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https://www.quantamagazine.org/fossil-discoveries-challenge-ideas-about-earths-start-20180122/ January 22, 2018

Fossil Discoveries Challenge Ideas About Earth’sStartA series of fossil finds suggests that life on Earth started earlier than anyone thought, calling intoquestion a widely held theory of the solar system’s beginnings.

By Rebecca Boyle

NASA’s Goddard Space Flight Center

How the young Earth might have looked during the Late Heavy Bombardment.

In the arid, sun-soaked northwest corner of Australia, along the Tropic of Capricorn, the oldest faceof Earth is exposed to the sky. Drive through the northern outback for a while, south of Port Hedlundon the coast, and you will come upon hills softened by time. They are part of a region called thePilbara Craton, which formed about 3.5 billion years ago, when Earth was in its youth.

Look closer. From a seam in one of these hills, a jumble of ancient, orange-Creamsicle rock spillsforth: a deposit called the Apex Chert. Within this rock, viewable only through a microscope, thereare tiny tubes. Some look like petroglyphs depicting a tornado; others resemble flattened worms.They are among the most controversial rock samples ever collected on this planet, and they mightrepresent some of the oldest forms of life ever found.

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Last month, researchers lobbed another salvo in the decades-long debate about the nature of theseforms. They are indeed fossil life, and they date to 3.465 billion years ago, according to John Valley,a geochemist at the University of Wisconsin. If Valley and his team are right, the fossils imply thatlife diversified remarkably early in the planet’s tumultuous youth.

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Jeff Miller (Epoxy mount); Courtesy of J. William Schopf, UCLA (microfossil)

A sliver of a nearly 3.5-billion-year-old rock from the Apex Chert deposit in Western Australia (top). An example ofone of the microfossils discovered in a sample of rock from the Apex Chert (bottom).

The fossils add to a wave of discoveries that point to a new story of ancient Earth. In the past year,separate teams of researchers have dug up, pulverized and laser-blasted pieces of rock that maycontain life dating to 3.7, 3.95 and maybe even 4.28 billion years ago. All of these microfossils — orthe chemical evidence associated with them — are hotly debated. But they all cast doubt on thetraditional tale.

As that story goes, in the half-billion years after it formed, Earth was hellish and hot. The infantworld would have been rent by volcanism and bombarded by other planetary crumbs, making for anenvironment so horrible, and so inhospitable to life, that the geologic era is named the Hadean, forthe Greek underworld. Not until a particularly violent asteroid barrage ended some 3.8 billion yearsago could life have evolved.

But this story is increasingly under fire. Many geologists now think Earth may have been tepid andwatery from the outset. The oldest rocks in the record suggest parts of the planet’s crust had cooledand solidified by 4.4 billion years ago. Oxygen in those ancient rocks suggest the planet had water asfar back as 4.3 billion years ago. And instead of an epochal, final bombardment, meteorite strikesmight have slowly tapered off as the solar system settled into its current configuration.

“Things were actually looking a lot more like the modern world, in some respects, early on. Therewas water, potentially some stable crust. It’s not completely out of the question that there wouldhave been a habitable world and life of some kind,” said Elizabeth Bell, a geochemist at theUniversity of California, Los Angeles.

Taken together, the latest evidence from the ancient Earth and from the moon is painting a pictureof a very different Hadean Earth: a stoutly solid, temperate, meteorite-clear and watery world, anEden from the very beginning.

Ancient CluesAbout 4.54 billion years ago, Earth was forming out of dust and rocks left over from the sun’s birth.Smaller solar leftovers continually pelted baby Earth, heating it up and endowing it with radioactivematerials, which further warmed it from within. Oceans of magma covered Earth’s surface. Backthen, Earth was not so much a rocky planet as an incandescent ball of lava.

Not long after Earth coalesced, a wayward planet whacked into it with incredible force, possiblyvaporizing Earth anew and forming the moon. The meteorite strikes continued, some excavatingcraters 1,000 kilometers across. In the standard paradigm of the Hadean eon, these strikesculminated in an assault dubbed the Late Heavy Bombardment, also known as the lunar cataclysm,in which asteroids emigrated to the inner solar system and pounded the rocky planets. Throughoutthis early era, ending about 3.8 billion years ago, Earth was molten and couldn’t support a crust ofsolid rock, let alone life.

