“You are not special,” the character Tyler Durden warns his followers in the movie Fight Club and in the namesake novel by Chuck Palahniuk. “You are not a beautiful or unique snowflake. You’re the same decaying organic matter as everything else.” Durden’s harsh but not inaccurate assessment lays the foundation for that story’s subsequent tumult. The same idea under the name the “Copernican principle” also happens to have been a linchpin of science for the past four centuries. (The first rule of the Copernican principle is, Do not talk about the Copernican principle, but....)

In 1543 Copernicus gave the establishment of his day a bloody nose by proposing that the best explanation for the observed motions of the stars and planets was to picture the sun, not Earth, as the center of known space. He had the prudent good sense to promptly die. Sixty years later the Vatican kayoed two astronomers who forced the point more aggressively: it burned Giordano Bruno at the stake and caged Galileo until he threw in the towel (while angling for a rematch with a mumbled “Eppur si muove”). Nevertheless, the facts were on the scientists’ side. Astronomers now develop their theories mindful that Earth most likely occupies an ordinary, unprivileged place in the cosmos.

So 11 years ago, when astronomers suddenly realized that the universe was not merely expanding but accelerating in its expansion, most of them concluded that some otherwise undetectable antigravity force,a “dark energy,” was shoving apart galaxies. An alternative possibility, however, can explain the observations as a fluke of cosmological geometry. It avoids invoking dark energy as an ad hoc cause but at the price of throwing out the Copernican principle: roughly speaking, it puts Earth, or at least our galaxy, back at the center of the observable universe. Timothy Clifton and Pedro G. Ferreira explore that idea in “Does Dark Energy Really Exist?

Even if the Copernican principle’s application to cosmology is subject to amendment, its application to other areas of science, notably biology, remains robustly well supported. (The second rule of the Copernican principle is, Do not talk about the Copernican principle...) It can nonetheless offend humans’ self-importance: witness creationists’ ongoing push-back against the evolutionary concept that people are simply another type of animal.

And yet biological evidence of our kinship with other creatures is everywhere we look. Gerald H. Jacobs and Jeremy Nathans reveal the literal truth of that statement in “The Evolution of Primate Color Vision”. Humans, apes and monkeys see a range of colors that other mammals do not; more tellingly, the genetic and biomolecular details of how humans and Old World primates (to whom we are most closely related) see color are different even from those of their New World cousins.

Our relatedness to other animals also leaves us with some common vulnerabilities. When an evolving viral disease hops a species barrier, it can sometimes cause horrific infections. Virologist Nathan Wolfe proposes in “Preventing the Next Pandemic” that health authorities monitor the status of animal diseases on the verge of leaping to humans. That fight is one we do not want to lose.

Editor's Note: This story was originally published with the title "Nothing Special"