The question of what caused bacteria to evolve into single-celled plants and animals--which they did more than a billion years ago--has long puzzled scientists. Now geologists claim to have evidence to bolster one intriguing theory: prolonged nutrient deprivation, which forced simple organisms called prokaryotes to cooperate and gradually merge as complex eukaryotic organisms.
The Precambrian era between 2 billion and 1 billion years ago has been described as "the dullest period in Earth's history," with nothing much going geologically, climatically, or biologically. But because a lack of geologic activity meant a paucity of erosion-generated phosphorus--the fundamental "biofuel" of the time--the stage was being set for the most momentous chapter in biological evolution, argue Martin Brasier of the University of Oxford and John Lindsay of the Australian Geological Survey in Canberra in the June issue of Geology.
The scientists examined rocks in Northern Australia near the Gulf of Carpentaria where Precambrian oceans left a 6-km-thick stack of limestone sediments. They measured carbon isotopes--which reflect total biological matter produced and buried in the oceans--at 10-meter intervals through 1.5 billion years of limestone and confirmed what others have inferred from the fossil record: a prolonged period of biological stasis. Brasier and Lindsay argue that the shortage of phosphorus, which is abundant only in younger rocks, pushed simple bacterial life into symbiotic associations. These in turn developed into more complex eukaryotes with nuclei--the ancestors of today's plants and animals.
The idea that "long-term geological events [that kept phosphorus scarce] drove the evolution of eukaryotes" has been floating around for some time, notes Julie Bartley, a geochemist at the University of West Georgia, Carrollton. However, she says, "this is the first good database to support the notion of nutrient deprivation as the driving force of this major evolutionary event."