ORIGIN OF LIFE 129 gested to contain fossil evidence of Martian life While many are unconvinced by these claims, the meteorite does provide evidence that material can be transferred from Mars to Earth with relatively good preservation, and suggests that something like a bacterial spore could seed life from one planet to another There is evidence that early Mars had liquid water and may have been at least as favourable as Earth as a site for the origin of life During the heavybombardment phase transfer of material from Mars to Earth would have been much more frequent than it is today Glossary ALH84001 Martian meteorite discovered in the Allan Hills ice-field in Antarctica in 1984 It has been suggested that this meteorite contains evidence of Martian microbial life Amino acid Simple organic molecule containing an amino group (NH2) and a carboxyl group (COOH) Amino acids link together in chains to form proteins Archaea One of three domains of life, the other two being the Bacteria and the Eucarya The archaea are prokaryotic organisms, which were shown to be distinct from the bacteria by molecular phylogeny using 16S ribosomal RNA The archaeal domain includes many extremophilic organisms Bacteria One of three domains of life, the other two being the Archaea and the Eucarya The bacterial domain includes all prokaryotic organisms not classified as archaea Base pairing (Watson–Crick base pairing) The pairing of the DNA and RNA bases through chemical bonds, which link the double-helix structure of DNA and allow the copying of information Adenine always pairs with thymine (or uracil), and cytosine pairs with guanine Bases The components of DNA and RNA that carry the genetic information in their sequence DNA uses adenine, guanine, cytosine, and thymine In RNA, thymine is replaced by uracil Chirality A chiral molecule is a molecule with an asymmetric structure that can exist in two mirrorimage forms (or enantiomers) In living organisms such molecules are usually found in only one of the two possible enantiomers (homochirality) Thus, amino acids are normally in the left-handed or L-enantiomer, while sugars are in the right-handed or D-enantiomer Cyanobacteria A class of bacteria that make use of oxygen-producing photosynthesis; commonly referred to as blue-green algae DNA Deoxyribonucleic acid; the molecule that constitutes the genome of living cells DNA molecules have a double-stranded helical structure built from a sugar–phosphate backbone and a set of four bases (adenine, guanine, cytosine, and thymine) The sequence of bases specifies the genetic information Domain The highest taxonomic division in the classification of living organisms The three domains are the Archaea, the Bacteria, and the Eucarya Domains are subdivided into kingdoms While the three-domain model is widely used in astrobiology, some biologists prefer other schemes, such as the five-kingdom system Enzyme A protein that acts as a catalyst Most chemical processes in living cells are enzyme catalysed Genetic code The set of rules by which three-letter ‘words’ in a DNA or RNA sequence describe an amino acid to be incorporated in a protein Genome The complete set of genetic information for a particular organism Heavy bombardment During the first few hundred million years after the formation of the solar system, the Earth and the other planets were subject to an intense bombardment by the debris left over from the formation of the solar system It is during this phase of heavy bombardment that most of the craters on the Moon were formed The emergence of life on Earth appears to coincide roughly with the end of the heavy bombardment Hyperthermophile An organism adapted to life at very high temperatures Hyperthermophiles have optimum growth temperatures above 80 C, and a number can grow at temperatures above 100 C Interstellar molecule Molecules in interstellar space are most commonly detected by means of radiofrequency emission lines coming from the gas in molecular clouds Molecules can also be detected from the infrared spectra emitted from dust Well over a hundred molecular species have been detected by these methods Some of the more complex molecules found include acetic acid, acetone, and ethanol Lateral gene transfer The transfer of genes between different species Lateral gene transfer may have been widespread in the early stages of life on Earth, and this complicates the interpretation of the tree of life LUCA Last universal common ancestor; the last common ancestor of all living organisms Martian meteorites Meteorites that originate from Mars, also known as SNC meteorites Their Martian origin is demonstrated by bubbles of gas trapped within them, which have identical composition to the atmosphere of Mars