by our own reckless behavior are going to reshape our species.  Because we're not somehow immune to evolutionary pressure; yes, we've eliminated a lot of the diseases and malnutrition that acted as selectors on our population in pre-technological times, but if we mess up the climate enough, we'll very quickly find ourselves staring down the barrel of natural selection once again.

Which won't be pleasant.  I'm pretty certain that whatever happens, we're not going extinct any time soon, but the ecological catastrophe we're increasingly seeming to be facing won't leave us unscathed.  I wonder what innovations and adaptations we'll end up with to help us cope?

My guess is whatever they are, they'll be even more drastic than the ones that occurred to our kin four hundred thousand years ago.

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