AN OVERVIEW OF THE ORIGIN OF LIFE THE CASE FOR BIOLOGICAL PROSPECTING ON MARS JULIAN A HISCOX School of Animal and Microbial Sciences PO Box 228 Reading RG6 6AJ UK E mail j a hiscox reading ac uk Abstract Studies of the Earth s earliest biosphere have suggested a close coupling between the evolution of early life forms and the physical and chemical evolution of the planetary surface From a biological perspective there were many similarities between early Earth and early Mars This has led to the idea that an origin of life event may have occurred on Mars leading to the development of microbial life Various theories have been advanced to explain the origin of life on Earth and these are reviewed with relevance to Mars If traces of past or present biogenic activity are to be found on Mars then the most likely place to prospect is several kilometers below the surface where liquid water might be stable Such prospecting may best lend itself to human exploration Keywords Biogenic traces exobiological prospecting human exploration origin of life Mars Raman spectroscopy robotic exploration 1 Introduction Whilst the way in which life evolves has been elucidated from the species to the molecular level the origin of life remains one of the most vexing issues in biology and philosophy Historically the question has been answered differently during different eras and at different stages of civilization We know it is almost certain that no steps towards the origin of life could have occurred on the Earth s surface until about 4 2 billion years ago after the Earth s crust formed and the amount of meteorite impacts had subsided so that the environment was no longer in constant upheaval Oberbeck and Mancinelli 1994 Schwartzman et al 1993 On the other hand the fossil record strongly supports the existence of cellular life 3 6 billion years ago Schopf and Parker 1987 and indirect evidence exists for biological activity as early as 3 8 to 4 billion years ago Mojzsis et al 1996 Schidlowski 1988 The time window for an origin of life event appears to be about 0 4 billion years Whilst molecular genetics and the fossil record have helped chart the evolution of modern day life Schopf 1989 Woese 1987 locating geological remains of the origin of life is probably impossible simply due to the reworking of the Earth s surface through plate tectonics and erosion How can we begin to investigate the origin of life Before asking what happened in this 0 4 billion year time window it is prudent to inquire what type of life resulted The oldest known remains of life appear Earth Moon and Planets 87 191 212 2001 2001 Kluwer Academic Publishers Printed in the Netherlands 192 JULIAN A HISCOX amazingly complex Presumably a molecule called deoxyribonucleic acid DNA as with all modern day organisms bar some viruses was used to store the genetic information DNA is composed of four chemical units called bases adenine A thymine T cytosine C and guanine G Like a computer program it is the sequential arrangement of these bases in groups of three that carries the genetic information Also these microfossils appear to be surrounded by some sort of membrane therefore everything inside them was enclosed More importantly because there are many similar bacteria all joined together this would imply that the bacteria were dividing replicating and therefore making daughter bacteria With this knowledge in hand the question of the origin of life now splits into five i How has the Earth remained continuously habitable for the past 4 billion years ii How did the building blocks of life arise iii How did they form into molecules that stored genetic information iv How did this information make copies of itself called replication v How did membranes evolve to encapsulate this information 2 Establishing the Initial Conditions from Which Life Emerged Habitable Zone Theory There is a close coupling between life and geological and atmospheric conditions McKay 1991 A planet such as the Earth must satisfy a number of conditions in order to support the evolution of life based on terrestrial paradigms It must have liquid water over a biologically significant period of time and other compounds including the so called CHNOPS elements These are carbon hydrogen nitrogen oxygen phosphorus and sulphur Liquid water not only acts as a medium for chemical reactions but is also an integral part of biological systems both at the structural and molecular level Therefore habitable planets and moons if they are to support surface liquid water must lie within an orbital zone that is thermally compatible with life where the average global temperature lies a little below the freezing point of water up to boiling point Kasting et al 1993 In the case of the Solar System the orbital zone compatible with surface liquid water the habitable zone HZ extends from just within the orbit of the Earth to just outside the orbit of Mars Earth has remained continuously habitable for at least 3 8 billion years despite a large increase in stellar luminosity with time The increase in solar flux was probably offset by a decrease in atmospheric carbon dioxide concentration caused by negative feedback in the carbon silicate geochemical cycle i e plate tectonics and vulcanism Kasting 1989 This same feedback mechanism implies that an Earth like planet could remain habitable out at least to the orbit of Mars Exceptions to planets as abodes for life are satellites orbiting giant planets within the HZ and satellites outside the HZ Williams et al 1997 but nevertheless contain internal stable bodies of liquid water Smith et al 1979 Squyres et al 1983 due to mechanisms such as tidal heating Reynolds et al 1987 PROSPECTING FOR BIOGENIC ACTIVITY ON MARS 193 3 Modern Theories for the Origin of Life How life may have originated on the Earth is unknown Two potential pathways are that life originated independently on Earth or that life was transported to the Earth from elsewhere This latter process is known as panspermia Arrhenius 1908 and it is possible that life i e microbes could have been transported from Mars to Earth or vice versa Melosh 1988 Moreno 1988 However whilst panspermia is feasible to explain the emergence of life on Earth Weber and Greenberg 1985 Horneck 1993 Parsons 1996 an origin of life event is still required to occur somewhere Modern theories of the origin of life on Earth in the most part reject the notion of panspermia and seek to explain how life could arise and evolved into what