The Why and When of Homochirality

Chiral molecules dominate organic chemistry. For example, if we consider the acyclic saturated aliphatic a-amino acids, we see that glycine, the only two-carbon member of the series, is achiral however, alanine, the C3 member of the series, is chiral one of the two C4 isomers is chiral all of the three C5 isomers (including valine) are chiral and six of the seven C6 isomers (including leucine and isoleucine) are chiral. In general, the more atoms there are in molecules, the more isomers there...

NaCa Na Ca

K-Na-Ca triangle for Archaean TTG (A) and modern continental crust (B). TTG plot in the trondhjemitic field (Tdh) whereas their modern equivalents are K-enriched and follow a classical calc-alkaline differentiation trend (CA). The field of Archaean TTG (Tdh) is from Martin (1994) RbBaTh U K TaNbLa Ce Sr Nd P Hf ZrSmTi Tb Yb Fig. 4.8. Primitive mantle normalized Archaean TTG (blue points) and modern continental crust (grey points) average compositions. Compared to primitive mantle, all...

Extreme Environments and Their Inhabitants 931 Extremophiles and Extremotolerants

It is beyond the scope of this review to decorticate in detail extremophiles and their natural environments. Table 9.1 summarises general features of ex-tremophilic and extremotolerant organisms their classification as a function of the physicochemical parameter considered, their most outstanding molecular adaptations, as well as some examples of species or groups of extremophiles and the habitats they colonise. Figures 9.3 to 9.9 illustrate different extreme environments. Temperature, pH,...

K

Seismic Profile Andean Fjord

Ne Ce Lid m Sm u Tb mi lift Nd SriEoGd Dy Lr Vbli Fig. 4.16. (A) K-Na-Ca triangle showing the trondhjemitic composition of liquids obtained by experimental melting of basalts. Blue field primitive continental crust Trondhjemitic (Tdh) and classical calc-alkaline (CA) modern juvenile crust, differentiation trends are also shown. (B) Chondrites normalized REE patterns for liquids generated by experimental basalt melting FSS tholeiite (Rapp et al., 1991) D15 low-K tholeiitic amphibolite (Wolf and...

Chronology from Radioactivity

For longer time scales, we resort to radioactivity studies. Radioactivities can be classified in two groups - short term radioactivities, (also called extinct radioactivities since the radioactive elements have totally decayed today) which, depending on their half-lifes (see Tab. 2.1), tell us about phenomena lasting from a few hundred thousand years to a few years. They allow a precise relative dating. - long term radioactivities are those for which the radioactive isotopes still exist, with...

Info

LiCt W MiGdTith' r bLi Lift M ioEiCJTtft Er Wj LiCt W MiGdTith' r bLi Lift M ioEiCJTtft Er Wj Fig. 4.12. Melting modelling for an Archaean tholeiite (basalt) metamorphosed into (A) garnet-free (G0) and (B) garnet-bearing (G25) amphibolite (Martin, 1987). Grey circles Archaean tholeiite Blue circles TTG Pale blue field domain of REE patterns for liquids generated by 10 to 50 melting of the source. (C) (La Yb)N vs. YbN diagram summarizing the different computed models of basalt melting. Source is...

Ozone as a Biomarker

In order to validate the astrobiological strategy of a Darwin TPF mission, it is necessary to estimate the risk of false-positive and false-negative occurrences. A false-positive case results from the detection of an abiotic feature that is wrongly attributed to some biological activity. On the other hand, a false-negative case occurs when an inhabited planet does not present any of the searched-for features, whether because the dominant metabolisms do not produce the expected bio-signature or...

Cometary Silicates

The nature of other refractory components is constrained by infrared observations of comets. Identification is achieved through a comparison of observations with laboratory spectra of different kinds of silicates and possibly carbonaceous grains, and it should be noted that there is no unique solution to the fitting problem. It appears that grains are made of a mixture of crystalline and amorphous silicate (Brucato et al., 1999, Crovisier et al., 1997, Harker et al., 2002, Hayward and Hanner...

Organic Molecules on the Primitive Earth

An exhaustive inventory of the origins of all the organic molecules occurring on the primitive Earth 4Gyr ago was carried out by Chyba and Sagan (1992). They distinguished three probable sources - Exogenous contribution from meteorites, micrometeorites, interplanetary dust particles (IDP), comets, etc. - Endogenous synthesis associated with impact of such bodies - Endogenous production associated with other available energy sources (solar UV, lightning, radioactivity, etc.) The total...

1

Compiled (La Yb)N ratios and YbN contents for Archaean TTG (blue points) and modern continental crust (grey points) from Martin (1986). A major change in continental crust composition clearly appears at the Archaean-Proterozoic boundary TTG generated prior to 2.5 Ga are characterized by low Yb contents (0.3 < YbN < 8.5) associated with strongly fractionated REE patterns (with high LaN YbN) (5 < (La Yb)N < 150) Continental crust younger than 2.5Ga has high Yb content (4.5 <...

