02/07/2003
Our Left-handed Heritage
I’ve just spent five minutes gazing at the palms of my two hands. I was struck by how similar they look, with the prominent lines in the palms matching much more closely than I had expected. Of course, those features that match are mirror images of each other. You might be wondering, has Bortrum finally flipped, wasting time looking at his two hands? Possibly, but mirror images and right- and left-handedness are subjects that have occupied many scientists over the years.
Obviously, right-handed individuals predominate in our general population. Just look at the right-hand placement of the writing surfaces on virtually all one type of school desks. How did most of us humans come to be right-handed? I have no idea; my concern here is not for human handedness. Molecules can also be right- or left-handed. Last week we discussed how fatty acids could be “cis” or “trans”, with the same chemical formula and structure except for a couple of hydrogen atoms in different positions. Other compounds can also have the same chemical compositions and structures but still differ in that one is the mirror image of the other. One is left-handed, the other right- handed.
Important examples of such compounds are certain amino acids. Let’s not worry ourselves about the chemical formulas of these acids. However, we should recognize that amino acids are the stuff of life, the compounds that we living critters use to make proteins. Proteins are the wave of the future now that the DNA has been mapped for ourselves and various other creatures. As we’ve mentioned before, now comes the hard part, correlating all those genes and other segments of DNA with the proteins they produce or control. But that’s a problem for the future. Let’s go back to the past. Regular readers will know that I can’t resist anything that has to do with our origins.
An article titled “Astrochemistry” by Joe Alper in the Winter 2003 issue of the American Chemical Society publication Chemistry prompted this column. Contrary to the preponderance of right-handedness in our human population, amino acids that Nature and our bodies use to synthesize proteins are overwhelmingly left-handed. Why is Nature left-handed? Where did the amino acids come from? When?
After the earth was formed some 4.6 billion years ago, it was a hellish place, under bombardment from all kinds of meteors and such. One impact apparently was sufficiently large so as to make us a moon. Amazingly, evidence suggest that some form of single-celled life was present around 4 billion years ago. It seems that this primitive life appeared a mere hundred million years or so after the bombardment of Earth from outer space settled down to a calmer pace. In that geologically short time, both the chemicals and the life had to be formed. But what if those amino acids and other chemicals didn’t have to be formed on Earth but were deposited from space? This is the subject of Alper’s article.
The universe had been around for roughly 10 billion years when Earth was formed. Huge numbers of galaxies had formed and zillions of stars had blown up, spewing out heavy elements like carbon, oxygen and nitrogen, not to mention iron and other metallic elements. All the elements were there to form the organic compounds needed for life. Indeed, with all our powerful telescopes and sensitive detectors, astronomers have identified scads of chemical compounds out there in dust clouds, comet tails, etc. It doesn’t seem far-fetched that our earth might have been the recipient of gobs of chemicals in those early days of its existence.
But wait, some of you may recall the experiment back in 1953 that was the subject of an earlier column. This was the famous experiment of Stanley Miller and Harold Urey that shook up the biological and other scientific communities half a century ago. Miller and Urey thought that early Earth was devoid of any molecular oxygen, a “reducing” atmosphere, and that the atmosphere contained compounds such as methane, water, hydrogen and ammonia. Miller set up a closed vessel containing these materials and electrodes. He then sparked the mixture repeatedly to simulate lightning. Sure enough, amino acids were formed. Later, ultraviolet light was shown to give the same effect as the “lightning”. This ingenious experiment was taken as proof that the necessary chemicals for life to arise could have formed on the earth itself.
Since Miller’s experiment, the composition of the early earth’s atmosphere has come into question. Instead of being “reducing”, with no molecular oxygen, it appears that there could have been molecular oxygen after all. The Alper article cites a NASA chemist, Joseph Guth, as saying the Miller-Urey chemistry would not have happened on an earth having no reducing atmosphere. But there was another problem with that experiment. It didn’t seem left-handed enough. The amino acids that Miller produced were about equally divided between right- and left-handed versions. If the amino acids were equally handed, how did Nature become left-handed? Miller thinks that in the course of evolution the left-handed amino acids were selected more or less by chance in the early life forms. Once formed, this left-handedness was propagated down the years.
But what if the amino acids came from space and they were predominantly left-handed? There would be no need to invoke a chance evolutionary origin of Nature’s lefty bent. Fortunately, unless you’re hit by one, we’re still being bombarded by meteorites, albeit much less frequently than in the early days. Just three years ago, a meteorite broke up over British Columbia, the pieces falling in a frozen environment and, fortuitously, some were recovered within a week from frozen ice. The finder was an amateur geologist who knew enough not to handle the ice chunks with his bare hands. The find provided a rare opportunity to look at pristine samples of a visitor from outer space.
Sandra Pizzarello of Arizona State University leads a team studying this meteorite material and they found a bonanza of biologically important compounds. They proved the compounds were not of earthly origin by analyzing the ratios of different carbon isotopes. And, what do you know, left-handed amino acids predominate! One of the compounds found in the meteorite was nicotinic acid. I have this vision of some otherworldly creatures out there on another planet trying desperately to give up smoking.
It seems that the left-handed molecules that give us life may indeed have come from the stars. But, lacking absolutely convincing evidence, the question remains - did lefty molecules from space drive evolution or, as Miller thinks, did evolution drive the lefty molecules? Personally, I prefer the space-driven scenario but I’m sure it will be a controversial subject for many years.
Sadly, something other than meteorites have fallen from the sky this past week. The Columbia disaster shows us once more how special are those individuals who enthusiastically and joyfully ride those rockets into space. As President Bush told the astronauts’ children, they can forever be proud of their parents.
Allen F. Bortrum
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