daniel_c_dennett's picture
Philosopher; Austin B. Fletcher Professor of Philosophy, Co-Director, Center for Cognitive Studies, Tufts University; Author, From Bacteria to Bach and Back

Why Some Sea Turtles Migrate   

My choice is an explanation that delights me. It may be true and may be false—I don't know, but probably somebody who reads Edge will be able to say, authoritatively, with suitable references. I am eager to find out. I was told some years ago that the reason why some species of sea turtles migrate all the way across the South Atlantic to lay their eggs on the east coast of South America after mating on the west coast of Africa is that when the behavior started, Gondwanaland was just beginning to break apart (that would be between 130 and 110 million years ago), and these turtles were just swimming across the narrow strait to lay their eggs. Each year the swim was a little longer—maybe an inch or so—but who could notice that? Eventually they were crossing the ocean to lay their eggs, having no idea, of course, why they would do such an extravagant thing.

What is delicious about this example is that it vividly illustrates several important evolutionary themes: the staggering power over millions of years of change so gradual it is essentially unnoticeable, the cluelessness of much animal behavior, even when it is adaptive, and of course the eye-opening perspective that evolution by natural selection can offer to the imagination of the curious naturalist. It also demonstrates either the way evolutionary hypotheses can be roundly refuted by discoverable facts (if it is refuted) or the way those hypotheses can be supported by further evidence (if in fact it is so supported).

An attractive hypothesis, such as this, is the beginning, not the end, of the inquiry. Critics often deride evolutionary hypotheses about prehistoric events as "just-so stories," but as a blanket condemnation this charge should be rejected out of hand. Thousands of such hypotheses—first dreamt up on slender evidence—have been tested and confirmed beyond a reasonable doubt. Thousands of others have been tested and disconfirmed.  They were just-so stories until they weren't, in other words. That's the way science advances.

I have noticed that there is a pattern in the use of the "just-so story" charge: with almost no exceptions it is applied to hypotheses about human evolution. Nobody seems to object that we can't know enough about the selective environment leading to whales or flowers for us to hold forth so confidently about how and why whales and flowers evolved as they did.  So my rule of thumb is: if you see the "just-so story" epithet hurled, look for a political motive. You'll almost always find one. While it is no doubt true that some evolutionary psychologists have advanced hypotheses about human evolution for which there is still only slender supporting evidence, and while it is also no doubt true that some evolutionary psychologists have been less than diligent in seeking further evidence to confirm or disconfirm their favorite hypotheses, this is at most a criticism of the thoroughness of some researchers in the field, not a condemnation of their method or their hypotheses. The same could be said about many other topics in evolutionary biology.