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Lucy Reading-Ikkanda/Quanta Magazine

But starting around a decade ago, this story started to change, thanks largely to tinycrystals called zircons. The gems, which are often about the size of the period at the end ofthis sentence, told of a cooler, wetter and maybe livable world as far back as 4.3 billionyears ago. In recent years, fossils in ancient rock bolstered the zircons’ story of calmerclimes. The tornadic microfossils of the Pilbara Craton are the latest example.

Today, the oldest evidence for possible life — which many scientists doubt or outright reject— is at least 3.77 billion years old and may be a stunningly ancient 4.28 billion years old.

In March 2017, Dominic Papineau, a geochemist at University College London, and hisstudent Matthew Dodd described tubelike fossils in an outcrop in Quebec that dates to thebasement of Earth’s history. The formation, called the Nuvvuagittuq (noo-voo-wog-it-tuck)Greenstone Belt, is a fragment of Earth’s primitive ocean floor. The fossils, about half thewidth of a human hair and just half a millimeter long, were buried within. They are madefrom an iron oxide called hematite and may be fossilized cities built by microbialcommunities up to 4.28 billion years ago, Dodd said.

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Dominic Papineau

The bright red rock in the Nuvvuagittuq Greenstone Belt appears to contain tube-shaped microfossils dating to atleast 3.77 billion years ago.

“They would have formed these gelatinous, rusty-red-colored mats on the rocks around thevents,” he said. Similar structures exist in today’s oceans, where communities of microbesand bloody-looking tube worms blossom around sunless, black-smoking chimneys.

Dodd found the tubes near graphite and with carbonate “rosettes,” tiny carbon rings thatcontain organic materials. The rosettes can form through varying nonbiological processes,but Dodd also found a mineral called apatite, which he said is diagnostic of biologicalactivity. The researchers also analyzed the variants, or isotopes, of carbon within thegraphite. Generally, living things like to use the more lightweight isotopes, so an abundanceof carbon 12 over carbon 13 can be used to infer past biological activity. The graphite nearthe rosettes also suggested the presence of life. Taken together, the tubes and theirsurrounding chemistry suggest they are remnants of a microbial community that lived neara deep-ocean hydrothermal vent, Dodd said.

Geologists debate the exact age of the rock belt where they were found, but they agree itincludes one of the oldest, if not the oldest, iron formations on Earth. This suggests thefossils are that old, too — much older than anything found previously and much older thanmany scientists had thought possible.

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Matthew Dodd

The microfossils resemble sea life that grows near deep-sea hydrothermal vents.

Then in September 2017, researchers in Japan published an examination of graphite flakesfrom a 3.95-billion-year-old sedimentary rock called the Saglek Block in Labrador, Canada.Yuji Sano and Tsuyoshi Komiya of the University of Tokyo argued their graphite’s carbon-isotope ratio indicates it, too, was made by life. But the graphite flakes were notaccompanied by any feature that looked like a fossil; what’s more, the history of thesurrounding rock is murky, suggesting the carbon may be younger than it appears.

Farther to the east, in southwestern Greenland, another team had also found evidence ofancient life. In August 2016, Allen Nutman of the University of Wollongong in Australia andcolleagues reported finding stromatolites, fossil remains of microbes, from 3.7 billion yearsago.

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Laure Gauthiez-Putallaz

Allen Nutman prospecting for ancient microfossils in the Isua belt in southern Greenland.

Many geologists have been skeptical of each claim. Nutman’s fossils, for example, comefrom the Isua belt in southern Greenland, home to the oldest known sedimentary rocks onEarth. But the Isua belt is tough to interpret. Just as nonbiological processes can formDodd’s carbon rosettes, basic chemistry can form plenty of layered structures without anyhelp from life, suggesting they may not be stromatolites but lifeless pretenders.

In addition, both the Nuvvuagittuq Greenstone Belt and the Isua belt have been heated andsquished over billions of years, a process that melts and recrystallizes the rocks, morphingthem from their original sedimentary state.

“I don’t think any of those other studies are wrong, but I don’t think any of them are proof,”said Valley, the Wisconsin researcher. “All we can say is [Nutman’s rocks] look likestromatolites, and that’s very enticing.”

Regarding his work with the Pilbara Craton fossils, however, Valley is much lesscircumspect.

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Allen Nutman/University of Wollongong

The stromatolites form small wavelike mounds in sedimentary rock. The vertical lines are cuts made by theresearchers.

