2.5 Billions Years of Earth History in 100 Square Feet

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Scalding hot water from an underground thermal spring creates an iron-rich environment similar to what existed on Earth 2.5 billion years ago. (Nerissa Escanlar)

Along the edge of an inlet on a tiny Japanese island can be found– side by side – striking examples of conditions on Earth some 2.4 billion years ago, then 1.4 billion years ago and then the Philippine Sea of today.

First is a small channel with iron red, steaming and largely oxygen-free water – filled from below with bubbling liquid above 160 degrees F. This was Earth as it would have existed, in a general way, as oxygen was becoming more prevalent on our planet some 2.4 billion years ago. Microbes exist, but life is spare at best.

Right next to this ancient scene is region of green-red water filled with cyanobacteria – the single-cell creatures that helped bring masses of oxygen into our atmosphere and oceans.  Locals come to this natural “onsen” for traditional hot baths, but they have to make their way carefully because the rocky floor is slippery with green mats of the bacteria.

And then there is the Philippine Sea, cool but with spurts of warm water shooting up from below into the cove.

All of this within a area of maybe 100 square feet.

It is a unique hydrothermal scene, and one recently studied by two researchers from the Earth-Life Science Institute in Tokyo – microbiologist Shawn McGlynn and ancient virus specialist Tomohiro Mochizuki.

They were taking measurements of temperature, salinity and more, as well as samples of the hot gas and of microbial life in the iron-red water. Cyanobacterial mats are collected in the greener water, along with other visible microbe worlds.

Shawn McGlynn, associate professor at the Earth Life Science Institute in Tokyo scoops some iron-rich water from a channel on Shikine-jima Island, 100 miles from Tokyo. (Nerissa Escanlar)

The scientific goals are to answer specific questions – are the bubbles the results of biology or of geochemical processes? What are the isotopic signatures of the gases? What microbes and viruses live in the super-hot sections? And can cyanobacteria and iron co-exist?

All are connected, though, within the broad scientific effort underway to ever more specifically understand conditions on Earth through the eons, and how those conditions can help answer fundamental questions of how life might have begun.

“We really don’t know what microbiology looked like 2.5 billion or 1.5 billion years ago,” said McGlynn, “But this is a place we can go where we can try to find out. It’s a remarkable site for going back in time.”

In particular, there are not many natural environments with high levels of dissolved iron like this site. Yet scientists know from the rock record that there were periods of Earth history when the oceans were similarly filled with iron.

Mochizuki elaborated: “We’re trying to figure out what was possible chemically and biologically under certain conditions long ago.

“If you have something happening now at this unusual place – with the oxygen and iron mixing in the hot water to turn the water red – then there’s a chance that what we find today was there as well billions of years ago. ”

Tomohiro Mochizuki at collecting samples directly from the spot where 160 degree F water pushes up through the rock at Jinata hot spring. (Nerissa Escanlar)

The Jinata hot springs, as the area is known, is on Shikine-jima Island, one of the furthest out in the Izu chain of islands that starts in Tokyo Bay. More than 100 miles from Tokyo itself, Shikine-jima is nonetheless part of Tokyo Prefecture.

The Izu islands are all volcanic, created by the underwater movements of the Philippine and Pacific tectonic plates. That boundary remains in flux, and thus the hot springs and volcanoes. The terrain can be pretty rugged: in English, Jinata translates to something like Earth Hatchet, since the hot spring is at the end of a path through what does look like a rock rising that had been cut through with a hatchet.

Hot springs and underwater thermal vents have loomed large in thinking about origins of life since it became known in recent decades that both generally support abundant life – microbial and larger – and supply nutrients and even energy in the form of electricity from vents and electron transfers from chemical reactions.

And so not surprisingly, vents are visited and sampled not infrequently by ELSI scientists. McGlynn was on another hydrothermal vent field trip in Iceland over the summer with, among others, ELSI Origins Network fellow Donato Gionovelli and ELSI principal investigator and electrochemist Ruyhei Nakamura..

McGlynn’s work is focused on how electrons flow between elements and compounds, a transfer that he sees as a basic architecture for all life. With so many compelling flows occurring in such a small space, Jinata is a superb laboratory.

