Life’s early traces

Western Australia’s Pilbara region isn’t known for its hospitality to life. Dry creek beds carve paths through dusty red earth, and razor-sharp grasses cover the area’s low hills. In this place with record-setting heat and months of minimal rainfall, hardy plants and animals eke out an existence.

But the Pilbara may have been prime real estate for the planet’s earliest complex life-forms. In the region’s 3.5-billion-year-old rocks, geobiologist Nora Noffke has found tiny, subtle patterns that may reflect communities of microorganisms. The patterns look just like those made by moist mats of microbes found along today’s shorelines. The markings Noffke spotted in Australia may be the earliest evidence of complex life on Earth.

Other scientists aren’t so sure. Early signs of life are hard to interpret, and are famous for stirring up controversy. Taken one at a time, every odd structure, chemical trace or microscopic fossil that is held up as the earliest relic of life is met with skepticism. But together with Noffke’s patterns, the combined evidence tells a story about life’s origins that scientists can mostly agree on: Microbial colonies probably lived around 3.5 billion years ago.

If Noffke’s patterns really were created by cells, the subtle sculptures might offer a new way to search for microbes on other planets, and they might help scientists better understand how quickly life developed here on Earth.

A nascent world

When the Earth formed around 4.5 billion years ago, life as we know it didn’t stand a chance. Asteroids smashing into the planet left behind oceans of molten rock and probably wiped the planet clean of any potential life. Scientists think the earliest cells emerged sometime after a planet-sized body slammed into Earth, creating the moon.

Since complex life takes time to evolve, those first cells must have existed well before microbial colonies appeared about 3.5 billion years ago, scientists reason. If so, Noffke’s find and others suggest that, on the timeline of life on Earth, early cells’ origins lie quite close to the planet’s tumultuous beginnings. “Every time we have a find like this it says to me that the process of life arising on a planet is pretty fast,” says geomicrobiologist Penelope Boston of New Mexico Tech in Socorro. “And that’s pretty amazing.”

Ancient signs of life can be hard to pin down. “It’s easy if you find a big old wonkin’ dinosaur bone,” says Boston. “But most fossils aren’t that.”

The oldest dinosaur fossils date back to about 230 million years ago. Yet those bones are brand-new compared with Noffke’s patterned structures. Time has been tough on tiny, ancient fossils: They rest in rocks that have endured billions of years of Earth’s geologic history. Heat and pressure may have long since pulverized any cellular remnants beyond recognition. What’s left is often microscopic and wide open to interpretation.

Scientists look for these minuscule clues in different layers of rock. “Think of a layer cake,” says astrobiologist Abigail Allwood of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. The oldest rock layers sit at the bottom, and younger ones lie on top. Geologists use chemical tests to figure out the age of each rock layer, and consider anything entombed inside to be the same age.

When searching for signs of early life, scientists comb ancient layers of rock for telltale structures, chemicals and fossils. Noffke sometimes has to hunker down close to the rock and peer through a magnifying glass to see her patterned sculptures. But she believes the tiny impressions stamped into ancient rocks match those formed by microbial mats today.

Modern mats lie atop sandy shores and ooze a sticky slime that glues particles together. When waves lap across the mats, the microbes rearrange bits of sand and the slime cements them into place, sculpting recognizable patterns. They’re like the grooves worms make as they cut through sandy soil, Noffke says. If the sand solidifies, evidence of the trail becomes set in stone. Microbial mats don’t push paths through sand like worms do, but the microbes can form characteristic sculptures, such as chips, crinkles, tufts and rolls. Eventually, the microbes die and their cells break down. But their sedimentary sculptures can stick around.

Noffke has studied microbial mats, and the sculptures they leave behind, in aquatic environments around the world, from tidal flats off the coast of Germany to lagoons in Tunisia. She has seen the sculptures in sediments just weeks old, in those formed thousands of years ago and in rocks of various ages. Until now, the oldest place Noffke had seen these features, called microbially induced sedimentary structures, or MISS, was in South Africa, in 3.2-billion-year-old rocks.

