How algae shaped life on Earth

Seaweed belongs to one of the oldest stories on the planet. Long before animals, forests or fish, it was algae — the ancestors of today’s seaweeds — that filled the seas with oxygen and made complex life possible. Here is the short version of how algae shaped life on Earth.

The first life was microscopic

The earliest traces of life appear in the rock record from around 3.5 billion years ago, in the form of single-celled microbes and the layered mats (stromatolites) they built in shallow seas. Among the most important microbes were cyanobacteria — often called blue-green algae — which mastered photosynthesis: turning sunlight, water and carbon dioxide into energy and releasing oxygen as a by-product. Exactly how early cyanobacteria themselves arose is still debated, but they are among the oldest forms of life known.

Algae gave the planet its oxygen

For hundreds of millions of years those photosynthetic microbes pumped out oxygen until, around 2.4 billion years ago, it began to build up in the atmosphere — an event geologists call the Great Oxidation Event. The air you breathe traces back to these early algae. Without that slow, ancient oxygen build-up, there would have been no path to animals or land life at all.

From single cells to seaweed

Later, some cells took a second leap. A larger cell absorbed a photosynthetic cyanobacterium and kept it alive inside itself — a partnership called endosymbiosis — and that captured cell became the chloroplast, the photosynthetic engine inside algal and plant cells today. From these early photosynthetic eukaryotes came the red and green algae, and in time the larger, many-celled seaweeds. Brown seaweeds — the kelps and wracks we cultivate — arrived by a more roundabout route: their ancestors gained photosynthesis later, by taking up a red alga. That is why brown seaweeds are only distantly related to green plants, and part of why they are so biologically distinctive.

The ancestors of every plant

One group of green algae eventually made it onto land, around 470–500 million years ago, and gave rise to mosses, ferns and ultimately every flowering plant and tree. In other words, the green algae are the ancestors of the entire land-plant kingdom.

Why this still matters today

Algae never stopped doing their original job. Marine photosynthesisers — from microscopic plankton to large seaweeds — are estimated to produce about half of the world’s oxygen, and they draw down carbon every day. Cultivating and restoring seaweed taps into one of the planet’s oldest and most reliable natural processes. It is also why seaweed restoration and blue-carbon projects have drawn so much interest.

Key facts

• The earliest traces of life on Earth date back around 3.5 billion years; cyanobacteria (blue-green algae) are among the oldest known life forms.
• Algal photosynthesis drove the Great Oxidation Event about 2.4 billion years ago, oxygenating the atmosphere.
• Chloroplasts — the photosynthetic engines in algae and plants — arose when one cell engulfed a photosynthetic cyanobacterium (endosymbiosis).
• Land plants evolved from green algae, making algae the ancestors of all plants.
• Red, brown and green seaweeds are large, many-celled algae descended from those early lineages.
• Marine algae and plankton are estimated to produce about half of Earth’s oxygen today.

Curious about the living descendants? Browse the seaweed we grow, read how seaweed reproduces, or talk to us about a project.

Sources

• Smithsonian National Museum of Natural History — Early Life on Earth (earliest microbial life and cyanobacteria).
• NASA Astrobiology — the Great Oxidation Event (rise of atmospheric oxygen ~2.4 billion years ago).
• Digital Atlas of Ancient Life — Origin of Land Plants (endosymbiosis, chloroplasts and green-algal ancestry of land plants).
• NOAA — How much oxygen comes from the ocean? (about half of Earth’s oxygen from marine photosynthesis).

Try it yourself: grow a kelp

Kelp begins life at a microscopic stage and races toward the light. See if you can grow one to the surface.

Last updated 16. juni 2026