3.8 billion-year-old rocks reveal nitrogen rise, forging complex life

By studying the nitrogen isotope of rock samples dating as far back as 3.8 billion years ago from the North China Craton, Kang has been trying to figure out what caused the rise of eukaryotes for over two years now. 

Eukaryotes were present in a "low-key status" for a very long time until they became prevalent around 820 million years ago, explained co-author Shuhai Xiao, professor of geobiology.

Why this was the case remained a mystery. 

Now, Kang and his team decided to consider the bigger picture after collecting data from the rock samples. He took the data, added it to a more extensive database, and analyzed it across a longer time scale spanning different geographic locations.

"Once we did this kind of integration and put it into a big picture, we saw the rise of nitrate (or biologically available nitrogen) through time which happened around 800 million years ago," said Xiao.  

The team then linked the data to biological developments, such as the emergence of eukaryotes.

Co-author Ben Gill argues that figuring out the part nitrogen played in the emergence of marine eukaryotes is significant as it helps to complete the wider evolutionary jigsaw.

 "Where we sit today, with life as it is on the planet, is the sum total of all the events that happened in the past," said Gill.

 "And this is a key event where we shift from dominantly prokaryotic ecosystems (cells that are much simpler than the ones in our bodies) to eukaryotes. If that did not happen, we would not be here today."

Even though ancient oceans are long gone, the events that took place there are still preserved in rocks. Xiao stated that by studying these rocks, we can make connections between our planet's past, present, and future.

"Geologists look at rocks for the same reason that stock traders look at the Dow Jones curve when they make decisions to sell or buy stocks. The geological history written in rocks gives us important context about global changes in the future," he concluded. 

The complete study was published in Science Advances on March 22 and can be found here. 

Study abstract:

The early Neoproterozoic Era witnessed the initial ecological rise of eukaryotes at ca. 800 Ma. To assess whether nitrate availability played an important role in this evolutionary event, we measured nitrogen isotope compositions (δ15N) of marine carbonates from the early Tonian (ca. 1000 Ma to ca. 800 Ma) Huaibei Group in North China. The data reported here fill a critical gap in the δ15N record and indicate nitrate limitation in early Neoproterozoic oceans. A compilation of Proterozoic sedimentary δ15N data reveals a stepwise increase in δ15N values at ~800 Ma. Box model simulations indicate that this stepwise increase likely represents a ~50% increase in marine nitrate availability. Limited nitrate availability in early Neoproterozoic oceans may have delayed the ecological rise of eukaryotes until ~800 Ma when increased nitrate supply, together with other environmental and ecological factors, may have contributed to the transition from prokaryote-dominant to eukaryote-dominant marine ecosystems.