How pre-life chemicals may have become biologically significant
Chemists studying the rise of life on earth have penetrated a little deeper into its mystery. Research reported earlier this month shows how a common mineral could have sorted some of life's precursor chemicals into biologically significant groups. Other recent experiments demonstrate how the earliest life forms might have reproduced themselves without the help of DNA or proteins, two ingredients thought to be essential in the evolution of life.
Molecular chemist David Bartel notes that "we will never be able to prove" how primordial life actually worked "because we can't go back in time." But scientists can do the next best thing. They can study the basic properties of relevant biological chemicals and "see if these are compatible" with the scenarios scientists invent, he says.
Dr. Bartel and associates at the Whitehead Institute for Biomedical Research in Cambridge, Mass., are exploring reproduction in this way. Today, DNA molecules encode an organism's genetic instructions. The closely related chemical RNA reads those instructions and transfers the information to the chemical machinery that makes proteins. These, in turn, enable life processes to work. When living cells reproduce, protein catalysts help their DNA make new copies of itself. The early-life puzzle challenges scientists to explain how the first life forms reproduced when DNA and proteins had not yet emerged.
Chemists have suspected that RNA might both encode genetic instructions and promote its own replication. If so, then RNA alone would have been able to jump-start organic life. Now, the Bartel team has shown, for the first time, that RNA can indeed replicate itself. No natural form of RNA can do this today. As the team explained in the journal Science, they used a process that mimics natural evolution to create an artificial RNA to do the job. The result is a form of RNA that can carry out the reactions needed to synthesize its own building blocks and hook these together.
The team has not yet copied a complete RNA molecule. But it has copied enough of an RNA template to feel it is on the right track. Team members consider this some of the strongest evidence yet that RNA chemistry could have promoted early life. The early life puzzle also challenges scientists to explain how certain pre-life chemicals became biologically significant. Non-life chemistry produced many different organic materials some 4 billion years ago on earth. These would have included amino acids, some of which are building blocks of proteins today.
Amino acids come in two forms that are mirror images of each other. Chemists call them left-handed and right-handed molecules. Non-life chemistry produces these forms in equal amounts, but life chemistry uses mainly left-handed forms. The challenge for scientists is to explain how left-handed forms stood out in the primordial 50-50 mix.
Robert Hazen thinks minerals are the key. Dr. Hazen and Timothy Filley at the Carnegie Institution in Washington, and Glenn Goodfriend at nearby George Washington University recently described research in the Proceedings of the National Academy of Sciences that makes this point. They worked with calcite, a mineral that forms limestone and seashells. When immersed in a 50-50 mix of amino acids, calcite crystals preferentially segregated left-handed amino acids on one crystal face and right-handed acids on another face.
Again, this doesn't prove pre-life chemistry worked that way. But it does encourage Hazen to pursue his thesis.
(c) Copyright 2001. The Christian Science Monitor