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.
"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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