DNA Evolution

Did Life Arise All at Once?

Researchers have attempted to reconstruct the tree of life by comparing the RNA sequences found in ribosomes (ribosomal RNA) in many diverse species of bacteria, archaebacteria, fungi, protozoans, animals and plants. Others have done the same by comparing the amino acid sequences of ancient proteins that are found in all living things. The results have been ambiguous.

       While few researchers have been bold enough to suggest that life arose all at once, the evidence from the reconstruction experiments suggest that it did. The tree as it is conventionally drawn in figure 13.1 suggests that bacteria diverged first, and then archaebacteria second. This means that archaebacteria are more closely related to man than bacteria. The drawing in figure 13.1 is based largely on the analysis of ribosomal RNA, but this is not consistent with the protein comparisons that show that archaebacteria and bacteria share many of the same genes that are not found in higher forms of life.¹

"If these two prokaryotic groups span the primary phylogenic divide and their genes are vertically (genealogically) inherited, then the universal ancestor must have had all of these genes, these many functions: This distribution of genes would make the ancestor a prototroph with a complete tricarboxylic acid cycle, polysaccharide metabolism, both sulfur oxidation and reduction, and nitrogen fixation; it was motile by means of flagella; it had a regulated cell cycle and more. This is not the simple ancestor, limited in metabolic capabilities, that biologist originally intuited. That ancestor can explain neither the the broad distribution of diverse metabolic functions nor the origin of early autotrophy implied by this distribution. The ancestor that this broad spectrum of metabolic genes demands is totipotent, a genetically rich and complex entity, as rich and complex as any modern cell -seemingly more so." - Carl Woese, The Universal Ancestor.

"For instance, transcription, translation and splicing machineries of the archaebacteria resemble those of the eukaryotes, while the majority of the functional genes, coding primarily for metabolic enzymes, transport systems and enzymes of cell wall biogenesis, resemble the eubacterial ones. Microbiologists have reviewed a number of possible explanations for this mosaic, but none of them seems to be, at the present time, particularly convincing." - Mayr, "Two empires of Three,"PNAS, 1998.

The ribosomal RNA data does not agree with the protein data. The ribosomal data suggest that the archaebacteria are more closely related to man than bacteria. Yet the proteins tell another story. Woese resolved the dilemma by suggesting that the common ancestor was not a single cell, but that instead a collection of many different very simple cells. None of these simple cells had the ability to live alone, but together by sharing and transferring genes, they managed. In essence, the primordial soup came alive. Instead of life emerging from the primordial soup, the soup itself was alive.

        This is an interesting idea, but it does not change the probability. One of the nice features about information is that it is additive. So 1 complex protein containing 500 bits of information has the same chance of evolving as 10 smaller proteins containing 50 bits each. It is far more likely that the common ancestor was a complete cell similar to life today.

        Mayr perhaps offered a better explanation by suggesting that life is composed of only two kingdoms not three. This of course still implies that the common ancestor to all life was not simple, but possessed a multitude of genes. It very well may have been more complex than any living cell found today.

         The difficulties associated with chemical evolution almost demand that the first living thing be robust and complex. No organism whose genetic structure is based on RNA or DNA can replicate itself unless it can synthesize the nucleic acids required to do so ( adenine, cytosine, guanine, thymine and uracil). And if ribose was used as the backbone in the first DNA/RNA, then it too must be synthesized. This implies photosynthesis and the Calvin cycle must also be present. The simplest living organisms today are parasitic. They obtain nourishment by absorbing nutrients from their host, and this process allows them the luxury of not having many genes that would otherwise be necessary. Given the dilute concentration of biological precursors in the soup (if it existed), the first living thing would not be able to rely on the soup for nourishment.

References:

4) Meyer, "The Origin of Bilogical Information and the Higher Taxonomic Categories," Proc. Of the Biological scociety of Washington, 117:213-239, 2004.

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