Newfound 3.77-billion-year-old fossils could be earliest evidence of life on Earth
Tiny, tubular structures uncovered in ancient Canadian rocks could be remnants of some of the earliest life on Earth, scientists say.
The straw-shaped "microfossils", narrower than the width of a human hair and invisible to the naked eye, are believed to come from ancient microbes, according to a new study in the journal Nature.
Scientists debate the age of the specimens, but the authors' youngest estimate - 3.77 billion years - would make these fossils the oldest ever found.
Claims of ancient fossils are always contentious. Rocks as old as the ones in the new study rarely survive the weathering, erosion, subduction and deformation of our geologically active Earth. Any signs of life in the rocks that do survive are difficult to distinguish, let alone prove. And some researchers dispute the age of the materials examined in the new paper.
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But the scientists behind the new study say their analysis should hold up to scrutiny. In addition to structures that look like fossil microbes, the rocks contain a cocktail of chemical compounds that is almost certainly the result of biological processes.
If the findings are confirmed, they will boost the belief that life arose very early in the history of Earth - and may find it just as easy to evolve on worlds beyond our own.
"The process to kick-start life may not need a significant length of time or special chemistry, but could actually be a relatively simple process to get started" said Matthew Dodd, a biogeochemist at University College London and the lead author on the paper. "It has big implications for whether life is abundant or not in the universe."
The microfossils were discovered in rocks from the Nuvvuagittuq belt in northeastern Canada. This strip of iron-rich jasper now cuts across the eastern shore of Hudson Bay, but it was once a hydrothermal vent on the ocean floor. Billions of years ago, Dodd and his colleagues say, ancient microbes flourished around those vents, taking advantage of their chaotic chemistry to generate fuel.
When the microbes died, iron in the water was deposited on their decaying bodies, replacing cellular structures with stone. The rocks that contained them were buried, heated, squashed, and then forced upward to form the part of North America where they now sit. Depending on the dating method used, the material could be as old as 3.77 billion years - or as stunningly ancient as 4.28 billion years.
When Dodd's UCL colleague Dominic Papineau visited the Nuvvuagittuq belt in 2008, he knew immediately he would have to bring some samples back to his lab. Once back in London, he and Dodd peered at very thin slices of the rocks, first with an optical microscope, then with a laser-based device called a Raman microscope.
The optical observations revealed complex fossil structures encased in hematite, a mineral that would have formed as iron in the seawater interacted with the microbe's decaying organic matter. John Slack, an emeritus scientist at the US Geological Survey who studies jaspers from ancient hydrothermal vents, said that the fossils look just like the ones he sees in younger rocks, and around modern-day vents.
The Raman analysis, which measures the vibrations of atoms as they are struck by a laser to figure out what molecules a material contains, showed that the rocks contain carbonate, apatite and magnetite - minerals that often form in the presence of organic matter.
Graphite in the rocks also contained telltale signs of life. The mineral disproportionately contained the isotope carbon-12, a form of the atom in which the nucleus has 6 protons and 7 neutrons. This form of carbon is preferred for biological processes, and is considered an isotopic signature of life.
"We can think of alternative explanations for each of these singular observations," said Dodd, "but why all of these features occur together can really only be explained by one thing, which is a biological interpretation."
Not everyone is so convinced. Tanja Bosak, a geobiologist at MIT, said that the authors of the Nature paper are missing some key evidence for their claims. For one thing, the authors don't include images of the site where they found the fossils, or a detailed explanation of their geologic setting.
"This is the very first thing we tell our students to look at the context and report the context and interpret the context carefully," she said. "Because if the context isn't right than everything else you do doesn't matter."
Bosak was also skeptical of the stated minimum age for the fossils. The Nuvvuagittuq belt is composed of metamorphic rock - stone that's been heated, squeezed and deformed by processes deep within the Earth. It is also bisected by veins of igneous, or volcanic, rock that intruded into the sediments during this process.
Dodd and his colleagues got their estimates of the formation's youngest age by dating zircon crystals in the igneous intrusions (the logic being that those intrusions had to have formed after the fossils did). Their oldest estimate - 4.28 billion years old - comes from a more contested dating method that measures the decay of the element samarium into neodymium.
Bosak said that the metamorphic processes the rocks would have undergone means the veins from which the fossils come could be much younger.
Findings like these are subject to intense scrutiny because they have potentially far-reaching implications for the study of early organisms on Earth and other planets. The oldest universally accepted evidence of life on Earth is dated to about 3.4 to 3.5 billion years ago. The new paper proposes pushing that date back by nearly 300 million years.
"That's a long time," Bosak said. Just think: in the most recent 300 million years of history, the Earth has seen three mass extinctions, a reshuffling of continents, the rise and fall of dinosaurs and the evolution of humankind.
An earlier start date for the history of life also means that organisms were evolving at a time when Earth would have been quite hostile. Between 4 and 3.8 billion years ago, the planet was subjected to what's called the "Late Heavy Bombardment," a time when asteroids and comets flew through the solar system and barraged every body they struck. If microbes were able to thrive in this chaotic time, that would imply that life can take hold even under the worst of circumstances.
Significantly, the Nature paper comes just six months after researchers working in Greenland reported finding ancient stromatolites in 3.7-billion-year-old rocks. Those conical structures are usually produced by photosynthetic bacteria living in shallow seas. If both papers are confirmed, Slack explained, that means life not only existed early in Earth's history, but was diverse enough to include both chemosynthetic and photosynthetic bacteria.
If organisms found it so easy to thrive here on Earth, why not elsewhere? Could it be, in the words of Nobel laureate Christian de Duve, a "cosmic imperative?"
That's what Dodd would like to know. He noted recent research suggesting that 3.77 billion years ago, when the fossils formed, Mars was warm and had oceans on its surface.
"It means we could expect to find evidence of life on Mars at this time," Dodd said. And if we don't - "that suggests that life is a result of some fluke or phenomenon on Earth."
- The Washington Post