She thought she might find MISS in even older rocks in the Pilbara, which signs suggest once was home to an ancient sabkha, a kind of microbe-friendly salt flat. During a brief initial visit, Noffke had spotted what she suspected might be MISS. “I thought, ‘I have to get back here and take a closer look,’ ” she says.

So in 2011, she and graduate student Daniel Christian, both of Old Dominion University in Norfolk, Va., trekked to a part of the Pilbara with 3.5-billion-year-old rocks, hauling in food, water and camping supplies, and settled in for three weeks of fossil hunting. For days, the two scoured the area for fresh outcrops of ancient rocks. Then, the pair searched for rock beds containing MISS.

Halfway up a low hill, Noffke first caught sight of the microbial sculptures. “I was so excited that I couldn’t stop talking about it for a whole week,” she says. Ragged blotches, called chips, freckled the rocks’ surfaces. The chips looked just like sculptures made by mats in modern salt flats.

“I was convinced right away,” she says.

She collected small samples and brought them back to the lab to inspect them with microscopes. Tiny patterns, as well as the rock’s mineral makeup, matched MISS that Noffke had studied before, she and colleagues reported in 2013 in Astrobiology.

Though the structures formed by microbial mats are subtle, Boston, who studies microbes that survive in hostile environments, can see the similarities between ancient and modern MISS. “I’m pretty convinced that what they’re seeing is microbial in origin,” she says.

Chemical evidence within the MISS also hints at life. Noffke teamed up with paleobiologist David Wacey of the University of Western Australia in Crawley to detect ancient carbon in the rock sculptures. Her samples were loaded with the element — a good sign for scientists looking for life. The carbon might be remnants of cells broken down long ago, Noffke says.

NASA’s Allwood, whose research on ancient rocks could aid the search for life on other planets, wants to see more analysis of the chemicals inside Noffke’s samples. Different tests could pinpoint other biological hallmarks, such as whether the MISS have a carbon signature indicative of life: a specific ratio of carbon isotopes. Cells tend to stock up on carbon-12, the light version of the element that serves as a building material for living organisms.

Though Noffke’s structures are “definitely intriguing,” Allwood adds, “it’s very difficult to prove that they are absolutely biological.”

Scientists tend to rely on a mixture of visible and chemical features when assessing evidence for early life, but “it’s quite hard to tick all the boxes,” says paleontologist Kathleen Grey, formerly of the Geological Survey of Western Australia in Perth. Researchers rarely see all the features they want to see, she says, which might explain why signs of ancient life can be so controversial.

Layers of life

One sign of ancient life has captivated scientists for decades. In the 1980s, bumpy structures found in 3.5-billion-year-old rocks hinted that groups of bacteria might have lived back then.

These structures, called stromatolites, are built by microbial mats in watery environments rich in carbonate, a chemical building block of limestone and coral reefs. Stromatolites form when carbonate grains, sediment and slimy mat layers stack on top of one another. As the microbes within the mats grow, they alter the temperature and acidity of the surrounding water. These changes can force dissolved minerals to build up on the mats’ surface. New mats pile on top, and the process repeats, creating layered structures that can be meters thick.

Modern stromatolites, such as those found in Shark Bay in Australia, are often covered with a thin mat of live cells, but their core has already solidified into rock, says Grey, who has collected stromatolite samples from around the continent. “It’s a bit like concrete that’s setting from the bottom,” she says. By slicing through these structures, scientists can see the distinctive wavy lines of different mat layers. These layers also appear in ancient stromatolites.

Like Noffke’s MISS, the stromatolites hint at the sophistication of life 3.5 billion years ago. The cells must have been organizing into communities that shaped their surroundings, says Boston. “That’s not what you get from a soup of DNA sloshing around.”

But some type of primordial soup evolved into the planet’s first cells. And for those cells to develop into communities probably took hundreds of millions of years. Evidence for this timeline resides in 3.8-billion-year-old rocks from another desolate part of the world, Greenland. The carbon isotope signature of these rocks suggests that organisms may have lived there some 300 million years before the microbes that formed early stromatolites and MISS.

