Biological evolution is commonly viewed as a gradual and largely predictable journey from simple, single-celled bacterial ancestors, to ever larger and more complex life forms. Similarly, the evolution of Earth's atmosphere and oceans (which is considered to play a crucial role in biological evolution) is generally viewed as a gradual, albeit stepwise, trajectory towards greater oxygenation, with each rise occurring in tandem with, and facilitating progressive increases in, biological size and complexity. This paradigm of a monotonous progression towards modern conditions and the implied linkage between key evolutionary steps and oxygenation has recently been challenged. However, relevant data to resolve the issue are scarce and ambiguous. As such, links between Earth surface oxygenation and early biological evolution have been particularly difficult to unravel. Recently discovered, large eukaryote fossils from North China are particularly difficult to understand in the light of current understanding of Earth's atmospheric evolution which suggests exceedingly low oxygen levels at this time of diversification.
Studies of early Earth environments have been severely hampered by the poor quality of geological samples. With this in mind, we will drill through superbly preserved mid-Proterozoic samples from the North China craton, to obtain fresh samples to reconstruct the oxygenation history of the ocean and to investigate how nutrients in the ocean interact with this history. This will be combined with new paleontological data to demonstrate how nutrients and redox constrained the early evolution of eukaryotes, the ancestral lineage of all extant animals. By specifically targeting the best quality samples that can be obtained across this crucial interval in the history of life on Earth, the research outlined in this proposal will shed fundamental new insight into the enigmatic mid-Proterozoic Earth system, including why it took so long for large, complex multicellular eukaryotes to dominate marine ecosystems.