Unraveling Life's Blueprint: From Molecules to Evolutionary Origins
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DNA, the cell’s instruction manual and evolutionary memory, is fiendishly complex, but it has only four distinct components, called nucleotides. The complexity—and the information—in DNA come from the linear arrangement of these nucleotides along the molecule. Like the amino acids in proteins, nucleotides in DNA form the alphabet in which DNA’s information is encoded. Nucleotides, in turn, can be broken down into even simpler components: a sugar, a phosphate ion, and a simple organic molecule called a base. The bases can be synthesized from simple compounds likely to have been present when the Earth was young. Moreover, sugars can be generated from simple precursors like formaldehyde, also thought to have been present on the ancient Earth. And phosphate ions would have been supplied by the chemical weathering of volcanic rocks. Combining these components to form nucleotides challenged scientists for decades, but in 2009, British chemist John Sutherland and his colleagues generated two types of nucleotides under plausible early Earth conditions.
Finally, there are lipids, molecular constituents of the membranes that bound all cells. Like proteins and DNA, lipids are made of simpler units, in this case long, chain-like molecules called fatty acids, again likely generated chemically on the early Earth. Remarkably, if you splash or evaporate water that contains dispersed fatty acids, they spontaneously come together to form spheroidal microstructures that have a lot in common with the membranes that bound bacteria.
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So, the principal building blocks of life, the molecules from which our cells are built, can form from natural processes under conditions plausibly found locally if not ubiquitously in our planet’s infancy. It is important to underscore that this conclusion is not merely theoretical, or, for that matter, experimental. We know that reactions of the type just outlined did occur billions of years ago; their record is preserved in meteorites, those remarkable relics of our solar system in statu nascendi… The chemistry from which life emerged may be widespread in the universe. So far, so good, but from here things get trickier. We know that amino acids can combine to form short linear molecules called peptides. And nucleotides can do much the same. Function and memory seem to be imminent in such molecules, but in living organisms, DNA provides the molecular instructions for protein synthesis, and proteins are needed to replicate DNA. How do we escape the chicken-and-egg dilemma of which came first?
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The answer may be that neither DNA nor proteins were present in the first evolving protoorganisms. Since 1970, the known diversity of RNA molecules has expanded amazingly, as has their documented range of function. RNA stores information, like its cousin DNA, but some RNAs act like enzymes, doing the cell’s molecular work in a way once thought to be the exclusive domain of proteins. Also, small RNA molecules are now known to play a role in regulating gene expression within cells… Finally, recent experiments have shown that RNA molecules synthesized in the laboratory can evolve, shaped by selection to perform specific tasks. The discovery that RNA molecules can store information, function as enzymes, and evolve leads to a big thought: perhaps the earliest entities that reproduced and evolved were made of RNA, not DNA and proteins.
