Somewhere between the rise of single-cell organisms from the primordial soup and the advent of dating apps, reproduction made the leap from cloning to sex.

A ghostly, bottom-dwelling ocean creature that came and went about 565 million years ago just may have been the first to cross that threshold, according to a study published this week in Nature.

"Fractofusus looked like nothing that is alive today," said the study's lead author, Emily Mitchell, a researcher at the University of Cambridge.

"They lived in very deep water - 2 kilometers - far below the photic zone, so they were not plants," Mitchell told AFP. "They had no mouths or any animal features, nor were they fungi."

But they were what biologists categorize as complex organisms.

Shaped like an oval skull cap, Fractofusus lived in communities, with generations spread outward in concentric circles and linked by spindly, branchlike connectors. Adults could reach 40 cm across, with babies a tenth that size.

While they appear to have gone extinct quickly, at least on a geological time scale, the enigmatic organisms did manage to colonize great swathes of ocean floor.

We know this because they left fossils, lots of them. Mitchell and colleagues used spatial and statistical analysis to examine more than 1,000 specimens spread across three sites on the Canadian island of Newfoundland.

"We already knew that Fractofusus had a non-random spatial pattern," Mitchell said. But it was only after the researchers mapped out the rock surfaced that they realized that the patterns were not caused by environmental forces such as currents, but a reproductive process.

The vast majority of individuals were clones that formed from runners, called stolon, similar to strawberry or spider plants today.

Up to that point in Earth's history - the Ediacaran age, stretching from 580 to 541 million years ago - that's how reproduction happened.

Animals and plants with clear sexual differentiation did not appear on the scene until the subsequent Cambrian explosion.

But cloning did not explain how Fractofusus got from one part of the ocean floor to another, and that is where things got interesting.