| Summary: | <p>Eukaryotic life arose around 1.6 billion years ago, but for much of Earth history has
been ecologically and biologically conservative. There is a gap of around a billion years between
the first eukaryotic fossils and their rise to ecological prominence around 800 million years ago.
The causes of this delay in eukaryote evolution are widely debated, but in recent years the role of
oxygen, has been proposed as a major driver in the evolution of complex life. Oxygen drives metabolism
for energy intensive organisms like eukaryotes, it is also a key component of eukaryotic
biomolecules. Oxygen is also a key player in the cycling of essential bio-limiting nutrients such as
phosphorus. Some have hypothesized that in the low oxygen Proterozoic world, phosphorus was
scarce and thus limited primary productivity, slowing the oxygenation of the Earth and locking the
planet into a billion-year long period of environmental stasis. Additionally, the iron rich oceans are
proposed to have sequestered phosphorus in sediments as iron phosphate minerals. To date, neither
of these hypotheses have been robustly tested against the sedimentary record.</p>
<p>To test these two hypotheses, I conducted a high-resolution study of exquisitely preserved
~1.4 - 1.3 Ga sedimentary rocks from the Roper Superbasin (RSB), northern Australia. I find no
evidence of sedimentary sequestration of phosphorus as Fe-phosphate in the examined sediments.
Additionally, I find that sediments were deposited under ferruginous and oxic conditions, contrary
to published work. Also, in contrast to previous work, I find that the RSB is a restricted basin and
underwent periods of disconnection from the global ocean. As an epicratonic basin enclosed on
multiple sides, the RSB is unsuitable for investigating global ocean conditions in the Proterozoic.
However, such locations appear to be commonnly preserved in this interval, and may have been
important sites for phosphorus deposition. Regardless of ocean connection the RSB, and locations
like it, are where we find eukaryote fossils, and so were obviously an important part of eukaryote
evolution. The questions raised by this study highlight how much is left to learn about Earths distant
past and the organisms that inhabited the planet at that time.</p>
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