Life on Earth exists on a scale that defies human imagination. The number of cells in living organisms today is a staggering 10³⁰, that’s one million times more cells than there are stars in the known universe. To put things in a more grounded perspective, the number of cells exceeds all the planet’s grains of sand by a factor of one trillion.

A new study extends this count to the past, estimating that somewhere between 10³⁹ and 10⁴⁰ cells have existed in Earth’s history. According to the scientists, the biosphere’s cumulative fixation is up to a trillion gigatons of carbon since life began. This figure surpasses Earth’s entire carbon stock by a factor of 100.

Beyond their sheer numbers, these calculations help scientists understand the planet’s life-sustaining mechanisms and its future limits.

Tallying Life Over the Eons

“This is the difference between being able to do science versus not,” Rob Phillips, a biophysicist at the California Institute of Technology, told Science. Quantifying life’s fundamental building blocks, he argues, lays the groundwork for answering critical questions about our planet’s ecology and future.

Primary productivity underpins Earth’s biosphere. Without it, there would be no breathable oxygen, no global food webs, and no carbon cycle as we know it. The research team, led by geologist Peter Crockford from Carleton University, reconstructed the history of this biological engine, stretching from the dawn of life to today.

Their research identifies critical leaps: the advent of oxygenic photosynthesis over two billion years ago, the rise of algae around 800 million years ago, and the terrestrial takeover by land plants about 450 million years ago.

By combining existing data on microbes and larger organisms, the researchers arrived at the jaw-dropping figure of 10³⁰ cells alive today. The human body alone contains around 30 trillion cells.

The next step was to look back in time. Using modern rates of primary productivity — the conversion of carbon dioxide into energy-rich compounds — the team reconstructed how life has cycled carbon through Earth’s history.

From Ancient Microbes to Modern Forests

The story begins over three billion years ago, when cyanobacteria first evolved. For nearly two billion years, these microbes were Earth’s primary photosynthetic organisms, capturing sunlight and transforming it into chemical energy. Around 800 million years ago, algae emerged, outpacing cyanobacteria in productivity. Then, 450 million years ago, land plants came into the picture and changed the carbon cycle, vastly increasing the planet’s biomass.

Although cyanobacteria have dominated primary production for most of life’s history, today, land plants dominate productivity, followed by marine algae.

To piece together this grand story of life on Earth, Crockford and his colleagues analyzed data on ancient photosynthesizers and modeled their productivity under different climatic and geological conditions. Ice ages, for instance, suppressed primary productivity, while the evolution of new organisms boosted it.

Their calculations reveal that life on Earth has cycled all of the planet’s carbon roughly 100 times. But the researchers also suggest a limit is in sight. Earth’s resources could only support up to 10⁴¹ cells before running out of the carbon needed to sustain them.

Alessio Collalti, a forest ecologist at the National Research Council of Italy, called the paper “a movie of the life on and of Earth, and how it has developed since the very beginning.”

A Sobering Look Ahead

As the Sun continues to age, its increasing brightness will drive geologic changes that gradually reduce atmospheric CO₂. In about a billion years, CO₂ levels may drop so low that photosynthesis will cease. Without plants and oceanic producers, life on Earth could grind to a halt.

Crockford’s research also invites new questions: How did early productivity rates influence the pace of evolution? How robust are these estimates given gaps in the fossil record? And what lessons can today’s carbon-stressed ecosystems draw from this ancient history?

For now, though, the planet remains teeming with an almost incomprehensible abundance of cells, each one a testament to the resilience and complexity of life.

The findings appeared in the journal Current Biology.