New evidence found in ancient rocks gives us a better picture of environmental conditions on Earth at a time when life was taking its first baby steps.
A research team led by Martin Homann from the Free University in Berlin, Germany, looked at rocks from the Barberton Greenstone Belt in South Africa that originally were deposited in a tidal flat 3.22 billion years ago. Their analysis, which included electron microscopy, showed well-preserved chains of cell-size molds that match the morphology of bacterial cells that exist today. Isotope analysis confirmed that organic matter in the samples is indeed of biological origin.
The ancient microorganisms lived in cavities in the sediments, presumably to protect themselves from ultraviolet radiation. At that time there was no free oxygen in the atmosphere and Earth had no ozone layer, so unfiltered radiation would have reached the surface and scorched any organisms that lived there. Many scientists have theorized that life in those early days would only have been possible at some depth below the ocean, with the overlying water column shielding the microorganisms beneath.
The research by Homann and co-authors changes the paradigm, as it strongly suggests that life was already thriving more than three billion years ago on land that was at least partially exposed. Microbes at this time apparently adapted to their hostile environment by retreating into natural cavities. Since the Barberton Greenstone Belts are the oldest well-preserved tidal deposits, one may wonder whether these kinds of microbes existed even earlier, perhaps as long as four billion years ago when the first life appeared on Earth. Tides at that time would have been drastically higher, and tidal landforms like beaches and mudflats would have been larger and more common, because the Moon was much closer at the time—around 25,000 kilometers from Earth compared to 400,000 km today. (The Moon still retreats by about 4 centimeters every year.)
Could life have originated in this kind of environment? Graham Cairns-Smith from the University of Glasgow, Scotland, thinks so. In the past he proposed that the first life on Earth might have been based on clay minerals. Tidal flats are typically rich in many types of clay, and the latest findings may give his hypothesis a boost. It would, however, still require that organic compounds, on which all life is based today, accomplished a genetic takeover at some later point in time. How this transition from (proposed) clay-based life to organic life based on DNA and RNA might have happened is, of course, another huge puzzle.