But additional specimens from other sites came to light over the past two decades, and many of them have withstood scrutiny. There is now solid evidence of life dating back about 3.5 billion years.
Earth was a billion years old by then, and scientists have long wondered if even older fossils might be found.
In August, Dr. Van Kranendonk and his colleagues reported discovering fossils in Greenland dating back 3.7 billion years. The scientists argued that the organisms were once mats of bacteria that grew in shallow coastal waters.
In the new study, published in the journal Nature, Mattew S.Dodd, Dominic Papineau and their colleagues at University College London studied rocks that were either slightly older or much older than those containing the Greenland fossils.
They came from a remote geological formation in Canada called Nuvvuagittuq, which stretches across four square miles on the coast of Hudson Bay. Geologists surveyed the formation for the first time in the 1990s.
Researchers have variously estimated its age at 3.77 billion years or 4.22 billion years — just 340 million years after the formation of the planet.
In 2008, Dr. Papineau collected rocks from the formation and found a number of clues indicating that they had formed around hydrothermal vents on the ancient sea floor that spewed iron and other minerals.
He also found hints that there might have been life there — tiny blobs of rock, for instance, that contained a compound called apatite, which can form from phosphorus released by dying organisms.
The tubes and other structures in the rock that Mr. Dodd found are also reminiscent of bacteria that live today around hydrothermal vents. They grow as filaments, feeding on iron compounds and creating tube-shaped cavities in the sediment.
Similar filaments contain iron compounds in the Nuvvuagittuq rocks, Mr. Dodd and his colleagues found, and they are attached to round clumps that resemble the tiny anchors bacteria use to hold on to rock surfaces. The rocks also contain forms of organic carbon that could have been created by bacteria.
The researchers argue that it would be unlikely for all of these features to have formed in the absence of life. “Then you’re left with one scenario — a biological origin,” Mr. Dodd said.
Such a discovery could have big implications for the understanding of life’s early evolution.
If these really are fossils 3.77 billion years old, then they show that life was already diversifying by that time, thriving in both the shallow ocean in what is now Greenland and the deep ocean in today’s Canada.
And if these are fossils 4.2 billion years old, then scientists will have evidence that life began quickly on Earth, not long after the oceans formed.
Yet Frances Westall, the director of research at the CNRS-Centre de Biophysique Moléculaire in Orléans, France, isn’t convinced these are fossils at all. “I am frankly dubious,” she said.
For one thing, she has argued, the filaments in the Nuvvuagittuq rocks are too big. She and her colleagues have found filaments formed by bacteria in rock dating back 3.3 billion years, and these are far smaller.
On the early Earth, bacteria were forced to stay small, Dr. Westall said, because the atmosphere did not yet have enough oxygen to fuel their growth.
Long after the Nuvvuagittuq rocks formed on the sea floor, they were heated to tremendous temperatures. Some experts doubted that microscopic fossils could have survived such a baking.
“These authors built their research on pushing speculative ideas and appear totally unaware of the considerable evidence against their interpretation,” said Wouter Bleeker, of the Geological Survey of Canada.
In response, Dr. Papineau observed that the type of rock studied, known as chert, is very hard and might have protected fossils from high temperatures.
“I think the authors have done a good job,” said David Wacey, who researches the origins and evolution of life at the University of Western Australia. With the new evidence, he said, “One comes up with a pretty convincing biological scenario” for the origins of the mysterious rock features.
Dr. Wacey was not surprised that the new work had drawn criticism. “It may be many years before a consensus is reached,” he said. “But this is how science progresses.”
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