One-Celled Organisms Laid the Foundations for Complex Life — Here’s How

Key Takeaways on One-Celled Organisms

• One-celled organisms began influencing life once their descendants achieved multicellularity.
• Researchers once thought that this multicellularity transition was rather explosive, but it turns out that the organisms only needed a few minor tweaks.
• During Earth’s early history, bacteria changed the atmosphere, which helped spark multicelled life.

Earth, as we know it, is largely defined by multicellular life. Plants form the verdant backdrop — grassland, tundra, forest, jungle — upon which animals roam, while fungi weave their mycelial threads through just about every inch of soil. But this lively scene, with its eruption of biological diversity, emerged from a much older world, one dominated by simpler organisms.

Life today is an elaboration of the ground rules laid down billions of years ago by our single-celled forebears. Everything from the structure of DNA to the fine details of metabolism originated with those biological pioneers, and some of them primed the planet for more complex creatures by oxygenating the atmosphere.

“The particular nature of the unicellular ancestors does have a big impact on how multicellularity evolves,” says Matthew Herron, an evolutionary biologist at the National Science Foundation’s Division of Environmental Biology, who studies the transition to multicellular life.

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The Beginning of One-Celled Organisms

The vast influence of single-celled organisms (aka one-celled organisms) becomes apparent once their descendants first achieve multicellularity. Scientists once thought this transition required a dramatic overhaul of the genome — an explosion in its size and a thorough reorganization.

But in recent years, Herron says, it’s become clear that when multicellularity evolves, “you’re taking an existing unicellular genome and you’re making relatively minor tweaks to it.”

In other words, multicellular organisms don’t have to reinvent themselves; they can just repurpose existing genes for radically new functions.

For example, single-celled organisms, like all living things, have some degree of phenotypic plasticity: the ability to change their physical traits and behaviors, or phenotype, in response to environmental changes, such as dogs growing thick fur in the winter and humans gaining muscle by lifting weights.

With the advent of multicellularity, some genes responsible for phenotypic plasticity came to be used instead for cellular differentiation, the process by which stem cells develop into the many specialized cell types that make up plants, animals, and fungi, according to a study in BioEssays.

That’s exactly what happened within the volvocine green algae, where a unicellular species like Chlamydomonas shares a common ancestor with Volvox, a green alga that’s closely related but multicellular.

A 2020 paper led by Aurora Nedelcu, an evolutionary biologist at the University of New Brunswick, reported that ancient stress responses in Chlamydomonas took up the cause of cellular differentiation in its Volvox, paving the way for more sophisticated biology based on division of labor between specialized cells — perhaps the same path taken by our own ancestors.

From One-Celled Organisms to Multiple

Multicellular life made plenty of innovations — leaves and flowers, eyes and wings. But the basic machinery that enabled these biological wonders got set in motion much earlier with the first eukaryotes, organisms whose cells contain a nucleus.

Volvox, for example, already had mitochondria, the “powerhouse” organelle that generates most of the energy for cellular functions.

“Think about all of the abilities that they must’ve inherited from their single-celled ancestors that they didn’t have to invent from scratch,” Herron says. At the most fundamental level, all multicellular organisms, from algae right up to humans, run on an operating system developed by unicellular life.

Another example is the ribosome, shared not only by all eukaryotes but also by all prokaryotes, like bacteria. Following the genetic code, ribosomes build everything from enzymes to antibodies, making them factories for the sorts of molecules that become fodder for future evolution.

These and other biological tools were perfected in the unimaginably distant past.

“When multicellularity evolves,” Herron says, “it’s always evolving from something that’s been adapting to the environment for millions of years, hundreds of millions of years, maybe billions of years.” Everyone alive today reaps the rewards of that ancient, thorough tinkering.

How Bacteria Made The World Hospitable

These achievements took place when Earth was a very different place. For the first half of our planet’s history, the atmosphere was devoid of oxygen, an essential gas for most life today, and particularly for complex organisms like ourselves.

Then, a little more than 2 billion years ago, single-celled cyanobacteria — which photosynthesize like plants but belong to an entirely different lineage — began to flood the atmosphere with oxygen. That era, known as the Great Oxidation Event (GOE), set the stage for the rise of multicellularity, according to the American Society for Microbiology.

“The evolution of multicellular forms,” wrote the authors of one 2013 paper in PNSA, “coincides with the onset of the GOE and an increase in diversification rates.”

To the naked eye, these new proliferating microbes wouldn’t have seemed like a big advance on the old blueprint. But fast-forward more than a billion years, after a long incubation, multicellular organisms finally start to scale up, becoming trilobites, weird fish, monstrous sharks, dinosaurs, and, eventually, human beings.

Once again, all our marvelous relatives and we have plain-old unicellular life to thank for preparing the way for us in the world, a world still governed by the biological laws they enacted so long ago.

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