The first eukaryotes, a lineage of life-forms that includes all plants and animals, appears to have arisen a billion years earlier than scientists had thought. The discovery of eukaryotic biochemicals in 2.7-billion-year-old rock, reported in tomorrow's issue of Science, raises questions about the sluggish origins of complex life-forms and the timing of when Earth's atmosphere first filled with oxygen.
Scientists have presumed that eukaryotes, defined by a cell nucleus wrapped in a membrane, evolved from prokaryotes, such as bacteria, that lack nuclear membranes. A team led by geologist Jochen Brocks of the University of Sydney in Australia has been searching for biochemical clues that may explain how both fundamental groupings of organisms arose.
The researchers have now found traces of organic molecules (C28-C30 steranes), made only by eukaryotes, in 2.7-billion-year-old shale near Wittenoom, in northwest Australia. Another type of hydrocarbon they found in the rock, 2?-methylhopanes, is today only made by cyanobacteria. This is the first strong confirmation that these prokaryotic photosynthesizers may have been the earliest organisms to pump oxygen into the air and make it breathable, the team says.
The finding "gives tantalizing insights" into early evolution, says Simon Conway Morris of the University of Cambridge in the United Kingdom. The older age for the origin of eukaryotes raises the question of why it took so long for more complex, multicellular organisms to evolve, he says. And the date for cyanobacteria is bound to be controversial, Conway Morris says: It "contradicts some new molecular clock analyses which have suggested that the origin of the cyanobacteria was significantly younger."