Researchers studied microfossils and the rocks that contain them in the McArthur and Birrindudu basins, Northern Territory, Australia. The deposits formed about 1.75 to 1.4 billion years ago, when the region was a shallow inland sea with lagoons, offshore mudflats and calm coastal waters. Atmospheric oxygen was only about 1% or less of modern levels, and oxygen in the ocean had a patchy distribution.
The team prepared and sorted microfossils from drill core material and combined sedimentology with geochemistry to characterise ancient environments. Sediment type allowed them to match taxa to four settings: lagoons, tidal areas, coastal regions and offshore waters. Mineral indicators — including iron pyrite (FeS2) and the concentrations of vanadium, molybdenum and uranium — helped reveal how much oxygen was present in each setting.
The fossils appear almost exclusively in rock formed from oxygenated seafloor environments. If the organisms had lived freely in the water column, their remains should also appear in anoxic seafloor sediments, but they do not. This pattern therefore suggests early eukaryotes needed oxygen for at least part of their life cycle and lived on or within the seafloor. Living on the seafloor would have kept ancestral cells close to other organisms and could have favoured the assimilation of bacterial partners into mitochondria. Nevertheless, eukaryote diversity remained low in absolute terms for nearly one billion years after they first appear in the fossil record.
Later major changes in Earth history include global cooling around 720 million years ago that led to Snowball Earth conditions until about 635 million years ago. Mass extinctions and the reopening of ecological niches may have helped trigger the later rise of diverse multicellular life in the Ediacaran. Authors are now studying older microfossils from the McArthur Basin and other basins to learn when eukaryotes first gained their complexity. The work is part of a project supported by the Simons Foundation, the Gordon and Betty Moore Foundation and NASA’s Exobiology program.
Difficult words
- microfossil — very small fossil remains of ancient organismsmicrofossils
- sedimentology — study of sediments and how they form
- geochemistry — chemical composition and processes in rocks and sediments
- taxon — a named group of related organismstaxa
- anoxic — without free oxygen present in the environment
- oxygenated — containing a normal or increased amount of oxygen
- seafloor — bottom surface of the ocean or sea
- mitochondrion — cell organelle that produces energy for cellsmitochondria
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Discussion questions
- How might living on the seafloor have helped ancestral cells acquire bacterial partners that became mitochondria?
- Why do you think eukaryote diversity remained low for nearly one billion years after they appear in the fossil record?
- How could global cooling and Snowball Earth conditions have affected the rise of diverse multicellular life?
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