Early life and the Tree of Life
Up until 1990 all organisms were classified as prokaryotes or eukaryotes. The former have no central nucleus, but instead the DNA is free-floating, while in the latter DNA is contained in a nucleus. All bacteria are prokaryotes and the more familiar complex organisms are eucaryotes.
Late in the 1970s, discoveries at hot ocean vents and in the extreme environments of Yellowstone National Park in the US, showed life not only can survive in extreme conditions, but thrives in them. Extremophiles – as they collectively came to be termed – are all single celled and prokaryotes, so were classed as bacteria.
In 1990 Carl Woese at the University of Illinois proposed a third branch called archaea because of their genetic differences to bacteria – they are closer to the genetic make-up of eukaryotes than to bacteria. Most have not been cultured in a laboratory.
The Tree of Life is characterised in different ways showing their relatedness to each other, the three branches meeting at what is known as LUCA, the Last Universal Common Ancestor. Organisms near the root of the Tree of Life are hyperthermophilic (heat-loving). The microbes that probably made the stromatolite structures in the ancient Pilbara are likely to have been anaerobic (oxygen hating) extremophiles due to the extreme conditions that must have existed on a world that would have been very alien to the one we know today.
Walter, M.R. (1999) The Search for Life on Mars, Allen & Unwin, Australia
Dyer, B.D. (2003) A Field Guide to Bacteria, Cornell University Press
Web link: NASA Astrobiology Institute