PARIS – Scientists unveiled fossils from west Africa Thursday that push back the dawn of multicellular life on Earth by at least 1.5 billion years.
Just how complex the newly discovered organisms are is sure to be hotly debated.
But there can be no doubt that the creatures unearthed from the hills of Gabon, visible to the naked eye, have upended standard evolutionary timelines.
"The cursor on the origin of complex multicellular life is no longer 600 million years ago, as has long been maintained, but more like 2.1 billion years," said Abderrazak El Albani, a researcher at the University of Poitiers and lead author of the study.
The findings were published in the British journal Nature.
Up to now, conventional scientific wisdom held that the planet was populated only by single-celled microbes until the so-called Cambrian explosion, a major surge of biodiversity that began some 600 million years ago.
Ever-more complex life forms emerged rapidly from there, eventually creating an evolutionary tree with homo sapiens atop one of its branches.
"Multicellularity represents one of the principle thresholds in evolutionary history," Philip Donoghue and Jonathan Antcliffe from the University of Bristol said in a commentary, also in Nature.
But the new organism, which appears to have lived in colonies, shows that the drive toward complexity began much sooner.
Shaped like cookies with ragged edges and a lumpy interior, more than 250 specimens have been found so far, El Albani said.
"They have different body shapes, and vary in size from one to 12 centimetres (0.4 to 5.0 inches)," he told AFP by phone.
The fossilised creatures may also have crossed another threshold of evolution far earlier than any other known organism.
Unlike simple bacteria, their cells appear to have membrane-bound nucleus housing and protecting its chromosomes, the genetic blueprints for life.
Geochemical analysis shows that the organisms lived in slightly-oxygenated ocean waters, leading the researchers to speculate that oxygen may have been an essential catalyst for the leap from single- to multi-cell life forms.
"The Proterozoic Eon saw two major events of oxygen build-up in the atmosphere and the oceans," El Albani explained.
The first of these would have occurred just before the Gabon specimens emerged, and the second ahead of the Cambian explosion.
Earth's earliest, primitive life forms are thought to have sparked to life about 3.9 billion years ago after the so-called Late Heavy Bombardment, a 100-million-year fusillade during which our young planet was pummeled by meteorites that blasted craters the size of Thailand and France.
Fossils reveal microscopic life forms 3.5 billion years old, and geochemical clues point to more primitive organisms -- thought by some to be the common ancestor to all things living -- 300,000 million years before that.