单选题
How living creatures evolve has been pretty well understood for the past 150 years. How they came to exist in the first place, though, remains a mystery. Part of the reason for this mystery is that subsequent evolution has done a good job of erasing the evidence. But not a complete one. Some features are shared by all organisms, and may thus go back to the beginning of life. And one of the most bizarre of these features is that a lot of the molecules of which life is made are left-handed.
A left-handed molecule is one that causes polarized light to rotate to the left (i.e., anticlockwise). Most molecules which behave this way have a right-handed equivalent that is, in its arrangement of atoms, their mirror image. Ordinary chemical processes cannot tell the difference between the two forms, so they are usually equally abundant. But the enzymes that govern biochemistry are such precise tools that, often, only one-handedness is acceptable. In the case of amino acids, the subunits of which proteins are made, the acceptable form is the sinister one. Many people feel that understanding why this is so would illuminate the origin of life—and two groups of researchers, pursuing separate lines of enquiry, have come up with what may be the pieces of the jigsaw.
One further puzzle is that the amino acids found in meteorites (which are assumed to be similar to those of the primitive Earth) have been modified by a process called methylation into a form that is biologically useless. Nevertheless, since such methylated amino acids are the starting point, it is where Ronald Breslow and his student Mindy Levine, who work at Columbia University, started.
A couple of years ago they revealed the first piece of the jigsaw when they found that an initial imbalance in favor of left-handed methylated amino acids in a solution can be amplified by repeated evaporation. During evaporation, the left-and right-handed molecules mate up and fall out of solution, leaving a left-handed excess. A mere two cycles of evaporation can push a starting ratio that is just 1% in favor of the left to one that is 90% left-handed.
Now, as Dr. Breslow has revealed to a meeting of the American Chemical Society, in New Orleans, Ms. Levine has discovered a process that favors the production of left-handed biologically active amino acids. The presence of copper in solutions that contain the chemical precursors of amino acids, together with left-handed methylated amino acids to seed the reaction, gives amino-acid formation a sinister bias. When Ms. Levine made an amino acid called phenylalanine this way she got 37% more of the left-handed form than the right-handed. With another, valine, the excess was 23% and with alanine, 20%.
The connection between the two pieces of work is that the left-handed methylated amino acids required to seed the second could have been provided by the evaporative process of the first—if, of course, a slightly biased supply of them had previously existed.
This is where Sandra Pizzarello of Arizona State University comes in. She has shown that the methylated amino acids found in meteorites do, indeed, have a bias of 1% or more in favor of the left-handed, suggesting that methylated amino acids kicking around on the primitive Earth would have shared a similar bias.
The mistake previous researchers made, therefore, was thinking of the methylated amino acids of meteorites as ingredients of life. Actually, if this work is pointing in the right direction, they were merely seeds. Taken together, these results argue that life formed in places with a lot of evaporation going on (suggesting heat) and a significant amount of copper present. This is speculation, of course, but it favors the idea that living things were created in land-locked ponds, rather than at sea, and probably in a volcanic environment. (Volcanic heat would drive the chemical reactions, as well as causing lots of evaporation.) It also suggests that biochemical left-handedness confers no selective advantage. What makes meteoritic amino acids left-handed has yet to be discovered. But it seems just a matter of chance that the living world is sinister.
单选题
What can be inferred from the results of the studies by Ronald Breslow and Mindy Levine?