Worm burrows

After other seemingly complex trace fossils have been identified as either xenophyophores or pseudofossils, there still remains a fair number of distinctive 'worm' burrows (Figure 10.6). For example, Nenoxites trails are found on top surfaces with an apparently pelleted wall, while Palaeophycus commonly is preserved as ahypichnial furrow. The positive epirelief of Aulichnites (Figure 10.6) indicates active backfilling, but the median furrow and angular bending would better fit a short, possibly...

Sources of amphiphilic molecules

The most striking examples of self-assembling molecules are called amphiphiles, because they have both a hydrophilic ('water loving') group and a hydrophobic ('water hating') group on the same molecule. Amphiphilic molecules are among the simplest of life's molecular components, and are readily synthesized by non-biological processes. Virtually any hydrocarbon having ten or more carbons in its chain takes on amphiphilic properties if one end of the molecule incorporates a polar or ionic group....

The permeable crust conditions for a Europan biosphere

The two major terrain-forming processes on Europa are (1) melt-through, creating chaos, and (2) tectonic processes of cracking and subsequent ridge formation, dilation, and strike-slip. These two major processes have continually destroyed preexisting terrain, depending on whether local or regional heat concentration was adequate for large-scale melt-through or small enough for refreezing and continuation of tectonism. The processes that create both chaotic and tectonic terrains (and probably...

How to use this book a users manual

The chapters of this book are derived from invited, one-hour review talks, as well as a few invited shorter talks, given at the conference. These constitute an outstanding set of lectures delivered by masters of fields such as planetary science, evolutionary biology, and the interdisciplinary links between the two. One of our major goals was to create a volume that would be useful for teaching an interdisciplinary audience at the level of senior undergraduate or junior graduate students. Our...

History

The spots in question were observed on images made by the Mars orbiter camera (MOC) on board the Mars Global Surveyor (MGS) spacecraft between 1998 and 1999 (images are credited to NASA JPL Malin Space Science Systems). These features appear in the southern and northern polar regions of the planet in the spring, and range in diameter from a few dozen to a few hundred metres. Malin and Edgett published their first observation on the Internet in 1999, and then in print (Malin and Planetary...

The emergence of primitive cells

Life on the Earth most likely arose from vast numbers of natural experiments in which various combinations of organic molecules were mixed and recombined to form complex interacting systems, then exposed to sources of energy such as light, heat, and oxidation-reduction potentials presented by donors and acceptors of electrons. This mixing and recombination probably did not occur in free solution, but rather in fluctuating environments at aqueous-mineral interfaces exposed to the atmosphere...

PH limits for life

The acidity of a solution, referred to as pH, quantifies the activity of hydrogen ions. In dilute solutions, the activity is approximately equal to the numeric value of the concentration of the H+ ion, denoted as H+ . Thus, Seawater (currently) has a pH of 8.2. This means that it has a H+ of 10-8 2 mol l-1, or approximately 6.31 x 10-9 mol l-1. While the pH scale is thought of as ranging from 0 to 14, a pH below 0 or above 14 is possible. For example, acid mine run-off can have a pH of -3.6,...

Protoplanetary Disks Orion Nebula

PRC95-45b ST Scl OPO November 20, 1995 M. J. McCaughrean (MPIA), C. Ft. O'Dell (Rice University), NASA Fig. 4.1. Hubble images of four protoplanetary disks in the Orion Nebula. These are seen as silhouettes against the bright background of the nebula. clustered star formation. They are also illuminated by ultraviolet radiation as well as X-rays from the central star, and this drives the production of organic molecules (as we discuss in the next section). Studies of the frequency of disks in...

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...

Protoplanetary disks and the formation of biomolecules

Well over 100 molecules, many of them organic, have been identified in interstellar gas. The clumps and cores in molecular clouds out of which stars form have densities ranging from 104 cm-3 to 108 cm-3 and low temperatures. In this state, cold gas chemistry can account for the formation of simple molecules (CO, N2, O2, C2H2, C2H4, and HCN). The surfaces of dust grains play host to the formation of more complex organic molecules, which include nitriles, aldehydes, alcohols, acids, ethers,...

