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Life's 



Expanding 



Realm 



Eveiy living organism 



rest's on a microbial 



foundation foi'med billions of 



years ago 



by Andrew Knoll 



We live on an ever changing planet, 

 where stability — as much as humans 

 might yearn for it — has no place. With 

 every change may come disruption, or 

 even extinction, for some forms of life. For 

 others, however, change may mean oppor- 

 tunity. The result of all this dynamism has 

 been more than just a constantly changing 

 cast of characters. Environmental change, 



along with the opportunities it brings, has 

 created an expanding Earth, not literally a 

 growing planet but one where the range of 

 environments available for colonization 

 has increased enormously over time and 

 where beneficiaries of one change have 

 been the progenitors of the next. 



To appreciate the biological importance 

 of the expanding environment, one must 

 take the long view of evolution, looking at 

 how ecosystems have developed over the 

 full extent of the earth's history. Wanting 

 to see what this planet might have been 

 like four billion years ago, and lacking a 

 time machine, I took a trip to the North 

 Pole. The trip was hot, dusty, and bone- 

 jarringly bumpy; only skillful driving 

 prevented collision with the kangaroos 

 encountered en route, for this particular 

 North Pole is in the remote outback of 

 western Australia. There, in the hills be- 

 yond an isolated sheep station, are 3.5-bil- 

 lion-year-old chert and lava formations. 

 The sediments they contain help us put to- 

 gether a picture of the primordial earth. 



Then, as now, oceans covered the globe, 

 a gray green expanse broken by small con- 



tinents and broad volcanic platforms that 

 rose out of the sea, as Iceland does today. 

 The North Pole sediments tell us that the 

 chemical content of seawater was deter- 

 mined not so much by erosion from the 

 land, as it is today, as by the circulation of 

 water through vents in the ocean floor; the 

 atmosphere contained abundant carbon 

 dioxide and very little oxygen. 



On such a planet, humans couldn't sur- 

 vive for an hour, but other organisms 

 could — and did. Du^ect evidence of these 

 early organisms exists in fossils of bacteria 

 preserved in chert and in stromatolites 

 (distinctively layered, often dome-shaped 



Once cyanobacteria (here a living form 

 magnified 100 times) began producing 

 oxygen as a byproduct ofpfiotosynthesis, 

 more than a billion years passed before 

 the atmosphere contained enough of the 

 precious element to allow the evolution of 

 oxygen-dependent organisms. 



Dwigtit R. Kuhn 



14 Natural History 6/94 