The volcanic Izu island chain, starting in Tokyo Bay and going out into the Philippine Sea.

For Mochizuki, the site turned out to be exciting but definitely not a goldmine. That’s because his speciality is viruses that live at very high temperatures, and even the bubbling hot spring in the iron trench measured about 73 degrees C (163 degrees F.) The viruses he incubates live at temperatures between closer to 90 C (194 F), not far from the boiling point.

His goal in studying these high-temperature (hyperthermophilic) viruses is to look back to the earliest days of life forming on Earth, using viruses as his navigators. Since life is thought by many scientists to have begun in a super hot RNA world, Mochizuki wants to look at viruses still living in those conditions today to see what they can tell us.

So far, he explained, what they have told us is that the RNA in the earliest lifeforms on Earth – denizens of the Archaean kingdom – did not have viruses. And this is puzzling.

So Mochizuki is always interested in going to sample hot springs and thermal vents to collect high temperature viruses, and to look for surprises.

Though the bubbling waters were so hot that both researchers had difficulty standing in the water with boots on and holding their collection vials with gloves, it was not hot enough for what Mochizuki is after. But that certainly didn’t stop him from taking as many samples as he could, including some for other ELSI researchers doing different work but still needing interesting samples.

Researchers often need to be inventive on field trips, and that was certainly the case at Jinata. When McGlynn first tried to sample the bubbles at the scalding spring, his hands and feet quickly felt on fire and he had to retreat.

To speed the process, he and Mochizuki built a funnel out of a large plastic water bottle, a device that allowed the bubbles to be collected and directed into the sample vial without the gloved hands being so close to the heat.   The booted feet, however, remained a problem and the heat just had to be endured.

Nearby the steaming bubbling of the hot spring were collections of what appeared to be fine etchings on the bottom of the red channel. These faint designs, McGlynn explained, were the product of a microbe that makes it’s way along the bottom and deposits lines of processed iron oxide as it goes. So while the elegant designs are not organic, the creatures that creates them surely is.

“Touch the area and the lines go poof,” McGlynn said. “That’s because they’re just the iron oxide; nothing more. Next to us is the water with much less iron and a lot more oxygen, and so there are blooms of (green) cyanobacteria. Touch them and they don’t go poof, they stick to your hand because they’re alive.”

Filaments created by microbes as they deposit iron oxide at the bottom of small channel. (Marc Kaufman)

McGlynn also collects some of the the poofs to get at the microbes making the unusual etchings. It may be a microbe never identified before.

As a microbiologist, he is of course interested in identifying and classifying microbes. He initially thought the microbes in the iron channel would be anaerobic, but he found that even tiny amount of oxygen making their way into the springs from the atmosphere made most aerobic, or possibly anaerobes capable of surviving with oxygen (which usually is toxic to them.)

He also found that laboratory studies that found cyanobacteria would not flourish in the presence of iron were not accurate in nature, or certainly were not accurate at Jinata onsen.

But it is that flow of electrons that really drives McGlynn – he even dreams of them at night, he told me.

One of the goals of his work, and that of his colleague and sometimes collaborator at ELSI, geobiochemist Yuichiro Ueno, is to answer some of the outstanding questions about that flow of electrons (electricity) from the core of the Earth. The energy transits through the mantle, to the surface and then often is in contact with the biosphere (all living things) before it enters the atmosphere and sometimes disappears into space.

He likened the process to the workings of a gigantic battery, with the iron core as the cathode and the oxygen in the atmosphere as the anode. Understanding the chemical pathways traveled by the electrons today, he is convinced, will tell a great deal about conditions on the early Earth as well.

It’s all important research in what is a chipping away of the many unknowns in the stories of the origins of Earth and the origin of life.

A boundary between where the very hot iron-rich water meets and the less hot water with thriving cyanobacteria colonies at Jinata.

The field work also illustrated the hit-and-miss nature of these kind of outings. While McGlynn has not come up with Jinata surprises or novel understandings, he was so taken with the setting that he wondered if a seemly empty building not too far from the site could be turned into an ELSI marine lab.

And while Mochizuki did not find sufficiently hot water for his work, he might still be coming back to the island, or others nearby. That’s because he learned of a potentially much hotter spring at a spot where the sea hits one of the island’s steep cliffs – a site that requires boat access that was unsafe in the choppy waters during this particular visit.