Signs of LifeThe tornadic microfossils lay in the Pilbara Craton for 3.465 billion years before being separatedfrom their natal rock, packed up in a box and shipped to California. Paleobiologist William Schopf ofUCLA published his discovery of the strange squiggles in 1993 and identified 11 distinct microbialtaxa in the samples. Critics said the forms could have been made in nonbiological processes, andgeologists have argued back and forth in the years since. Last year, Schopf sent a sample to Valley,who is an expert with a super-sensitive instrument for measuring isotope ratios called a secondaryion mass spectrometer.

Valley’s team found that some of the apparent fossils had the same carbon-isotope ratio as modernphotosynthetic bacteria. Three other types of fossils had the same ratios as methane-eating ormethane-producing microbes. Moreover, the isotope ratios correlate to specific species that hadalready been identified by Schopf. The locations where these isotope ratios were measuredcorresponded to the shapes of the microfossils themselves, Valley said, adding they are the oldestsamples that look like fossils both physically and chemically.

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Jeff Miller / UW-Madison

John Valley in his mass spectrometer laboratory at the University of Wisconsin, Madison.

While they are not the oldest samples in the record — supposing you accept the provenance of therocks described by Dodd, Komiya and Nutman — Schopf’s and Valley’s cyclonic miniatures do havean important distinction: They are diverse. The presence of so many different carbon isotope ratiossuggests the rock represents a complex community of primitive organisms. The life-forms must havehad time to evolve into endless iterations. This means they must have originated even earlier than3.465 billion years ago. And that means our oldest ancestors are very, very old indeed.

Watery WorldFossils were not the first sign that early Earth might have been Edenic rather than hellish. The rocksthemselves started providing that evidence as far back as 2001. That year, Valley found zircons thatsuggested the planet had a crust as far back as 4.4 billion years ago.

Zircons are crystalline minerals containing silicon, oxygen, zirconium and sometimes otherelements. They form inside magma, and like some better-known carbon crystals, zircons are forever— they can outlast the rocks they form in and withstand eons of unspeakable pressure, erosion anddeformation. As a result, they are the only rocks left over from the Hadean, making them invaluabletime capsules.

Valley chipped some out of Western Australia’s Jack Hills and found oxygen isotopes that suggestedthe crystal formed from material that was altered by liquid water. This suggested part of Earth’s

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crust had cooled, solidified and harbored water at least 400 million years earlier than the earliestknown sedimentary rocks. If there was liquid water, there were likely entire oceans, Valley said.Other zircons showed the same thing.

“The Hadean was not hell-like. That’s what we learned from the zircons. Sure, there were volcanoes,but they were probably surrounded by oceans. There would have been at least some dry land,” hesaid.

Zircons suggest there may even have been life.

In research published in 2015, Bell and her coauthors presented evidence for graphite embeddedwithin a tiny, 4.1-billion-year-old zircon crystal from the same Jack Hills. The graphite’s blend ofcarbon isotopes hints at biological origins, although the finding is — once again — hotly debated.

“Are there other explanations than life? Yeah, there are,” Bell said. “But this is what I wouldconsider the most secure evidence for some sort of fossil or biogenic structure.”

Imaging is by Crystal Shi (Department of Earth, Energy, and Environmental Sciences, Stanford University, Stanford,CA)

An X-ray of a 4.1-billion-year-old sample of zircon reveals dark spots made by carbon deposits.

If the signals in the ancient rocks are true, they are telling us that life was everywhere, always. Inalmost every place scientists look, they are finding evidence of life and its chemistry, whether it is inthe form of fossils themselves or the remnants of life’s long-ago stirrings. Far from fussy anddelicate, life may have taken hold in the worst conditions imaginable.

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“Life was managing to do interesting things at the same time Earth was dealing with the worstimpacts it’s ever had,” said Bill Bottke, a planetary scientist at the Southwest Research Institute inBoulder, Colorado.

Or maybe not. Maybe Earth was just fine. Maybe those impacts weren’t quite as rapid-fire aseveryone thought.

Evidence for a BeatingWe know Earth, and everything else, was bombarded by asteroids in the past. The moon, Mars,Venus and Mercury all bear witness to this primordial pummeling. The question is when, and forhow long.

Based largely on Apollo samples toted home by moonwalking astronauts, scientists came to believethat in the Earth’s Hadean age, there were at least two distinct epochs of solar system billiards. Thefirst was the inevitable side effect of planet making: It took some time for the planets to sweep upthe biggest asteroids and for Jupiter to gather the rest into the main asteroid belt.