Biogenic Isotopes

C isotope values for samples from the Early Archaean formations in Barber-ton and the Pilbara range from -14 o to -42 o (Hayes, 1983 Walsh, 1989 Strauss and Moore, 1992 Walsh and Lowe, 1999 Westall et al., 2001 Ueno et al., 2001 Brasier et al., 2002). Although these values (-14 o to -42 o) are consistent with a biologically produced signal, Hayes (1983) and Robert (1989) showed that post depositional processes could alter the original isotopic record. Moreover, Brasier et al. (2002) hypothesise...

Chirality Basic Concepts

I call any geometrical figure, or group of points, chiral, and say it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself. This definition of chirality was given by Kelvin in May 1893, at the time of a conference of the Oxford University Junior Scientific Club (Kelvin, 1904). The definition was illustrated by the following comment Two equal and similar right hands are homochirally similar. Equal and similar right and left hands are...

Elementary Prebiotic Chemistry in Aqueous Solution

The laboratory simulations of the early Earth are also important to understand the mechanisms by which biologically relevant molecules are made from the raw materials. These chemical pathways are generally referred to as prebiotic chemistry (Miller, Orgel, 1974 Raulin, 1990 Brack, 1998). Important in this regard are the roles of a limited number of key organic molecules such as nitriles (mainly HCN), and aldehydes (mainly formaldehyde). It is the chemical transformations of these compounds in...

A

The first steps of life evolution on Earth based on molecular biology data the transformation of a ribonucleotide into a deoxyribonucleotide and the appearance of DNA from RNA tion of enzymatic proteins necessary to deoxyribonucleotide polymerisation (reverse transcriptase and DNA polymerase), and the formation of deoxythymidine monophosphate (dTMP) starting from uracyl monophosphate (dUMP), leading to the replacement of uracyl by thymine in DNA. The fact that this transition occurred...

C

C (alpha) Carbon atom linked to a chemical function we are interested in. More specifically, the carbon atom directly linked to the carboxylic function in amino acids. In alpha amino acids, the amino group is linked to the alpha carbon. The Greek letters alpha, beta, gamma . . . are used to describe carbon atoms separated from the function by one, two, three other carbon atoms. CAI (Ca-Al Inclusions) Ca-and Al-rich inclusions abundant in some chon-drite meteorites. Caldera Circular km-sized...

Early Fractionation of the Earths Crust

For most of its history, at least since about 4.4-4.2Ga, it appears that the early Earth has been a water-covered planet. This inference was made on the basis of the existence of zircon crystals that formed 4.4-4.2Ga ago but now occur in younger sedimentary rocks in western Australia (Wilde et al., 2001 Mojzsis et al., 2001)1. Zircon crystals form by fractionation of magmatic crustal rocks that have been recycled (melted and fractionated) in the upper mantle by tectonic processes. Small amounts...

Ttg

Number Sio2 Diagram Rapp 1991

Classical calc-alkaline High (4.5 < YbN < 20) The second part of this chapter will try to discuss the reasons why hot subduction was prominent during Archaean and what was the terrestrial dynamics during this period. 4.2 Evolution and Dynamic of the Primitive Continental Crust 4.2.1 Introduction The Archaean Specificity The primitive Earth is not only characterized by specific processes of continental-crust genesis, but several other features make it different from modern Earth. Among these...

Archaea

Upper limits of life on Earth for the three major groups of cellular organisms Fig. 7.3. A character specific to hyperthermophiles the reverse gyrase. This enzyme, made up by the fusion of two 'classic' proteins, a helicase and a topoisomerase, transforms relaxed DNA into positive supercoiled DNA by the use of ATP energy. This enzyme might help to prevent denaturation of the double-helix structure at high temperature Fig. 7.3. A character specific to hyperthermophiles the reverse...

About the pH of Primitive Oceans 5231 Requirements from the Primary Pump

The pH of the aqueous phase (the primitive ocean) is a critical parameter of our scenario, especially for steps (1) and (5). Concerning step (1), the N-carbamoylation of amino acids occurs at any pH comprised between 5 and 12, with a maximum rate at pH 7-8. CAA are stable over the whole pH 5-12 range. Concerning step (5), the polycondensation of NCA in water has been studied from the middle 1950s. (Bartlett and Dittmer 1957 Bartlett and Jones 1957). Below pH 4, only NCA hydrolysis occurs. Above...

Looking for the Most Primitive Life Forms Pitfalls and Progresses

Simonetta Gribaldo, Patrick Forterre Two complementary approaches can be applied to try to understand how life appeared on our planet. The first, illustrated by Miller's seminal experiment, consists in trying to reproduce - either theoretically or and practically - the conditions of primitive Earth, and to figure out which possible scenarios might have led from such conditions to the apparition of life as we know it today. This is the type of approach frequently adopted by chemists and...