Marks of life on ancient Greenland also pop up as another element: Red streaks of iron cut through its oldest rocks. The red hue hints that enough oxygen may have been around to produce rust. And if oxygen existed 3.8 billion years ago, photosynthesizing bacteria may have been releasing the gas into the air as they converted sunlight into sugar.

But chemical clues aren’t entirely convincing, Wacey says. He’s skeptical that the Greenland rocks’ carbon traces were actually left behind by living organisms. “There are no structures within these rocks that you could ever say are fossils,” he says. What’s more, even a shred of modern plant matter can contaminate ancient rock samples with lifelike carbon signatures. These signatures can even be forged where no life exists, in volcanoes and hydrothermal environments.

For decades, the field has had trouble establishing criteria for what constitutes solid evidence of early life. Ideally, scientists would like to find fossils shaped like cells and outlined by something that looks like a membrane. For some scientists, that kind of evidence is the keystone for building a convincing case for ancient signs of life. So far, no one has found that evidence within 3.5-billion-year-old stromatolites or MISS.

“Life is enclosed within a cellular membrane,” says paleobiologist Martin Brasier of the University of Oxford. “That’s the crunch point for me.”

Direct evidence

Scientists thought they had found convincing evidence of ancient cells in 1987. J. William Schopf and Bonnie Packer reported in Science finding fossilized cells in an Australian rock layer just less than 3.5 billion years old. The team claimed that tiny spheres wrapped in sheaths were micro­fossils of bacteria.

Most scientists accepted the claim until 2002, when Brasier, a veteran at evaluating ancient fossils, shook up the field. He examined the specimens and claimed in Nature that the structures weren’t fossils at all: They were mineral growths. “People simply looked for funny shapes that reminded them of cells,” Brasier says. Shapes are important, he says, but so is sketching out a picture of the ancient cells’ environment, and finding evidence of lifelike behaviors. “You put all these things together, and then you’ve got a story,” he says.

In 2011, Brasier, Wacey and colleagues reported finding stronger evidence for microfossils of cells. Their fossils rest in 3.4-billion-year-old sandstone just 50 kilometers from where Schopf and Packer discovered their specimens.

In the younger rock, the researchers found clumps of rounded cells that look like bunches of grapes and elongated cells connected like sausage links. But these microfossils don’t just look like cells; Brasier contends that the clumps acted like cells too. The organisms appear to have stuck themselves to sand grains. And tiny crystals of a mineral called pyrite speckled in and around the microfossils may be a by-product of sulfur metabolism. Like some modern microbes living in low-oxygen environments, these organisms may have “breathed” the element instead of oxygen, the researchers reported in Nature Geoscience.

Though Noffke hasn’t yet found actual cells within the MISS from the Pilbara, she has seen microscopic textures that resemble filaments, long skinny threads of bacteria. No one has confirmed that these filaments are organic in origin, but Brasier thinks MISS are worth a closer look. “If we were to find signs of cells in and around these MISS, that would make me very happy,” he says.

All together, the evidence for complex life 3.5 billion years ago is piling up, says geomicro­biologist Maud Walsh of Louisiana State University in Baton Rouge, who has been following the early life debate for years. “It’s building more lines of evidence that we had pretty robust ecosystems of bacteria.”

Grey, who studies stromatolites, agrees. “It’s like putting a detective case together.” Again and again, she says, the clues keep hinting at life early in Earth’s history. And because MISS are made by such complex ecosystems, cellular life must have existed for millions of years before the structures appear in the fossil record.

The ancient structures embedded within the rocks of the Pilbara may even help scientists searching for life on other planets, says Walsh. Researchers could examine photos snapped by the Mars rovers for both stromatolites and MISS like those nestled in Earth’s oldest rocks.

Noffke is hopeful because Earth and Mars both hold evidence of ancient sabkhas. “The likelihood that there are MISS on Mars is very big,” she says.

Even if the case for such ancient life falls apart, the vigorous discussion it whips up is healthy, Brasier says. “How terrible it would be if we found life on Mars, and nobody could actually agree,” he says. “We’ve got to rehearse these debates here.”