New research examined microfossils and their host rocks from the McArthur and Birrindudu basins in Northern Territory, Australia. The fossils date to about 1.75–1.4 billion years ago, when the area was a shallow inland sea with lagoons, mudflats and calm coastal waters. Atmospheric oxygen was only about 1% or less of modern levels and oxygen in the ocean was unevenly distributed.
Scientists prepared and sorted microfossils from drill cores and used sedimentology and geochemistry to characterise the ancient environments. By matching sediment types to fossil taxa, they placed organisms in four environments: lagoons, tidal areas, coastal regions and offshore waters. Mineral indicators such as iron pyrite and concentrations of vanadium, molybdenum and uranium helped show local oxygen conditions.
The fossils occur almost exclusively in rock formed where the seafloor had oxygen. This suggests early eukaryotes needed oxygen for part of their life cycle and lived on or within the seafloor rather than floating freely. Living on the seafloor would have kept ancestral cells close to other organisms, which may have favoured the early acquisition of mitochondria. Even so, eukaryote diversity remained low in absolute terms for nearly one billion years after their first appearance.
Authors are now studying older microfossils from the McArthur Basin and other basins. The work is supported by the Simons Foundation, the Gordon and Betty Moore Foundation and NASA’s Exobiology program.
Difficult words
- microfossil — very small fossil of ancient microscopic lifemicrofossils
- sedimentology — study of layers of rock and sediments
- geochemistry — study of chemical elements in rocks and water
- taxon — group of organisms with similar characteristicstaxa
- seafloor — ground at the bottom of the sea
- eukaryote — cell with a nucleus and complex structureseukaryotes
- mitochondrion — cell structure that makes most of energymitochondria
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Discussion questions
- How might living on or within the seafloor help small organisms meet and interact?
- Why is oxygen presence in the seafloor important for the life cycle of eukaryotes?
- The article says eukaryote diversity stayed low for nearly one billion years. Why do you think diversity remained low for so long?
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