In addition, McGlynn and Mochizuki did make some surprising discoveries, though they didn’t involve microbes, electron transfer or viruses.

During a morning visit to a different hot spring, they came across a team of what turned out to be officials of the Izu islands – all dressed in suits and ties. They were visiting Shikine-jima as part of a series of joint islands visit to assess economic development opportunities.

The officials were intrigued to learn what the scientists were up to, and made some suggestions of other spots to sample. One was an island occupied by Japanese self-defense forces and generally closed to outsiders. But the island is known to have areas of extremely hot water just below the surface of the land, sometimes up to 100 C (212 F.)

The officials gave their cards and told the scientists to contact them if they wanted to get onto that island for sampling. And as for the official from Shikine-jima, he was already thinking big.

“It would be a very good thing,” he said, “if you found the origin of life on our island.

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Could High-Energy Radiation Have Played an Important Role in Getting Earth Ready For Life?

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A version of this article first appeared in Astrobiology Magazine, www.astrobio.net.

The fossil remains of a natural nuclear reactor in Oklo, Gabon.  It entered a fission state some 2 billion years ago, and so would not have been involved in any origin of life scenario.  But is a proof of concept that these natural reactors have existed and some were widespread on earth Earth.  It is but one possible source of high energy particles on early Earth. The yellow rock is uranium oxide. (Robert D. Loss, Curtin University, Australia)

Life on early Earth seems to have begun with a paradox: while life needs water as a solvent, the essential chemical backbones of early life-forming molecules fall apart in water. Our universal solvent, it turns out, can be extremely corrosive.

Some have pointed to this paradox as a sign that life, or the precursor of life, originated elsewhere and was delivered here via comets or meteorites. Others have looked for solvents that could have the necessary qualities of water without that bond-breaking corrosiveness.

In recent years the solvent often put forward as the eligible alternative to water is formamide, a clear and moderately irritating liquid consisting of hydrogen, carbon, nitrogen and oxygen. Unlike water, it does not break down the long-chain molecules needed to form the nucleic acids and proteins that make up life’s key initial instruction manual, RNA. Meanwhile it also converts via other useful reactions into key compounds needed to make nucleic acids in the first place.

Although formamide is common in star-forming regions of space, scientists have struggled to find pathways for it to be prevalent, or even locally concentrated, on early Earth. In fact, it is hardly present on Earth today except as a synthetic chemical for companies.

New research presented by Zachary Adam, an earth scientist at Harvard University, and Masashi Aono, a complex systems scientist at Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology, has produced formamide by way of a surprising and reproducible pathway: bombardment with radioactive particles.

 

In a room fitted for cobalt-60 testing on the campus of the Tokyo Institute of Technology, a team of researchers gather around the (still covered) cobalt-60 and vials of the chemicals they were testing. The ELSI scientists are (from left) Masashi Aono,  James Cleaves, Zachary Adam and Riquin Yi.  (Isao Yoda)

The two and their colleagues exposed a mixture of two chemicals known to have existed on early Earth (hydrogen cyanide and aqueous acetonitrile) to the high-energy particles emitted from a cylinder of cobalt-60, an artificially produced radioactive isotope commonly used in cancer therapy. The result, they report, was the production of substantial amounts of formamide more quickly than earlier attempts by researchers using theoretical models and in laboratory settings.

It remains unclear whether early Earth had enough radioactive material in the right places to produce the chemical reactions that led to the formation of formamide. And even if the conditions were right, scientists cannot yet conclude that formamide played an important role in the origin of life.

Still, the new research furthers the evidence of the possible role of alternative solvents and presents a differing picture of the basis of life. Furthermore, it is suggestive of processes that might be at work on other exoplanets as well – where solvents other than water could, with energy supplied by radioactive sources, provide the necessary setting for simple compounds to be transformed into far more complex building blocks.

Formamide is a clear liquid which is miscible with water and has an ammonia-like odor.

“Imagine that water-based life was preceded by completely unique networks of interacting molecules that approximated, but were distinct from and followed different chemical rules, than life as we know it,” said Adam.

