ABSTRACT
BACKGROUND: THE ORIGIN OF LIFE AND THE FIRST 3 BILLION YEARS Life on Earth arose some 3.5 billion years ago. There is some debate concerning what actually constitutes ‘life’ and, indeed, whether life actually arose on this planet or originated extraterrestrially in a simple form and evolved further here. Nevertheless, the earliest fossil evidence of single-celled prokaryotes (simple cells with no nucleus or organelles) akin to modern cyanobacteria (previously known as blue-green algae) comes from 3,500 million year-old cherts in Western Australia and South Africa. For nearly 2,500 million years after its origin, life evolved very slowly and planet Earth was home to nothing but bacteria, although there was a huge array of different types, which actually constitute two separate kingdoms, the Eubacteria and the Archaebacteria. The former are mostly photosynthesizing bacteria, but many of the latter are unusual forms able to survive in extreme conditions, such as the ‘hyperthermophiles’ which can live in water as hot as 100°C, and the ‘chemophiles’ which thrive in a cocktail of toxic chemicals. Both these sets of conditions are found around submarine hydrothermal vents (or ‘black smokers’) and it has been suggested that life may have originated in such a setting, in which case the archaebacteria are our oldest ancestors.
