THE ORIGINS OF LIFE
The Invisible Hand of Physics
By Davide Castelvecchi
Who needs nature to be so complicated? Why are there as many as four fundamental forces?
After all, virtually everything we experience is ruled by three forces only. Gravity keeps our feet on the ground; the electromagnetic force creates chemistry and keeps our molecules and bodies together; and the strong nuclear force keeps our atoms from blasting apart.
Meanwhile, the fourth force, the weak nuclear force, shows up mostly in certain kinds of radioactivity. (This year, scientists have even shown that it’s not unthinkable to have a universe that has no weak nuclear force at all, but is still roughly similar to our universe.)
But what if, instead, this underdog of physics had helped shape life as we know it? A controversial proposal came out at the very end of last year (but I only found out about it in 2006), when a team of chemists suggested that, through a subtle effect on the chemistry of water, the weak nuclear force could have caused one of the most significant and enigmatic properties of the molecules of life: the ability to distinguish left and right. That’s when the shape of a molecule is different than the shape of its mirror image (think of a screw, or the difference between you left and right hand).
In most cases when left and right matter, life tends to choose one kind of handedness versus the other, and to stick to it, mostly — a property known as chirality. For example, some bacteria have learned to live in our guts using in their membranes an amino acid with reversed chirality compared to the one our bodies are used to — and different than the one our enzymes are able to dissolve. (Penicillin breaks apart the bugs’ version of the amino acid, but not the more common one, and that’s how it kills bacteria while leaving our cells intact.)
Life’s chirality has long puzzled scientists. Some have suggested that it could have all started in space, where circularly polarized radiation would have selected organic molecules with a particular handedness, before those molecules fell on Earth and (hypothetically) served as the ingredients of life. Other scientists have posited that if the ancestor of all life forms lived on the surface of a crystal, and the crystal had a “handed” structure, that’s how life could have started out speaking one particular chemical language.
The research published late last year hints at a new, less fortuitous mechanism for introducing chirality. The weak nuclear force causes a subtle effect on the magnetic properties of water. Although it’s tiny, this effect has macroscopic consequences: It makes it harder to unfold molecules of left-handed (but not right-handed) polyglutamic acid, the scientists found. If this effect is confirmed and is shown to appear in other chemical reactions, it could explain why organisms have evolved with chiral preferences.
But chiral effects can often appear in the lab from subtle contaminations, skeptics say. As Robert Hazen, a mineralogist at the Carnegie Institutution of Washington and the author of the crystal-surface hypothesis of the origin of life, told me, “Even the neon lights in my lab can introduce a tiny chirality in glass, so my beakers could introduce a tiny bias in this kind of experiment.”
But Meir Shinitzky, of the Weizmann Institute of Science in Rehovot, Israel, and one of the authors of the study, says that the effect disappears when regular water is replaced with heavy water, i.e., water made with a heavier isotope of hydrogen. This is consistent with the fact that in heavy water the magnetc effects of the nuclear force are absent. Shinitzky says that this will end up convincing most skeptics. “It questions the basis of their criticism,” Shinitzky wrote in an email.