From bacteria to blue whales, the number of cells in living things exceeds the estimated number of sand grains on Earth by a factor of a trillion. It’s 1 million times larger than all the stars in the universe. And the number of cells that have ever lived is 10 orders of magnitude larger still, according to new estimates researchers reported last week in Current Biology. These calculations aren’t just an exercise in superlatives. They could also help scientists better understand our planet’s fecundity and predict how lifeforms may use carbon in the future.
“These efforts are absolutely indispensable,” says Rob Phillips, a biophysicist at the California Institute of Technology who was not involved with this work but has collaborated with some of the authors. Quantifying such baselines is crucial to scientists’ ability to ask meaningful questions. “Simply counting and measuring things [is] the difference between being able to do science versus not.”
Peter Crockford, a geologist at Carleton University, and his colleagues began their inventory by combining existing estimates of the number of microbes currently in the ocean, soil, and Earth’s subsurface with the number of cells in larger organisms. The result was the number of cells alive today. That number—an eye-popping 1030 cells, the majority of them cyanobacteria—was the starting point for calculating the total number of cells that have ever lived.
The key to that calculation was primary productivity, the conversion of carbon dioxide (CO2) into the carbon-based compounds that fuel all of life. Those compounds—sugars and starches for example—move up the food chain: Plants and photosynthesizing microbes are eaten by other organisms, which in turn are eaten by even bigger organisms. All of these die and are consumed and broken down by insects and microbes. Those consumers return CO2 to the atmosphere as they breathe and die, completing the carbon cycle.
To understand how primary productivity has changed over Earth’s geologic history, Crockford and colleagues combed the scientific literature for estimates of the numbers and types of photosynthesizing organisms at different points in time and how much “food” they produced. Knowing the primary productivity of modern cells, the researchers then were able to wind the clock back to calculate how many cells would have been required to sustain past levels of productivity. They adjusted the calculation based on factors such as when different lifeforms evolved and how ice ages dampened the activity of those organisms.
The first photosynthesizers were microbes called cyanobacteria, which evolved somewhere between 3.4 billion and 2.5 billion years ago. Sometime between 800 million and 650 million years ago, their productivity was surpassed by algae, Crockford says. And when land plants evolved 450 million to 350 million years ago, their productivity surged past that of algae.
Putting all the math together, somewhere between 1039 and 1040 cells have ever lived. Together, those photosynthesizers have cycled through all of Earth’s carbon about 100 times. And the researchers’ calculations suggest these numbers are approaching an upper limit. Earth simply doesn’t have the resources to support more than 1041 cells, they say.
The numbers seem “reasonable and realistic,” says Alessio Collalti, a forest ecologist at the National Research Council of Italy’s Institute for Agricultural and Forestry Systems in the Mediterranean. The paper reads “like a movie of the life on and of Earth, and how it has developed since the very beginning,” he says.
However, Donald Canfield, a geobiologist at the University of Southern Denmark cautions “these estimates become much more uncertain as we go further back in time.”
Crockford and colleagues also run their movie into the future. As the Sun ages it will brighten, driving geologic processes that will gradually lower the amount of CO2 in the air. About 1 billion years from now, according to the authors’ scenario, CO2 levels will fall so low that photosynthesis will falter. Plants will die. Ocean temperatures will rise so much that primary producers there won’t survive either. Soon thereafter, Earth’s biomass will plummet and life as we know it would grind to a halt.
“I hope they are wrong,” says Jeffrey Cameron, a biochemist at the University of Colorado Boulder. Even so “it is a great thought exercise to do these calculations and debate them.”