What Can We Expect to Find in the Earliest Rock Record?

The earliest preserved rock record, although fragmentary, provides us with unique evidence for testing models of when and where life first appeared on Earth. It is widely agreed that life emerged on our planet prior to 3,000 million years ago, but there currently exists no consensus as to the earliest fossil evidence of life on Earth.

In order for researchers to be able to work to a consistent baseline, we must first define “ what is life?” This in itself is not a simple task (see Cleland and Chyba, 2002). For example, the current NASA definition of life as: “ a system which is self-sustained by utilising external energy or nutrients owing to its internal process of component production and coupled to the medium via adaptive change that persist during the time history of the system” (Luisi, 1998) is both vague and awkward. For the purpose of this book a clearer, if more restrictive, definition is required which is tailored towards evidence that may feasibly be retrieved from the rock record. Namely that fossil life is “ a complex structure that encodes evidence of biological behaviour and processing (e.g., growth, decay, and community tiering), and who's distribution and abundance is controlled by biologically significant variables such as light levels, temperature and nutrient gradients.”