Their work was presented at recent gatherings of the International Society for the Study of the Origin of Life, and the Astrobiology Science Conference.

The team of Adam and Aono are hardly the first to put forward the formamide hypothesis as a solution to the water paradox, and they are also not the first to posit a role for high-energy, radioactive particles in the origin of life.

An Italian team led by Rafaelle Saladino of Tuscia University recently proposed formamide as a chemical that would supply necessary elements for life and would avoid the ‘water paradox.’ Since the time that Marie Curie described the phenomenon of radioactivity, scientists have proposed innumerable ways that the emission of particle-shedding atomic nuclei might have played roles, either large or small, in initiating life on Earth.

Merging the science of formamide and radioactivity, as Adam and Aono have done, is a potentially significant step forward, though one that needs deeper study.

“If we have formamide as a solvent, those precursor molecules can be kept stable, a kind of cradle to preserve very interesting products,” said Aono, who has moved to Tokyo-based Keio University while remaining a fellow at ELSI.

Aono and technician Isao Yoda in the radiation room with the cobalt-60 safely tucked away. (Nerissa Escanlar.)

The experiment with cobalt-60 did not begin as a search for a way to concentrate the production of formamide. Rather, Adam was looking more generally into the effects of gamma rays on a variety of molecules and solvents, while Aono was exploring radioactive sources for a role in the origin of life.

The two came together somewhat serendipitously at ELSI, an origins-of-life research center created by the Japanese government. ELSI was designed to be a place for scientists from around the world and from many different disciplines to tackle some of the notoriously difficult issues in origins of life research. At ELSI, Adam, who had been unable to secure sites to conduct laboratory tests in the United States, learned from Aono about a sparingly-used (and free) cobalt-60 lab; they promptly began collaborating.

It is well known that the early Earth was bombarded by high-energy cosmic particles and gamma rays. So is the fact that numerous elements (aluminum-26, iron-60, iodine-129) have existed as radioactive isotopes that can emit radiation for minutes to millennium, and that these isotopes were more common on early Earth than today. Indeed, the three listed above are now extinct on Earth, or nearly extinct, in their natural forms

Less known is the presence of “natural nuclear reactors” as sites where a high concentration of uranium in the presence of water has led to self-sustaining nuclear fission. Only one such spot has been found —in the Oklo region of the African nation of Gabon — where spent radioactive material was identified at 16 sites separate sites. Scientists ultimately concluded widespread natural nuclear reactions occurred in the region some 2 billion years ago.

That time frame would mean that the site would have been active well after life had begun on Earth, but it is a potential proof of concept of what could have existed elsewhere long before

Adam and Aono remain agnostic about where the formamide-producing radioactive particles came from. But they are convinced that it is entirely possible that such reactions took place and helped produce an environment where each of the backbone precursors of RNA could readily be found in close quarters.

Current scientific thinking about how formamide appeared on Earth focuses on limited arrival via asteroid impacts or through the concentration of the chemical in evaporated water-formamide mixtures in desert-like conditions. Adam acknowledges that the prevailing scientific consensus points to low amounts of formamide on early Earth.

“We are not trying to argue to the contrary,” he said, “but we are trying to say that it may not matter.”

If you have a unique place (or places) on the Earth creating significant amounts of formamide over a long period of time through radiolysis, then an opportunity exists for the onset of some unique chemistry that can support the production of essential precursor compounds for life, Adam said.

“So, the argument then shifts to— how likely was it that this unique place existed? We only need one special location on the entire planet to meet these circumstances,” he said.

Zachary Adam, an earth scientist in the lab of Andrew Knoll at Harvard University. (Nerissa Escanlar)

After that, the system set into motion would have the ability to bring together the chemical building blocks of life.

“That’s the possibility that we look forward to investigating in the coming years,” Adam said.

James Cleaves, an organic chemist also at ELSI and a co-author of the cobalt-60 paper, said while production of formamide from much simpler compounds represents progress, “there are no silver bullets in origin of life work. We collect facts like these, and then see where they lead.”

Another member of the cobalt-60 team is Albert Fahrenbach, a former postdoc in the lab of Harvard University’s Nobel laureate Jack Szostak and now an associate principal investigator at ELSI.

