It came from the ocean.

Often those words conjure images of web-footed monstrosities in the old B-movie reels of our imaginations. But this time, scientists are taking up the refrain. Something did emerge from the depths, about 650 million years ago, and forever changed the course of life on this planet.

In an article published in the scientific journal Nature, geochemists say that something was algae — a simple, single-cell organism that revolutionized the planetary food web.

And, in turn, that allowed the world’s first tiny animals to evolve.

"This is one (of) the most profound ecological and evolutionary transitions in Earth's history," lead researcher Jochen Brocks told the BBC.

Like so many of history’s fleeting conquerors, there’s precious little evidence of their empire left. No fossils apparently exist of the algae itself. It was only the recent discovery of trace biomolecules in the Australian desert that fueled the new theory.

"These molecules tell us that it really became interesting 650 million years ago,” Brocks said in a statement. “It was a revolution of ecosystems, it was the rise of algae."

Sometimes, the tiniest pieces can fill in a lot of the big picture of the jigsaw puzzle that is biological history. Even the famed English naturalist Charles Darwin, the BBC notes, drew a blank when it came to that pivotal point — called the Cambrian Explosion — when life took a relatively sudden turn for the complex, myriad and multi-celled.

“The transition from dominant bacterial to eukaryotic marine primary productivity was one of the most profound ecological revolutions in the Earth’s history, reorganizing the distribution of carbon and nutrients in the water column and increasing energy flow to higher trophic levels,” notes one of the scientists in the report.

When algae showed up

In other words, for the roughly 3.8 billion years that preceded the "explosion," life on this planet may generously be characterized as boring. Mostly, it was vast oceans teeming with bacteria and not much else.

Then, according to the authors, the algae armada sailed onto the scene. The microscopic life forms had actually been around for at least a billion years previously. But their ranks exploded in the wake of a massive ice age that had gripped the planet.

Scientists say the great melt may have played a role in the algae mega-bloom.

And that algae went on to do powerful work here on Earth.

illustration of graptolites floating in water Life took a turn from single-celled algae and bacteria to more complex organisms during the Cambrian Explosion. These long-extinct graptolites floated in shallow waters between 485 and 443 million years ago. (Photo: AuntSpray/Shutterstock)

"This rise in algae happens just around the time the first animals appeared on the scene," Brocks explained to the BBC. “It was algae at the bottom of the food web that created this burst of energy and nutrients that allowed larger and more complex creatures to evolve.”

And as life pitter-pattered along its evolutionary path, guess who showed up around 7 million years ago.

Which is why we may owe our former algae overlords a world of appreciation. They may have left us with the greatest legacy of any conquering army.