Everything was ready for the celebratory feast. Weeks earlier, the alien fleet had entered Earth’s orbit and made radio contact, and now the visitors would receive their official welcome. Dozens of heads of state would greet humanity’s guests during an official dinner at the White House.

The aliens looked remarkably similar to us—apart from their green scales, that is. Moreover, the chemistry of their bodies and of ours was also very similar, scientists from the two worlds had concluded. Sure, the alien cells produced molecules not found in human biochemistry, but the building blocks of those molecules were essentially the same carbon-based amino acids and sugars as the ones in our bodies.

But on the eve of the great meeting, the science adviser to the President burst into the Oval Office. “The dinner must be called off, or the aliens might die!” the adviser told the startled President. “We forgot to check their chirality!”

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WHAT’S WRONG WITH THIS PICTURE? (See illustrations below for the answer)

That’s a word that few politicians—indeed, few people outside science—will know. But chirality, or handedness, is an essential characteristic of the molecules of life. Most naturally occurring organic molecules are chiral, meaning that they are distinct from their mirror images in the same way that our right and left hands differ.

In the lab, chemical reactions that synthesize amino acids and sugars create the right- and left-handed versions of the molecules in equal amounts. Life on Earth, however, uses one version almost exclusively, preferring what are conventionally called right-handed sugars and left-handed amino acids. A molecular preference for only one handedness is what chemists call homochirality.

In principle, organisms could exist that use both kinds of molecules or that exclusively adopt the opposite forms from those used in life on Earth. This is what troubled the scientists in the alien state dinner scenario: Just as our bodies can’t absorb organic matter of the wrong handedness, so too the aliens might find our food equally indigestible—and perhaps even toxic.

Most scientists believe that Earth life’s choice of chemical handedness was purely random. “The most plausible idea is that it was an accident,” says biochemist David Deamer of the University of California, Santa Cruz. It’s possible, then, that the chemistry of some alien forms of carbon-based life—assuming such things exist—may well have the opposite chirality to ours. We might find alien pizza even harder to digest than deep-dish.

On the other hand, a few scientists say that something more fundamental might be going on. They argue that throughout the universe, nature might consistently choose one handedness over the other. That intrinsic preference, these scientists suggest, might originate from the influence of the weak nuclear force, the only fundamental force of nature that can tell left from right. In recent years, a number of experiments have provided tentative—if controversial—support for this proposal.

(Read the rest of my article “Alien Pizza, Anyone?” freely available on the Web site of Science News.)

This was the original 1953 photo of Francis Crick (pointing) and James Watson, as they contemplated a model of the 3D structure of DNA they figured out. All life forms on earth have DNA, which in normal conditions winds up in a right-handed double helix (called this way in analogy with a right-handed screw).

The DNA’s backbone is made of sugary molecules. Earth life forms use right-handed sugars; in principle, life forms could exist that use left handed sugars. DNA based on these molecules would wind around as a left-handed screw. If alien versions of Crick and Watson exist somewhere in the universe, they may have been wondering why the DNA they discovered was all left handed.

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