Sedimentary rocks often contain remains of dead marine organisms. If you have an old rock and use the right techniques, you can even extract some of the ancient organic material.
This organic material in old rocks has revealed chemical traces of eukaryotic organisms in samples up to 780 million years old - but not older than that. Therefore, it is reasonable to conclude that eukaryotes became dominant in marine ecosystems by 780 million years ago. But what about before this time? What if the organic material in rocks loses its resolution over time and doesn’t tell the whole story? According to Professor Donald Canfield, it is possible that the organic matter record is not revealing the real importance of eukaryote marine ecosystems before 780 million years ago.
What you typically look for in old rocks to reveal the presence of eukaryotes are sterane biomarkers, where steranes are chemical remnants of ancient eukaryote organisms. These steranes are typically extracted from the rocks along with other organic molecules using chemical solvents. These extracts are called bitumen. It is this technique that has revealed eukaryotes in rocks up to a maximum of 780 million years ago. The organic matter not removed by organic solvents and left in the rocks is known as kerogen. The kerogen may contain important information as to the nature of organisms in the ancient environment where the sediments first deposited, but they are more difficult to access.
Nevertheless, Donald Canfield and his colleagues from Petrochina in Beijing analyzed some 1.4-billion-year-old sedimentary rocks from the Xiamaling Formation in China , focusing on the kerogen. To do this they dissolved all the rock with acid, so that in the end only organic material remained. After extracting with organic solvent, and revealing no steranes, the remaining kerogen was transferred to a gold tube which was heated, breaking down the kerogen into components that could then be further removed in organic solvents. These extracts revealed steranes from algae that were not found in the bitumen extracts indicating that eukaryotic algae were part of the ancient marine ecosystem 1.4 billion years ago.