o 



o 



About 2.8 billion years ago, 

 cyanobacteria formed the distinctive 

 layers visible in this stromatolite fivm the 

 Nullagine Range in Western Australia. 



Reg Morrison 



Structures formed by cyanobacteria and 

 other microbes). Indirect evidence of life 

 exists, too. Carbon atoms come in two sta- 

 ble forms that differ by a single neutron. 

 Because photosynthetic organisms prefer- 

 entially incorporate the lighter form, they 

 have a chemical signature that can be read 

 even in the carbon preserved in North Pole 

 rocks. This view of early life on Earth, al- 

 though fragmentary, is enough to show 

 that three billion years before trilobites 

 first graced the oceans, life existed in the 

 form of complex microbial communities. 

 Some of the organisms that evolved in 

 our planet's long infancy are still with us. 

 In the damp mud of swamps, deep in the 

 Black Sea, at the mouths of hydrothermal 



vents and elsewhere in the oceans, and 

 even in our own digestive tracts, oxygen- 

 free environments harbor bacteria whose 

 physiologies evolved to exploit the ancient 

 North Pole habitats and other primeval 

 seas. Those survivors from a bygone 

 world suggest that the eaith's very earliest 

 biota comprised bacterialike microbes that 

 lived in hot, oxygen-poor envu^onments 

 and derived their energy from chemical re- 

 actions or the fermentation of organic mol- 

 ecules. Early on, some lineages evolved 

 the ability to use energy from sunlight to 

 drive the formation of organic matter from 

 carbon dioxide dissolved in seawater. This 

 innovation — photosynthesis — was eco- 

 logically liberating and enabled life to 

 cover the globe. 



Most photosynthetic bacteria rely on 

 hydrogen sulfide and similar molecules 

 for the electrons needed in photosynthesis; 

 but one lineage, the cyanobacteria, learned 

 to use a much more common substance — 

 water. As a result, cyanobacteria, the blue- 

 green scum in birdbaths and ponds, be- 

 came the most abundant producers of 



organic matter on the planet. And because 

 they produce oxygen as a byproduct of 

 photosynthesis, these tiny organisms set a 

 new course for the earth's environmental 

 history, paving the way for the many kinds 

 of creatures, including humans, with oxy- 

 gen-dependent, or aerobic, metabolism. 



But the oxygen revolution didn't happen 

 quickly. Cyanobacteria may have begun 

 releasing oxygen into the atmosphere as 

 early as three and a half billion years ago 

 (at the time of the North Pole sea), but 

 signs of atmospheric change first show up 

 in soils formed about 2.1 billion years ago. 

 By that time atmospheric oxygen had 

 passed a crucial threshold, from less than 

 about 1 percent of present-day levels to 10 

 percent or more. The implications of this 

 change are enormous: Above 1 percent of 

 today's level, the atmosphere contains 

 enough oxygen to allow the evolution of 

 aerobic organisms. Also at the higher lev- 

 els of oxygen, stratospheric ozone (itself a 

 form of oxygen) eifectively shields the 

 earth from lethal ultraviolet radiation. 



The biological consequences of this 



1 6 Natural History 6/94 