The second came later. It began sometime between 500 and 700 million years after the solar systemwas born and finally tapered off around 3.8 billion years ago. That one is called the Late HeavyBombardment, or the lunar cataclysm.

As with most things in geochemistry, evidence for a world-rending blitz, an event on the hugestscales imaginable, is derived from the very, very small. Isotopes of potassium and argon in Apollosamples suggested bits of the moon suddenly melted some 500 million years after it formed. Thiswas taken as evidence that it was blasted within an inch of its life.

Zircons also provide tentative physical evidence of a late-era hellscape. Some zircons do contain“shocked” minerals, evidence for extreme heat and pressure that can be indicative of somethinghorrendous. Many are younger than 3 billion years, but Bell found one zircon suggesting rapid,extreme heating around 3.9 billion years ago — a possible signature of the Late HeavyBombardment. “All we know is there is a group of recrystallized zircons at this time period. Giventhe coincidence with the Late Heavy Bombardment, it was too hard not to say that maybe this isconnected,” she said. “But to really establish that, we will need to look at zircon records at otherlocalities around the planet.”

So far, there are no other signs, said Aaron Cavosie of Curtin University in Australia.

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NASA

Craters on the moon have been taken as evidence for the Late Heavy Bombardment, but reassessments of thegeological evidence from Apollo moon rocks casts doubt on whether the asteroid bombardments during the Hadeanera were as severe as was thought.

Moon RocksIn 2016 Patrick Boehnke, now at the University of Chicago, took another look at those originalApollo samples, which for decades have been the main evidence in favor of the Late HeavyBombardment. He and UCLA’s Mark Harrison reanalyzed the argon isotopes and concluded that theApollo rocks may have been walloped many times since they crystallized from the natal moon, whichcould make the rocks seem younger than they really are.

“Even if you solve the analytical problems,” said Boehnke, “then you still have the problem that theApollo samples are all right next to each other.” There’s a chance that astronauts from the six Apollomissions sampled rocks from a single asteroid strike whose ejecta spread throughout the Earth-facing side of our satellite.

In addition, moon-orbiting probes like the Gravity Recovery and Interior Laboratory (GRAIL)spacecraft and the Lunar Reconnaissance Orbiter have found around 100 previously unknowncraters, including a spike in impacts as early as 4.3 billion years ago.

“This interesting confluence of orbital data and sample data, and all different kinds of sample data —lunar impact glass, Luna samples, Apollo samples, lunar meteorites — they are all coming togetherand pointing to something that is not a cataclysmic spike at 3.9 billion years ago,” said NicolleZellner, a planetary scientist at Albion College in Michigan.

Bottke, who studies asteroids and solar system dynamics, is one of several researchers coming up

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with modified explanations. He now favors a slow uptick in bombardment, followed by a gradualdecline. Others think there was no late bombardment, and instead the craters on the moon and otherrocky bodies are remnants from the first type of billiards, the natural process of planet building.

“We have a tiny sliver of data, and we’re trying to do something with it,” he said. “You try to build astory, and sometimes you are just chasing ghosts.”

Life Takes HoldWhile it plays out, scientists will be debating much bigger questions than early solar-systemdynamics.

If some of the new evidence truly represents impressions of primeval life, then our ancestors may bemuch older than we thought. Life might have arisen the moment the planet was amenable to it — themoment it cooled enough to hold liquid water.

“I was taught when I was young that it would take billions and billions of years for life to form. But Ihave not been able to find any basis for those sorts of statements,” said Valley. “I think it’s quitepossible that life emerged within a few million years of when conditions became habitable. From thepoint of view of a microbe, a million years is a really long time, yet that’s a blink of an eye ingeologic time.”

“There is no reason life could not have emerged at 4.3 billion years ago,” he added. “There is noreason.”

If there was no mass sterilization at 3.9 billion years ago, or if a few massive asteroid strikesconfined the destruction to a single hemisphere, then Earth’s oldest ancestors may have been herefrom the haziest days of the planet’s own birth. And that, in turn, makes the notion of life elsewherein the cosmos seem less implausible. Life might be able to withstand horrendous conditions muchmore readily than we thought. It might not need much time at all to take hold. It might arise earlyand often and may pepper the universe yet. Its endless forms, from tubemaking microbes tohunkering slime, may be too small or simple to communicate the way life does on Earth — but theywould be no less real and no less alive.