An organic chemist, Fahrenbach was a late-comer to the project, brought in because Cleaves thought the project could use his expertise.

“Connecting the origins of life, or precursors chemicals, with radiolysis (or radioactivty) was an active field back in the 70s and 80s,” he said. “Then it pretty much died out and went out of fashion.”

Fahrenbach said he remains uncertain about any possible role for radiolysis in the origin of life story. But the experiment did intrigue him greatly, it led him to experiment with some of the chemicals formed by the gamma ray blasts, and he says the results have been productive.

“Without this experiment, I would definitely not be going down some very interesting paths,” he said

 

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One Planet, But Many Different Earths

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Artist conception of early Earth. (NASA/JPL-Caltech)
Artist conception of early Earth. (NASA/JPL-Caltech)

We all know that life has not been found so far on any planet beyond Earth — at least not yet.  This lack of discovery of extraterrestrial life has long been used as a knock on the field of astrobiology and has sometimes been put forward as a measure of Earth’s uniqueness.

But the more recent explosion in exoplanet discoveries and the next-stage efforts to characterize their atmospheres and determine their habitability has led to rethinking about how to understand the lessons of life of Earth.

Because when seen from the perspective of scientists working to understand what might constitute an exoplanet that can sustain life,  Earth is a frequent model but hardly a stationary or singular one.  Rather, our 4.5 billion year history — and especially the almost four billion years when life is believed to have been present  — tells many different stories.

For example, our atmosphere is now oxygen-rich, but for billions of years had very little of that compound most associated with complex life.  And yet life existed.

The same with temperature.  Earth went through snowball or slushball periods when most of the planet’s surface was frozen over.  Hardly a good candidate for life, and yet the planet remained habitable and inhabited.

And in its early days, Earth had a very weak magnetic field and was receiving only 70 to 80 percent as much energy from the sun as it does today.  Yet it supported life.

“It’s often said that there’s an N of one in terms of life detected in the universe,” that there is but one example, said Timothy Lyons, a biogeochemist and distinguished professor at University of California, Riverside.

“But when you look at the conditions on Earth over billion of years, it’s pretty clear that the planet had very different kinds of atmospheres and oceans, very different climate regimes, very different luminosity coming from the sun.  Yet we know there was life under all those very different conditions.

“It’s one planet, but it’s silly to think of it as one planetary regime. Each of our past chapters is a potential exoplanet.”

 

A rendering of the theorized "Snowball Earth" period when, for millions of years, the Earth was entirely or largely covered by ice, stretching from the poles to the tropics. This freezing happened over 650 million years ago in the Pre-Cambrian, though it's now thought that there may have been more than one of these global glaciations. They varied in duration and extent but during a full-on snowball event, life could only cling on in ice-free refuges, or where sunlight managed to penetrate through the ice to allow photosynthesis.
A particularly extreme phase of our planet’s history is called  the “Snowball Earth” period.  During these episodes, the Earth’s surface was entirely or largely covered by ice for millions of years, stretching from the poles to the tropics. One such freezing happened over 700 to 800 million years ago in the Pre-Cambrian, around the time that animals appeared. Others are now thought to have occurred much further back in time. They varied in duration and extent but during a full-on snowball event, life could only survive in ice-free refuges, or where sunlight managed to penetrate through the ice to allow photosynthesis.

Lyons is the principal investigator for one of the newer science teams selected to join the NASA Astrobiology Institute (NAI), an interdisciplinary group hat calls itself “Alternative Earths.”

Consisting of 23 scientists from 14 institutions, its self-described mission is to address and answer these questions: How has Earth remained persistently inhabited through most of its highly changeable history?  How has the presence of very different kinds of lifeforms been manifested in the atmosphere, and simultaneously been captured in what would become the rock record? And how might this approach to early Earth help in the search for life beyond Earth?

“The idea that early Earth can help us understand other planets and moons, especially in our solar system, is certainly not new,” Lyons said.  “Scientists have studied possible Mars analogues and extreme life for years.  But we’re taking it to the next level with exoplanets, and pushing hard on the many ways that conditions on early Earth can help us study exoplanet atmospheres and habitability.”