The formation of planetesimals

In the first phase of terrestrial planet formation, solids condense out of the protostellar gas disk as dust grains, settle to the disk midplane, and eventually form kilometre-sized planetesimals. The actual mechanism of planetesimal formation remains unclear. Goldreich and Ward (1973) put forward a model in which the dust layer becomes sufficiently thin that it undergoes gravitational instability, contracting (in the terrestrial region) into kilometre-sized clumps and thus producing...

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...

Tides

The activity evidenced on Europa's surface is driven by tides. Tides generate periodic global stresses that crack and displace surface crust, as the thin shell of ice must accommodate to the changing global shape of the ocean below it. Moreover, tidal friction provides the dominant internal heat source, which keeps most of the thick water layer in the liquid state, as well as allowing occasional local or regional melting of the ice crust, either all the way through or just varying the ice...

The Ediacaran world

The terminal period of the Proterozoic, called Ediacaran after a locality in the Flinders Ranges of South Australia or Vendian in the Russian terminology , marks the first appearance of undoubted macrofossils. Because they are seemingly complex and because they have been studied mainly by palaeozoologists , all Ediacaran macrofossils were interpreted originally as early multicellular animals metazoans . The title of Martin Glaessner's 1984 monumental book, The Dawn of Animal Life, expresses...

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...

References

Bapteste, E., and Brochier, C. 2004 . On the conceptual difficulties in rooting the tree of life. Trends Microbiol., 12, 9-13. Bapteste, E., Boucher, Y., Leigh, J., and Doolittle, W.F. 2004 . Phylogenetic reconstruction and lateral gene transfer. Trends Microbiol., 12, 406-411. Beiko, R. J., Harlow, T. J., and Ragan, M.A. 2005 . Highways of gene sharing in prokaryotes. Proc. Natl. Acad. Sci. USA, 102, 14332-14337. Brown, J. R., and Doolittle, W. F. 1995 . Root of the universal tree of life...

Vendobionts as giant protozoans

As mentioned above, the view that the largest, most common and most diversified organisms of the Ediacaran biota were giant protozoans rather than metazoans was based mainly on their allometric compartmentation. Partly overturned fronds show that quilting patterns were identical on upper and lower surfaces. The penetration of two infaunal Pteridinium specimens by another individual and their growth responses to this accident Figure 10.1 are hard to reconcile with metazoans. So are the resting...

Environmental extremes Temperature

Why should temperature make a difference to organisms Organisms rely on biomolecules, all of which will break down at some temperature. For example, chlorophyll degrades near 75 C Rothschild and Mancinelli, 2001 . The melting temperature of nucleic acids depends on the base composition A-T bonds are less thermostable than are G-C bonds , the length of the polymer, and the solvent including the concentrations of ions in solution, but even DNA normally will degrade well below 100 C. Table 6.2....

What is an extremophile

The word 'extremophile', coined by Bob MacElroy 1974 from the Latin 'extremus' and the Greek 'philos', literally means 'lover of extremes'. Unfortunately, even the definition is not as clear as one might think. Extreme is usually taken to refer to physical, chemical, or, rarely, biological conditions. But what is extreme There is an objective definition of extreme, owing to the fact that life on Earth is based on organic carbon with water as a solvent. Thus, conditions that disrupt the...

Phylogenetic implications

Hot volcanic environments are among the oldest biotopes existing on Earth. What is known about the phylogenetic relationships of the organisms living there Based on the pioneering work of Carl Woese, the small subunit ribosomal RNA ss-rRNA is widely used in phylogenetic studies of prokaryotes Woese and Fox, 1977 Woese et al., 1990 . It consists of approximately 1500 nucleotides and is homologous to its eukaryotic counterpart. On the basis of sequence comparisons, a phylogenetic tree is now...

Protein evolution

The previous section envisages early proteins composed of a small set of amino acids. Several studies have considered whether such proteins could be functional. Using only three amino acids Gln, Leu, and Arg , proteins with strong helical structure were found Davidson et al., 1995 . Doi et al. 2005 found that random sequences of the five most abundant early amino acids according to our ranking, Gly, Ala, Asp, Glu, and Val, were more soluble than random sequences of 20 amino acids or Gln-Leu-Arg...