Looking for what kind of evidence for early life?
Science is about seeking evidence to support hypotheses, and then analyzing and interpreting that data. One line of evidence is not enough. And in circumstances where pieces of the evidence cannot tell the whole story – as with the Pilbara stromatolites – then it demands many lines of evidence. These are the areas scientists have determined or are investigating:
- Dating: Volcanic rocks are easier to date than sedimentary rocks – that is because the clock provided by the radioactive decay of the uranium to lead is locked into the crystallization of minerals at high temperatures. But it is sedimentary rock that is more likely to bear the signs of early life.
- Context: Rocks also bear a record of shapes that describe the environment – pillow lavas, seen in the Pilbara, form underwater; ripple marks from ancient shorelines are embedded forever in the rock, as so forth. Stromatolites form only in water.
- Maps: A number of types of map tell the scientist about the kinds of rocks (geological), the heights of the rocks (topographic), the magnetic and the mineralogical differences and so on. All contribute to the story the rocks can speak about.
- Isotopic: Many elements have lighter and heavier versions of the standard. This is true of carbon, the basic stuff of life. Life favours the lighter version, Carbon 12 over Carbon 13. So concentrations of more C12 than C13 suggest biogenic activity, but do not prove it.
- Spectroscopy: Infra Red data speaks to the nature of the mineralization of the rocks, and the geological circumstances for life.
- Microscopy: Much of the evidence is microscopic – so scientists make thin slices of rock for study under a light microscope and study samples in a 3D Scanning Electron Microscope, sometimes using an X-Ray Electron Dispersion Spectrometer, which can identify the elemental constituents.
- Stratigraphy: Carefully made stratigraphic columns (diagrams of the layers of rock) over a given area, help to match and date strata over relatively short distances so the construction of the strata and the way they are oriented can be determined. Folds and faults can put older strata on top of younger strata, making careful mapping an important part of this research.
- Geochemistry: This is where the most effort will be concentrated in the coming years. Biomarkers – hopanes, steranes and other molecules (the stuff left after life has long departed) – can be detected in sedimentary rocks up to 2.8 billion years old. But older than that has been difficult, although instrumentation and techniques are improving – inching towards showing the Pilbara stromatolites do have biomarkers with them.
- Microbiology: Modern day studies of systems similar to those that probably existed on Early Earth provide a useful comparison.
- Palaeobiology: Studies of ancient fossils provides information about the likely biology of the Pilbara stromatolites.
- Astronomy: Provides an understanding of solar system and planetary formation processes and provides comparisons with other planets such as Mars.