First Life

The origin of life on earth remains one of the key questions facing scientists today. Studies of ancient life face several obstacles. Firstly, we know surprisingly little about conditions facing organisms on the primordial earth. What was the early atmosphere like? How did plate tectonics work? The earth is estimated to be about ~4.45 billion years old (from dating of meteorites), yet the oldest rocks we find on earth are 'only' ~3.8 billion years old. Another problem is that the few old rocks which we do find on earth have been strongly altered in a process geologists call metamorphism. When rocks are subjected to changing conditions of pressure and/or temperature, the majority of vulnerable biological signatures are obliterated.

The most primitive forms of life we find on earth today are prokaryotes: single-celled organisms with no membrane-bound structures. Unlike more evolved organisms, many prokaryotes show special adaptations to living in oxygen-absent or oxygen-depleted environments. An example of such an adaptation is the ability to oxidise hydrogen sulfide (H2S) in order to provide energy for cellular processes. During chemotrophic respiratory processes such as these, organisms tend to preferentially fix the lighter isotopes of elements such as carbon and sulfur. A detailed analysis of biologically important isotopes, therefore, may yield clues to early life on Earth.

Some of the oldest uncontroversial evidence for life comes from ~3.5 billion year old rocks in Australia's Pilbara region and the Barberton, South Africa. Scientists continue to debate whether life existed earlier, for example during deposition of the ~3.8 billion year old rocks in Isua, south-west Greenland. By combining a comprehensive study on metamorphic petrology with detailed geochemical analysis, we hope to shed some light on the vital question of early life on Earth.

Part II: Life beyond Earth

Planet	Venus	Earth	Mars
Density [g/cm^3]	5.24	5.52	3.94
Radius [km]	6052	6378	3397
Mass [x10^24 kg]	4.86	5.97	0.64
Sidereal Orbit Period [Earth years]	0.62	1.00	1.88
Sidereal Rotation Period [Earth days]	243	1.00	1.03
Surface temperature [K]	730	288-293	186-268
Major atmospheric constituents	CO2, N2	N2, O2	CO2, N2, Ar
Distance from Sun [A.U.]	0.72	1.00	1.52

Our closest planetary neighbours in the Solar System, Mars and Venus, have several features in common with Earth. They are of similar size and composition, and have comparable distances from the Sun. Yet their surfaces hold little promise for life: Mars is an icebox, while Venus' atmosphere is under the control of a run-away greenhouse effect with daytime temperatures averaging 460°C. But in the past, things may well have been different. For one, geomorphological features on Mars suggest that it was not always the dry and cold place it is today. Unfortunately, 3.5 billion years of volcanic activity and meteorite impacts will have obscured all obvious traces of life. So how do we know what to look for?

Well, it just so happens that there is also a place on Earth where early traces of life have undergone shock metamorphism due to a large meteorite impact. About 2.05 billion years ago, a large meteorite crashed into 3.5 billion year old life-bearing rocks in South Africa. A careful study of the preserved chemical signatures in these rocks should give us a good idea of how these signatures react to metamorphism - this is turn will tell us what to look out for in Martian rocks, and perhaps meteorites as well.

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