Intelligent space dinosaurs: How worried should we be?

An eminent chemist concluded an article in an academic journal with a fanciful note, positing the existence of advanced dinosaurs on other worlds. How plausible is his assertion? 

It happens to every writer: You've penned a nice article, checked your facts, and made sure all the commas are in the right place. Now you just need to come up with a punchy ending that ties it all together. 

For Columbia University chemist Ronald Breslow and his journal article about the origins of organic molecules on Earth, that ending invoked extraterrestrial dinosaurs. 

Professor Breslow, it should be noted, is no crackpot. He holds Columbia's highest academic rank, served as president of the American Chemical Society, is a recipient of the National Medal of Science and many other top honors, and is generally regarded as an all-around Eminent Scientist. 

But none of this stopped him from embarking on a flight of fancy in the last paragraph of his article for the Journal of the American Chemical Society titled "Evidence for the Likely Origin of Homochirality in Amino Acids, Sugars, and Nucleosides on Prebiotic Earth," which reads:

An implication from this work is that elsewhere in the universe there could be life forms based on D amino acids and L sugars, depending on the chirality of circular polarized light in that sector of the universe or whatever other process operated to favor the L α‐methyl amino acids in the meteorites that have landed on Earth. Such life forms could well be advanced versions of dinosaurs, if mammals did not have the good fortune to have the dinosaurs wiped out by an asteroidal collision, as on Earth. We would be better off not meeting them.

Breslow was writing about a topic that has long puzzled scientists. Pretty much all of the proteins, sugars, and genetic material on Earth exists in one of two possible orientations, or chiralities, as they are called. With a few small exceptions, all of the amino acids – the building blocks of proteins – are "left handed." Almost all sugars are "right handed."

Nobody knows why this is so. It's easy to imagine an Earth with right-handed amino acids and left-handed sugars. Indeed, organic molecules found in meteorites appear to contain a mix of right- and left-handed chiralities. Breslow is a proponent of the hypothesis that life on earth was "seeded" by one or more meteorites containing basic organic molecules, and he speculates that these molecules just happened to be left handed. As Breslow notes, it could very well be that carbon-based life on other planets, if it exists, could be made of right-handed proteins.  

It's possible for polarized ultraviolet light to degrade molecules of one chirality while leaving molecules of their mirror image intact. One conjecture, explored by Breslow, proposes that starlight striking magnetically polarized interstellar dust could have broken down the right-handed organic molecules on our primordial space rock, leaving the left-handed ones undamaged.  

Okay, but what to make of Breslow's terminal foray into astropaleontology? Could there really be a planet ruled by brainy velociraptors? 

It's unlikely, to say the least. When it comes to evolution, nothing is inevitable. If we were to somehow wind the clock back some 3.5 billion years or so, all the way back to the last common ancestor of all life on Earth, there's nothing that we know of in nature that guarantees the emergence of animals, much less dinosaurs. Or humans, for that matter.  

Still, the universe is a big place, and given enough habitable worlds, anything is possible. But even if a craft filled with spacefaring, molecularly inverted Tyrannosaurs were to touch down in Central Park tomorrow, it probably wouldn't be as bad as you might imagine. For as long as the alien dinosaurs' proteins are the mirror image of our own, they would be unable to metabolize Earthlings' flesh. Our planet would be to them like one of those rest stops that has a gas station and perhaps a nice view, but nothing to eat. Not even a vending machine.

In other words, we have nothing to worry about, at least not until the dark matter space wombats get here.