The Micro-Cauldron of Life?

May 30th, 2007 Placozoan

THE QUESTION of the origin of the first living organism has fascinated scientists for decades. Currently the RNA world hypothesis, which proposes RNA as the first self-reproducing molecule, is the most promising. One difficulty is that this hypothesis require a high concentration of simple organic molecules, which were probably present in low concentrations in the earliest oceans. Some suggest adsorption onto solid surfaces as one solution to this problem, now a new solution has been proposed.

There are some hypotheses that the earliest life evolved around hot springs in the ocean floor, which now harbor unusual eubacteria and archaea. These hot springs build up mineral mounds around them that are riddled with pores. Baaske and coworkers demonstrate that a simulated hydrothermal pore system under a thermal gradient (from hot spring to cold ocean) can concentrate small molecules including nucleotides up to 10⁸ times, a hundred times higher than needed for intermolecular interaction.

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Compounding Complexity Passively

May 18th, 2007 Placozoan

I THOROUGHLY AGREE with the statement “it is well known that most biologists abhor all things mathematical” given in the article that I will cover today. The author deals with population genetics, and speeds through some basic equations in a page and a half. I will certainly not be talking about any equations today, but will attempt to restate the concepts in this fascinating paper.

The author, Michael Lynch, rejects the idea that organismal complexity arose through an adaptive mechanism (I previously discussed another paper suggesting a nonadaptive mechanism here). Key in his paper is an analysis of the differences between prokaryote and eukaryote evolution.

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A New Theory of Genome Evolution

May 17th, 2007 Placozoan

VERTEBRATE GENOMES are composed of islands of GC-rich DNA surrounded by a sea of GC-poor DNA. The GC-rich and GC-poor regions, called isochores, can be subdivided into five classes. The majority of vertebrate genes are contained in two GC-rich isochores, with the remainder spread throughout the other isochores. When compared to the genomes of fish and amphibians, the gene-rich isochores of mammals and birds are comparatively enriched in GC, while reptiles’ isochores are intermediate.

This is of interest for several reasons. First, the mutational tendency is for conversion from GC to AT as cytosine, which is commonly methylated when in a CpG dinucleotide (C followed by G), deaminates to produce thymidine. Subsequent DNA repair can then convert the GT mismatch to an AT. An increase in GC content requires a reversal of this mutational bias. Secondly, this GC increase must have occurred independently twice in order to appear in both mammals and birds.

A variety of mechanisms have been suggested for the origin of isochores, both selectionist and neutral, but no consensus has yet been reached. Now a new model has been proposed.

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The Early Tetrapod Bites the Worm

May 1st, 2007 Placozoan

RECENT DISCOVERIES of primitive tetrapod fossils continue to shed light on the transition from sea to land (for a discussion of some recent developments see our discussion of Gogonasus). The discovery last year of the tetrapod-like fish Tiktaalik provided additional information on the development of bony, articulated forelimbs and changes in the structure of the mouth, including hints at the evolution of the first neck. In later tetrapods the culmination of these changes can be seen.

Certainly the evolution of legs was an important innovation, but perhaps more complicated were the changes that had to occur in the structure of the mouth in order to allow success on land. Once again we see the domino effect of evolutionary change. The transition from the use of pharyngeal and opercular pumping of water through gills to buccal and costal pumping of air through primitive lungs required the loss of the opercula and supporting structures. But this creates another difficulty for the transitional tetrapodomorphs. The fish that preceded the tetrapods captured prey by a suction mechanism, sealing their opercula closed and opening the mouth to draw prey in on a flood of water. Obviously any tetrapod attempting to use this method of feeding on land would be sorely disappointed. So how did biting evolve as a means of prey capture?

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