The parasite connection

For most of us, endoparasites and pathogens have a negative connotation, and we think that the world would be better without them. In a less anthropocentric view, however, they appear to be essential for the long-term survival of all ecosystems. Like the arms race between predator and prey species at the trophic level, a constant race takes place between hosts and their parasites. But as we know, there is no ultimate winner, because most defence mechanisms (and drugs) work only at the level of...

The rise and fall ofArchezoa

As for Metazoa, advances in phylogenetic analyses have greatly modified our view of the eukaryotic tree. Originally, both morphological (Cavalier-Smith, 1987) and molecular studies (mainly based on rRNA) (Sogin, 1991) suggested that several simple lineages (e.g., devoid of mitochondrion, Golgi apparatus, or Fig. 8.6. Most common views of the universal tree of life. A. Schematic representation of Woese's paradigm. The root is located on the bacterial branch and the mitochondrial endosymbiosis...

The homology criterion

The reconstruction of the genealogical relationships of a set of species first requires the definition of a series of characters that are comparable across all studied organisms. This problem was solved in 1843 by the palaeontologist Richard Owen based on the principle of connectivity previously established by the anatomist Planetary Systems and the Origins of Life, eds. Ralph E. Pudritz, Paul G. Higgs, and Jonathon R. Stone. Published by Cambridge University Press. Cambridge University Press...

The nature of photosynthesis and the microbial consortium

Central to our hypothesis is the assumption that the primary productivity of this ecosystem rests on photosynthesis. Since water plays a central role, it is natural to assume that the source of reducing power is the photolysis of water. Such a process happens in cyanobacteria, for example, and oxygen is produced as a byproduct. Where is the oxygen then This objection could be very serious, since, according to Lovelock (1979), the best astronomical sign for life is an atmosphere out of...

How was the genetic code optimized

There is considerable evidence that the canonical code has evolved to minimize the effect of errors (Freeland et al., 2003). Mutations create errors in the gene sequence that are passed to the protein sequences translated from this gene. Translational errors due to codon-anticodon mispairing or mischarging of a tRNA will introduce occasional non-heritable errors into proteins. These errors are likely to involve only one of the three bases in the codon most of the time. Errors can be minimized...

The primary eclipse

The vast majority of the currently known extrasolar planets have been detected using the radial velocity technique.1 As detailed in the previous chapter, the radial velocity method searches for periodic motion of a star caused by the gravitational pull of an orbiting companion. Figure 1.1 shows a sketch of a typical periodic radial velocity signal and the basic geometry of the planetary system. This method is sensitive only to movement of the star towards and away from the observer, i.e., along...

A genome fusion at the origin of eukaryotes

Starting a century ago with Mereschkowski's symbiogenesis (1905), several genome fusion or cell fusion scenarios have been proposed to explain the origin of eukaryotes (Lopez-Garcia and Moreira, 1999 Margulis, 1971). While these scenarios were originally grounded in cell biology, they gained acceptance when the rise of sequencing techniques suggested a chimerical nature for the eukaryotic cell, i.e., a greater similarity of metabolic genes to Bacteria and of informational genes to Archaea. The...

The growth of planets

With the protoplanets (or 'oligarchs') now starved of planetesimals, the final stage of terrestrial planet formation requires them to agglomerate with one another. For this to happen, their orbits have to cross. This is referred to as the giant-impact (or chaotic) phase. We will begin with a description of how this unfolds in the 'standard model', and then touch on some more recent efforts to tie up the loose ends. Traditionally, N-body simulations have been the tool of choice for...

A complex prebioticlike chemistry

In the atmosphere of Titan, CH4 chemistry is coupled with N2 chemistry resulting in the formation of many organics - hydrocarbons and N-containing organic compounds - in the gas and particulate phases. These compounds are hydrocarbons, nitriles, and complex refractory organics. Several photochemical models describing the chemical and physical pathways involved in the chemical evolution of the atmosphere of Titan and the resulting vertical concentration profiles of the different molecules...

The growth of protoplanets

This is a necessary but not sufficient condition a would-be satellite approaching a planet too fast will simply pass through the Hill sphere with some deflection, rather than be captured. If the relative velocities between neighbouring planetesimals are dispersiondominated - i.e., the main contribution to their relative velocities is their mutual perturbations, rather than the difference in Keplerian speed between neighbouring orbits - their growth rate can be estimated using a simple 'particle...