How and when life on Earth first appeared has proven to be an exceptionally difficult and therefore disputative question to answer. As evidence for possible explanations has mostly been destroyed by tectonic activity, very little is known and understood (Schopf, 1993). However based on the limited evidence that has thus far been discovered many theories exist as to where life originated.
In Western Australia some of the oldest and most well preserved sedimentary and volcanic rocks can be found from the Pilbara region (Fig. 8) (Schopf, 1993; Nisbet & Sleep, 2001). It was from this region that a 3.2 billion year old volcanogenic massive sulphide deposit was discovered containing the oldest fossils ever to have been found, later identified to be filamentous bacteria (Nisbet & Sleep, 2001; Javaux, 2006). This discovery along with others suggests that bacteria were the first to inhabit Earth over 3.2 billion years ago during the Archaen, spanning a time between 4-2.5 billion years ago (Javaux, 2006). Volcanogenic massive sulphide deposits are associated with deep sea hydrothermal environments (Nisbet & Sleep, 2001), which are thought to have been abundant during the Archaen (Rasmussen, 2000). All together the evidence has led to the development of a theory for the origin of life by sulphur-reducing (Vergas et al., 1998), chemoautotrophic bacteria at hydrothermal vents (Rasmussen, 2000; Martin & Russell, 2002; Javaux, 2006; Wächtershäuser, 2006).
However, the evidence is circumstantial therefore life could have originated in a completely different manner (Nisbet & Sleep). Other theories have proposed that life has originated elsewhere in the solar system and brought to Earth by asteroids, comets or meteorites (Ehrenfreund et al., 2002). While the origin of life still remains uncertain, one thing that can be agreed upon is that the first organisms to evolve on Earth were likely to have been microscopic bacteria (Vergas et al., 1998; Rasmussen, 2000; Nisbet & Sleep, 2001; Martin & Russell, 2002; Javaux, 2006; Wächtershäuser, 2006).