The importance of this work was apparent at a recent workshop on biosignatures held by NASA’s initiative, the Nexus for Exoplanet System Science (NExSS.)  As Earth scientists, Lyons and his group are expert at finding proxy records in ancient rocks that hold information important to exoplanet scientists (among others) want to know.

Those proxy fingerprints occur as elemental, molecular, and isotopic properties preserved in rocks that correspond to ancient characteristics in the ocean or atmosphere that can no longer be observed directly.

“We can’t measure the pH in ancient oceans, and we can’t measure the composition of ancient atmospheres,” Lyons said.  “So what we have to do is go to the chemistry of ocean and land deposits formed at the same time and look for the chemical fingerprints locked away and preserved.”

At the exoplanet biosignatures workshop, Lyons was struck by how eager exoplanet modelers were to learn about the proxy chemicals they could profitably put in their models for clues about how distant planet atmospheres might form and behave.  It’s clear that no single element or compound will be a silver bullet for understanding whether there’s life on an exoplanet, but a variety of proxy results together can begin to tell an important story.

chromium
The element chromium and its isotopes have become important proxies for the measurement of oxygen levels in the atmosphere of early Earth and have led to some revised theories about when those concentrations jumped.  Understanding the potential makeup of early Earth’s atmosphere and oceans is a pathway to understanding exoplanets.

“We told them about the range of things they should be modeling and, wow, they were interested.  I was thinking at the time that ‘you guys really need us — and vice versa.'”

Some of the researchers most intrigued by potentially new geochemical proxies from the University of Washington’s Virtual Planetary Laboratory,  They’ve been a pioneer in modeling how different atmospheric, geological, stellar and other factors characterize particular kinds of planets and solar systems and their possibilities for life.

In keeping with the growing connection between exoplanet and Earth science, Lyons just brought one of the VPL top modellers, Edward Schwieterman, to UC Riverside for a postdoc as part of the Alternative Earths project.

Among his initial projects will take the new data being generated by the Alternative Earths team about the atmosphere and oceans of early Earth, and model what would happen on a planet with that kind of atmosphere if it was orbiting a very different type of star from our own.

“It’s a direct use of early Earth research on exoplanet studies, and is exactly the kind of work we plan to do be doing,” Lyons said. “Eddie is the perfect bridge between the lessons learned from early Earth and their implications for exoplanets.”

Banded iron formations Karijini National Park, Western Australia. The layers of reddish iron show the presence of oxygen, which bonded with the iron to form a rust-like iron oxide. These formations date most commonly from the period of 2.4 to 1.9 billion years ago, after the Great Oxidation Event.
Banded iron formations at Karijini National Park, Western Australia. The layers of reddish iron point to an early ocean poor in oxygen and rich in dissolved iron. These formations date most commonly from the periods just before and right after the Great Oxidation Event, which spanned from about 2.4 to 2.0 billion years ago. Their distributions over times and their chemical properties are key proxies for the tempo and fabric of the earliest permanent oxygenation of Earth’s atmosphere.

Lyons, along with colleagues Christopher Reinhard of Georgia Tech and Noah Planavsky of Yale and other members of their Alternative Earths team, are especially focused on an effort to understand Earth’s atmosphere—as tracked in the rock record—over the eons and especially the levels of oxygen present.

The concentration of oxygen in the atmosphere is now about about 21 percent and, by some estimates, reached as high as 35 percent within the past 500 million years.

In comparison, early Earth had but trace amounts of oxygen for two billion years before what is called the Great Oxidation Event—when marine O2-producing photosynthesis outpaced reactions that consumed O2 and allowed for the beginnings of its permanent accumulation in the atmosphere.  Estimated to have occurred 2.4 billion years ago, it began (or was part of) an oxidizing process that led to ever more complex life forms over the following one to two billion years.

Timothy Lyons, distinguished professor of geobiochemistry at the University of California, Riverside. He is also the principal investigator of a National Astrobiology Institute project xxx.
Timothy Lyons, distinguished professor of biogeochemistry at the University of California, Riverside. He is also the principal investigator of a National Astrobiology Institute project “Alternative Earths:  Explaining Persistent Inhabitation on a Dynamic Early Earth.