N

Nanobacteria Hypothetical bacteria, whose size could be around few nanometres, smaller than any known bacteria. Their existence is very much debated. N-carbamoyl-amino acids A molecule showing many similarities with amino-acids except that one of the H atoms of the amino group is substituted by the carbamoyl polyatomic group (-CO-NH2). N-carbamoyl-amino acids could have been prebiotic precursors of some amino acids. Neutral (mutations) (genetics) Term coming from the neutral theory of molecular...

Endogenous Organic Matter

The endogenous production of organic matter is unknown due to the absence of any fossil record. It must have greatly depended on the environment, especially on how reducing or oxidizing this environment was. Knowledge of the different sources of available energy and their respective contributions to the endogenous production of organic matter, has evolved over time. While solar UV has always been considered to have played an important quantitative role (Chyba and Sagan 1992) the origin of...

References

L., Schleicher, D. G., Osip, D. J., Birch, P. V. (1995). The ensemble properties of comets Results from narrowband pghotometry of 85 comets, 1976-1992. Icarus, 118, 223-270. A'Hearn, M.F., Boehnhardt, H., Kidger, M., West, R.M. eds (1999). Proceedings of the first international conference on Comet Hale-Bopp. Earth, Moon and Planets, 77, 78 & 79. Allamandola, L. J., Sandford, S. A., Valero,G. J. (1988). Photochemical and thermal evolution of interstellar precometary...

Primitive Atmosphere

The abundance of atmospheric nitrogen oxides (NOx) is a limiting factor for the efficiency of nitrosation and thus for the production of peptides through the primary pump. Unfortunately, the nature (pressure, temperature, composition) of the prebiotic atmosphere remains obscure in the absence of geological archives, preventing us from addressing quantitatively this point. Still, we can discuss the availability of NO considering the few constraints on the primitive terrestrial environment. The...

Formation of the Terrestrial Planets and Primordial Sculpting of the Asteroid Belt

After a few 105 years of runaway growth, the embryos in the terrestrial planet region and in the asteroid belt region have lunar to Martian masses. They govern the local dynamics and start perturbing each other. The system becomes unstable, and the embryos' orbits begin to intersect (Chambers and Wetherill, 1998). Because of mutual close encounters, the embryos' dynamical excitation (increase of eccentricity and inclination5 ) moderately increases, and accretional collision among embryos starts...

The Hertzsprung Russell Diagram Stellar Evolution at a Glance

Stellar Evolution Ttauri

The astronomers Einar Hertzsprung and Henry Norris Russell noticed very early on (towards the beginning of the 20th century) and independently, that stars tend to gather in specific parts of a diagram in which their luminosities are plotted as a function of their effective (surface) temperatures. Modern theory shows that stars evolve, and that they actually move as a function of time in such a diagram, widely referred to as the Hertzsprung-Russell diagram, or HR diagram for short. This...

Bibliography

Avalos M., Babiano R., Cintas P., Jimenez J.L., Palacios J.C. (2000). From parity to chirality Chemical implications revisited. Tetrahedron Asymmetry, 11, 2845-2874. Bouchiat M.A., Pottier L. (1984). An atomic preference between left and right. Scientific American, 76-86. Brack A. (July, 1998), L'asymetrie du vivant in Pour la Science - Dossier Hors Serie, Paris (France) Les symetries de la nature (this special issue contains many interesting articles for those who wish to integrate chirality...

Endogenous aAmino Acids

It was the historic experiment of Miller (Miller 1953) that showed that a-amino acids could have been formed on the primitive Earth from a reductive atmosphere. In the absence of reliable information on the composition of this atmosphere, the experiment was repeated on numerous occasions (Miller 1998) under different conditions, intended to simulate many of the different possible compositions CH4, NH3, H2, H2O , CH4, NH3, H2O , CH4, H2, N2, NH3, H2O , CH4, H2, N2, H2O , CO, H2, N2, H2O , CO2,...

Zoic

Synthetic geological time scale The Hadean extends from Earth accretion until the age of the oldest known rocks the 4.03-Ga Acasta gneisses. In other words, except for zircon crystals from Australia, direct geological memory for this 0.55-Ga long period does not exist. The Archaean-Proterozoic boundary corresponds to a period of transition in Earth dynamics, indeed present-day-like geodynamic processes started to operate since Proterozoic times (2.5Ga). These changes will be described...

Fate of Primary and Secondary Amines R3NH2 R3R4NH

Primary amines R3NH2 follow the same reaction pathways as ammonia, with some limitations. Indeed while they can take pathways B and C to form a-NR3-amino acids VIIId, they are blocked on pathway D by the impossibility of forming isocyanates Xb, so that hydantoins Xc substituted on nitrogen-1 are inaccessible. A roundabout way to obtain such substituted hydantoins is to react cyanate with a-amino acids-NR3 according to pathway F , then to dehydrate the NR3-carbamoylamino acids Xd thus obtained....