Origins of life research, or etiobiology as I like to call it, is an odd discipline. It is rarely done under any direct funding, instead being done as a side effect of other, mostly molecular, biological research, especially since NASA shut down the bulk of its Astrobiology Program under massive budget cuts. A few researchers, however, make it a primary focus. One of these is the laboratory of Andrew Ellington, AKA "deaddog". Michael Robertson, a researcher in his lab at the University of California Santa Cruz has discovered the structure of enzymes that replicate RNA.
This is bigger than it sounds. In a test tube, nucleic acids just sit there and slowly denature. To do anything, they need to be able to be copied and expressed, and what does this are enzymes known as ribozymes. But how to get these things before you have RNA - which is thought to be the original genetic material under the RNA World Hypothesis? It seems like an insoluble chicken and egg problem. Robertson's work shows that you can get ribozymes to do the copying very early on.
Starting with a mixture of randomly synthesized RNA molecules and selecting for the desired properties, researchers are able to evolve RNA enzymes from scratch. In the Ellington lab, Robertson evolved the ligase ribozyme (called the L1 ligase) and determined which parts were critical for its function and which parts could be removed to create a "minimal construct."
Here's the link: RNA enzyme structure offers a glimpse into the origins of life from PhysOrg.com
Researchers at the University of California, Santa Cruz, have determined the three-dimensional structure of an RNA enzyme, or "ribozyme," that carries out a fundamental reaction required to make new RNA molecules. Their results provide insight into what may have been the first self-replicating molecule to arise billions of years ago on the evolutionary path toward the emergence of life. [...]
There are several competing hypotheses about the origins of life. The RNA World is one, but another is the so-called "Metabolism First" account championed by Robert Shapiro in a recent Scientific American article. According to this view, chemical reactions formed that generated self-sustaining reactions that divided into copies, probably via mechanical instability of the lipid membranes that surrounded them. Others think this is unlikely, because of the so-called "Error Catastrophe" identified by Eörs Szathmáry, a Hungarian theoretical evolutionary biologist. On this view, random variations in the samples of the chemistry contained in the new "cell" would end up nonfunctional without some sort of high fidelity inheritance to prevent it.
A third hypothesis is the "Clay First" hypothesis in which the complex structures of clay crystals act as the "anchor" of the protobiotic reactions, until genetic material evolves by selection. Moreover, clay may have encapsulated protobiotic reactions before cell membranes evolved.
I personally do not think that these are mutually exclusive hypotheses. No chemical reaction involving nucleic acids will not have had something akin to "metabolism" (in the sense of employing the latent energy of the source molecules and free energy of the environment, to maintain the reaction), but some sort of stable catalyst is needed to keep the reaction cycle from drifting into error. I do not think either came first, but rather they came together, and clays and other structured minerals may have had something direct to do with it. But knowing that a ribozyme can evolve very early on makes the crucial role of RNA more likely.
Comments
How very cool. And I agree that the various hypotheses are not mutually exclusive. After all, with an entire planet and vast expanses of time to play with, how could there not be a lot of different stuff going on in all sorts of places? I'm beginning to think that we may never figure out how life began on Earth, not because there are no good hypotheses, but because there are too many to choose from.
Posted by: arensb | March 18, 2007 2:04 PM