There is a spirited scientific debate underway now about whether that “Great Oxidation Event” triggered permanently high levels of oxygen in the atmosphere and the oceans, or whether it began an up and down process through which the presence of oxygen was quite unstable and still well below current levels until relatively recent times.

Lyons and Reinhard are of the “boring billion” school, arguing that oxygen levels did not head continuously upwards after the Oxidation Event, but rather stayed relatively stable and still very low for most of a billion and half years after the Great Oxidation Event and continued to challenge O2-requiring life—for almost a third of Earth history.

This would be primarily an Earth science issue if not for the fact that oxygen — on its own and in conjunction with other compounds — is among the most prominent and promising biosignatures that exoplanet scientists are looking for.

Christopher Reinhard, Georgia Institute of technology. (Brad...
Christopher Reinhard of Georgia Institute of Technology and Alternative Earths project. (Ben Brumfield/Georgia Tech.)

In fact, not that long ago, it was widely accepted that a discovery of oxygen and/or ozone in the atmosphere of a planet pretty much proved, or at least strongly suggested, the presence of some sort of biology on the planet below.  That view has been modified of late by the identification of ways that free oxygen can be formed abiotically (without the presence of photosynthesis and life), potentially producing false positives for potential life.

While the field is a long way from an active search for direct, in situ fingerprints of life on exoplanets light years away, oxygen and its relationship with other atmospheric gases remains a lodestar in thinking about what biosignatures to search for. The technology is already in place for characterizing the compositions of very distant atmospheres.

And this is where, for Lyons, Reinhard and others, things get both interesting and complicated.

For more than a billion years before the Great Oxidation Event Earth demonstrably supported life.  It consisted mostly of anaerobic microbes that did just fine without oxygen, but in many cases needed and produced methane, an organic compound with one carbon atom and four hydrogens.

So if an exoplanet scientist from a distant world were to search for life on Earth during that period via the detection of oxygen only, they would entirely miss the presence of an already long history of life.  Searching for a potentially large-scale presence of methane might have been more productive, though that is a source of rigorous debate as well.

 

An image of a rock with fossilized stromatolites, tiny layered structures from 3.7 billion years ago that are remnants from a community of microbes. Found in a newly melted part of Greenland, Australian scientists reported in the journal Nature that the stromatolites lived on an ancient seafloor at a time when Earth's skies were orange and its oceans green. They describe the stromatolites as perhaps the oldest fossil found so far on Earth. (Allen Nutman/University of Wollongong)
An image of a rock with fossilized stromatolites, tiny layered structures from 3.7 billion years ago that are remnants from a community of microbes. Found in a part of Greenland new exposed by melting glaciers, Australian scientists reported in the journal Nature that the stromatolites lived on an ancient seafloor at a time when Earth’s skies may well have been orange and its oceans green. They describe the stromatolites as perhaps the oldest fossil found so far on Earth, although chemical suggestions of life may extend further back in time . (Allen Nutman/University of Wollongong)

Because both oxygen and methane can be formed without life, a current gold standard for detecting future biosignatures on exoplanets is the presence of the two together.  As a result of the way the two interact, they would remain in an atmosphere together only if both were being replenished on a substantial, on-going scale.  And as far as is now understood, the only way to do that is through biology.

Yet as described by Reinhard, the most current research suggest that oxygen and methane were probably never in the Earth’s atmosphere together at levels that would be detectable from afar.  There is some evidence that Earth’s atmosphere held a lot of methane in its early times, and there has been a lot of oxygen for the past 600 million years or so, but as one grew in concentration the other declined — and during the “boring billion” both were likely low.

“So we have a complicated situation here where using the best exoplanet biosignatures we have now, intelligent beings looking at Earth over the past 4.5 billion years would not find a convincing signature of life for most, or maybe all, of that time if they relied only on co-occurrence of oxygen and methane,” Reinhard said.  Yet there has been life for at least 3.7 billion years, and those beings studying Earth would have come up with a very false negative.

Lyons insists this is should not be a source of pessimism in the search for life on exoplanets, instead it is a “call to arms for new and more creative possibilities rather than the lowest hanging fruit.” It’s a challenge “to help us sharpen our thinking in a search that was never going to be easy.”

And the best test bed available for coming up with different answers, he said, may very well be the many different Earths that have come and gone on our planet.

 

 

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