Geology Reference
In-Depth Information
Chapter 7
From Microbes to Cell Giants
Gaia has evolved and changed throughout her long life, but these changes have not al-
ways been smooth and gradual—there have been critical transitions or tipping points that
have reconfigured her atmosphere, oceans and land surfaces beyond all recognition, be-
cause of tightly coupled evolutionary developments amongst her living beings and her
rocks, atmosphere and water cycle. The first such transition took place at her birth, when
the first life forms, the ancient bacteria, spread all over the young planet some 3,500 mil-
lion years ago at the beginning of the Archean eon. The bacteria ruled unaided for a bil-
lion years until the beginning of the Proterozoic eon some 2,500 million years ago, when
we are reasonably certain that the first nucleated cells appeared. Large multicellular be-
ings composed of great colonies of nucleated cells did not arise until the beginning of the
Phanaerozoic eon some 600 million years ago, during which the general trend, bar five
mass extinctions, has been a gradual increase in biodiversity, culminating in today's cli-
mactic profusion of life. The key point to contemplate is that the appearance of new life
forms in each eon changed things so much that an alien civilisation inspecting samples of
air, water and rock taken at different times would have found it hard to believe that they
could have come from the same planet.
During the Archean eon, long before there were multicellular beings such as trees, people
and elephants, tiny bacteria ruled the world, as they still do today. Bacteria recycle gases
