“In his autobiography, the brilliant physicist John Archibald Wheeler makes this confession of faith: "Whatever can be, is." He goes further: "Whatever can be, must be." Anything not prohibited by the laws of nature, exists, he says. Well, that's an extravagent claim, but it passed my mind once when my colleague Maura T. invited me up to the Science Building to see a strange creature she had just added to the aquarium.
Some years ago, in a Globe column, I mentioned Dr. Seuss's Grickily Gractus, a bird "that lays eggs on a cactus," as an example of a wildly improbable creature. A reader then sent me a photograph of a bird on the island of Bonaire perched - where else? - in a nest on a cactus. Apparently not even Dr. Seuss can think up a creature too odd to exist.
A biologist friend who had returned from a field trip to the upper Rio Negro in Brazil told me about a school of Curimata fish, in their tens of thousands, that passed under his boat, filling the water and air with a "metallic buzz saw sound" (caused, my friend discovered, by the stridulation of the fish's air-filled swim bladder by a bone). Singing fish! Not even Seuss could think of that.
Cactus-laying birds and singing fish are fine lessons in the diversity of life on Earth. Every niche in every habitat is filled. And for every creature alive today, a thousand others, even more improbable because they are less familiar, have lived before and become extinct. All life on Earth is related by common descent, and there has not been enough time to exhaust the possibilities. Nor are terrestrial habitats infinite in number. Still, the astonishing diversity of life bears witness to the truth of Wheeler's conjecture, at least in broad outline.
The chemistry of carbon-based life presumably applies throughout the universe, and the universe presents us with the prospect of hundreds of billions of galaxies, chockablock with stars and planets. The number of worlds, and therefore habitats, is unimaginably large, perhaps infinite. Who is willing to bet against any possibility in all that vastness?"