How Much Life Has Ever Existed on Earth, and How Much Ever Will?

Filamentous cyanobacteria from a tidal pond at Little Sippewissett salt marsh, Falmouth, Mass. Image Credit: Argonne National Laboratory, CC BY-NC-SA

Peter Crockford in Singularity Hub: All organisms are made of living cells. While it is difficult to pinpoint exactly when the first cells came to exist, geologists’ best estimates suggest at least as early as 3.8 billion years ago. But how much life has inhabited this planet since the first cell on Earth? And how much life will ever exist on Earth? In our new study, published in Current Biology, my colleagues from the Weizmann Institute of Science and Smith College and I took aim at these big questions.

Carbon on Earth

Every year, about 200 billion tons of carbon is taken up through what is known as primary production. During primary production, inorganic carbon—such as carbon dioxide in the atmosphere and bicarbonate in the ocean—is used for energy and to build the organic molecules life needs. Today, the most notable contributor to this effort is oxygenic photosynthesis, where sunlight and water are key ingredients. However, deciphering past rates of primary production has been a challenging task. In lieu of a time machine, scientists like myself rely on clues left in ancient sedimentary rocks to reconstruct past environments. In the case of primary production, the isotopic composition of oxygen in the form of sulfate in ancient salt deposits allows for such estimates to be made. In our study, we compiled all previous estimates of ancient primary production derived through the method above, as well as many others. The outcome of this productivity census was that we were able to estimate that 100 quintillion (or 100 billion billion) tons of carbon have been through primary production since the origin of life. Big numbers like this are difficult to picture; 100 quintillion tons of carbon is about 100 times the amount of carbon contained within the Earth, a pretty impressive feat for Earth’s primary producers.

Primary Production

Today, primary production is mainly achieved by plants on land and marine micro-organisms such as algae and cyanobacteria. In the past, the proportion of these major contributors was very different; in the case of Earth’s earliest history, primary production was mainly conducted by an entirely different group of organisms that doesn’t rely on oxygenic photosynthesis to stay alive. A combination of different techniques has been able to give a sense of when different primary producers were most active in Earth’s past. Examples of such techniques include identifying the oldest forests or using molecular fossils called biomarkers.

In our study, we used this information to explore what organisms have contributed the most to Earth’s historical primary production. We found that despite being late on the scene, land plants have likely contributed the most. However, it is also very plausible that cyanobacteria contributed the most. More here.