Paris - Researchers said Tuesday they had uncovered fossils showing that complex life on Earth began more than 1.5 billion years ago, nearly a billion years earlier than previously thought.
But the evidence, published in Nature Communications, immediately provoked debate, with some scientists hailing it as rock solid, and others saying they were wholly unconvinced.
After first emerging from the primordial soup, life remained primitive and unicellular for billions of years, but some of those cells eventually congregated like clones in a colony.
Scientists even took to calling the later part of this period the "boring billion", because evolution seemed to have stalled.
But at some point there was another huge leap -- arguably second in importance only to the appearance of life itself -- towards complex organisms.
This transition progressively gave rise to all the plants and animals that have ever existed.
Exactly when multi-cell eukaryotes -- organisms in which differentiated cells each contain a membrane-bound nucleus with genetic material -- showed up has inflamed scientific passions for many decades.
The new study is sure to enrich that tradition.
"Our discovery pushes back nearly one billion years the appearance of macroscopic, multicellular eukaryotes compared to previous research," Maoyan Zhu, a professor at the Nanjing Institute of Geology and Palaeontology, told AFP.
The fossils were uncovered in Hebei Province's Yanshan region, where Mao Zedong and his communist army hunkered down during World War II before coming to power.
Zhu and colleagues found 167 measurable fossils, a third of them in one of four regular shapes -- an indication of complexity.
No stranger to controversy
The largest measured 30 by eight centimetres (12 by three inches).
Taken together, they are "compelling evidence for the early evolution of organisms large enough to be visible with the naked eye," said Zhu.
"This totally renews current knowledge on the early history of life."
Up to now, eukaryotes of comparable size have not shown up in the fossil record until about 600 million years ago, when a multitude of soft-bodied creatures inhabited the world's oceans.
Phil Donoghue, a professor of palaeobiology at the University of Bristol, described the discovery as a "big deal".
"They are not the oldest eukaryotes, but they are certainly the oldest demonstrably multicellular eukaryotes," he told AFP.
Their very existence 1.56 billion years ago would mean that "oxygen levels were sufficiently high to allow for such large organisms to subsist."
But other experts were more sceptical.
"There is nothing here to suggest that the specimens are eukaryotic, as opposed to bacterial," said Jonathan Antcliffe, a senior researcher in the University of Oxford's department of zoology.
Bacteria are, by definition, unicellular, and do not have distinct nuclei containing genetic material.
'Not enough detail'
Antcliffe suggested the fossils more likely corresponded to colonies of bacterial cells, rather than a single complex organism.
Truly multicellular creatures display three-dimensional form in which only some cells are in direct contact with the environment.
This is "critically important for function because it introduces transport problems for oxygen, nutrients, and signalling molecules" needed by the internal cells, Andrew Knoll of Harvard University explained in an article reviewing scientific literature on the origins of complex life.
Another researcher, Abderrazak El Albani of the University of Poitiers in France, said there simply wasn't enough detail in the study to back up the claim.
"The morphological measures, on their own, are absolutely insufficient to tell us if these organisms were multicellular, eucaryotes or complex," he told AFP when asked to comment.
El Albani is himself no stranger to controversy on this topic.
A 2010 study he published in Nature's flagship journal -- claiming to have discovered the remains of 2.1 billion year old cell colonies in Gabon -- has been widely challenged, including by Zhu and Antcliffe, who described it as "largely discredited."