EVIDENCE FOR ANCIENT EXTRATERRESTRIAL LIFE: WHAT TO LOOK FOR AND WHERE TO SEARCH FOR IT

It is somewhat ironic that renewed interest in the possibility of extraterrestrial life resulted from investigations of a meteorite (ALH84001) with an extended residence time on Earth. Given that Earth has been infected with life for as far back in time as the rock record extends, it would be almost impossible to confirm that any organic evidence for past life in a meteorite such as this was not derived from its environment of preservation. The best opportunity to search for life elsewhere will be to visit other planetary bodies, assuming that we can do so without contaminating their surfaces during landing and subsequent exploration and analysis. Previous investigations of extraterrestrial materials (e.g. meteorites, lunar soils) have focused much attention on the stereochemistry of amino acids. If amino acids are racemic, they are assumed to have formed via abiotic processes. Nonracemic amino acids (L-enantiomer excess) are assumed to reflect a terrestrial overprint. However, amino acids derived from once living organisms racemize with the passage of time, and a variety of abiotic pathways have been proposed to account for the occurrence of nonracemic amino acids in extraterrestrial materials. Phenylalanine, lysine, histidine and arginine are common protein amino acids that occur in all organisms on Earth, past and present. These amino acids have not been formed during laboratory simulation experiments and have never been documented as indigenous components of carbonaceous meteorites. Thus, their occurrence on an extraterrestrial body could be compelling evidence for the existence of life as we know it.