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Picture:Seth Shostak
Picture:Seth Shostak

Older than the hills

by Seth Shostak


It’s a mystery that has baffled a generation of experts, because the clues are indescribably old and maddeningly ambiguous. At what point did the vital phenomenon we call life first appear, changing our planet from an anonymous rock to an inhabited home? Scientists are betting that the answer lies in the fly-blown outback of northwest Australia.

Imagine stepping into a time machine that could take you back to those yellowed days when Earth was only one-fourth as old as now, a countermarch of 3.5 billion years. If you could take a ride in such a machine, you would be transported to our planet’s distant youth, an era when the world was both strange and lethal.

The familiar geography of today, a blue globe tiled by large, continental plates, themselves carpeted by greenery and animals, would still be only a tentative dream. The young Earth offered none of this; it was a water world, the gray expanse of its tepid oceans broken only by scattered chains of volcanoes where molten rock from the mantle squirted through our planet’s liquid skin like hot toothpaste. Volcanoes were slowly, and haphazardly building up rocky masses that, in the slow grind of time, would eventually become continents.

There was little sound: no insect hum, no chirping birds. Not even the rhythmic whoosh of leaves. Only the naked wind broke the silence. The atmosphere was oppressively warm and replete with carbon dioxide. Oxygen levels were so low, you could suffocate in seconds.

This strange and hostile place – in many ways as alien as another planet – was our Earth during the so-called Archaean period, a more-than-billion-year long adolescence during which the world witnessed dramatic change. The continents formed, and biology began.

The Archaean was Earth’s second chapter. It was preceded by the literally hellish Hadean period, which was ushered in with the solar system’s traumatic birth, 4.6 billion years ago. Soon thereafter, a cataclysmic asteroid impact wrenched the moon from our planet’s still-soft crust. For nearly a billion years, as the Hadean wore on, a thunderous hail of rocks from space pitted the landscape. And then, rather quickly (at least, as judged by the lavish timescales of geology), the cosmic weather cleared. The rain of rocks – the left-over debris from the solar system’s beginnings – abated. The Earth, formed and formative, was able to stumble into its florid destiny without the constant havoc of projectiles from space. The Archaean period had begun. And with it, came life.

Our imaginary time machine has taken us back 3.5 billion years, only three hundred million years after the Archaean’s start. We have dialed in this particular temporal destination because this is the epoch during which paleontologists think biology may have taken its first, tiny swims. When, exactly, the subtle gels of life first appeared is, of course, an important question to specialists – but also one that has profound implications for those who search for life elsewhere in the cosmos. If life was already extant and extensive 3.5 billion years ago – if our planet had inhabitants, even simple microscopic inhabitants, then we might infer that life is a very common phenomenon. After all, if it arose so shortly after our world became tranquil – a period of time that is no more than a sigh in a planet’s history – then biology would seem less likely to be a miraculous occurrence. If Nature could so quickly conjure up life on Earth, then perhaps something similar has happened on Mars; and if not Mars, then on some of the hundreds of billions of other rocky planets we believe are strewn across the Milky Way galaxy.

So the question of precisely when life appeared on Earth is more than a technical point for a few bespectacled academics. Knowing the answer might give an important hint as to whether our universe is filled with inhabited worlds, or whether biology is no more than occasional foam on the empty seas of space. In addition, if we should prove unable to decipher the earliest fossil record of our own planet, what chance do we have of learning whether Mars or any other world hosted life long ago? We are investing billions of dollars in robotic hardware tossed towards the Red Planet, but this is likely to be a futile exercise if we’re incapable of unambiguously separating biology from geology.

Without doubt, knowing when the first earthlings arose is an important palaeontology puzzle. And critical clues are thought to lie in a remote corner of Australia, a thousand miles north of Perth; a patch of nearly uninhabited real estate known as the Pilbara. It’s scrub country, dry and covered in iron-red powder, as if someone had collected the world’s rust, ground it up, and dumped it over the desert. The landscape is bleak, only occasionally relieved by lonely stands of eucalyptus. Spinifex – the Australian equivalent of sagebrush – is everywhere; an aggressive plant with the appearance, spikes, and intent-to-do-harm of squat, faint-green porcupines.

The Pilbara would seem to be an unremarkable place for palaeontology except for a fortuitous, and singular circumstance: here are exposed well-preserved rocks from Earth’s earliest times. There are places – such as Isua, Greenland – where older rocks can be found, rocks dating well into the Hadean. But these have been melted and twisted, “cooked” as the geologists like to say, by the turmoil of a hot, young crust. Any record of ancient life they might contain has been irredeemably mutilated.

The Pilbara, on the other hand, sits atop an unusually thick continental nub, or craton. The craton formed because a “hot spot” lurks below, a thin place in our planet’s skin. Billions of years ago, molten rock from the semi-liquid mantle sundered the crust and percolated to the surface, forming a rocky pimple on the young Earth’s watery face. The mass of effused rock grew to be 30 miles deep, top to bottom, far thicker than most continents.

Thickness is a good defense against the tectonic forces that often destroy the geological record. Most large bodies of land, urged on by continental drift, slide over oceanic crust at their margins, causing the crust to subduct back into the mantle. As an example, the west coast of the Americas is riding over ocean crust at the rate of a half-inch or so a year, deflecting the floor of the Pacific downward where it can heat up, melt, and routinely break to the surface in volcanic eruptions. But the Pilbara craton, a deep wall of rock, is simply too massive for oceanic crust to subduct. Consequently, the old volcanic rock and the bedded layers formed in watery inlets during the Pilbara’s distant past – the mute records of what was alive in the sea at that time – were spared tectonic destruction.

Of course, ancient rocks, even if undisturbed, are hard to examine if they’re miles below the landscape, as they normally would be. Fortunately, the Pilbara hot spot has also oozed hot granite in relatively recent times, and this rocky eruction tilted up the Archaean sedimentary layers in the way that a flat metal lid of a tin can would be ruptured and bent upwards if its contents were set to boil. Today, the lifted layers from 3 billion years ago march like ruined Roman walls up and down the hills of the Pilbara, baring the hoary past to anyone who cares to look. Sure, countless complex animals – from rough-scaled reptiles to mighty mammals – have pawed and clawed their way over these lands since. But by the time of their arrival, the sea level had fallen, and their flesh and bones withered away on dry land without being fossilized.

“Frankly, there are rocks in South Africa, in the Barberton Mountains, that are as old as these,” notes Malcolm Walter, a professor of geology and the director of Australia’s Centre for Astrobiology in Sydney. “But there’s little doubt that the best record we have of these truly ancient times – the best preserved rock – is in northwest Australia, in the Pilbara. When you walk this landscape, everything is the same as it was more than three billion years ago – aside from the vegetation, of course.”

Clearly, if you’re searching for the preserved remains of Earth’s earliest inhabitants, then the Pilbara is the place to look.


Recognising Early Life

But what should we look for? What would the oldest life on our planet be like? Unlike the dinosaurs and their beastly kin, the primeval organisms of Earth had no bones, teeth, horns, shells, or other durable body parts that could easily fossilize. Life was microbial, and consisted of single-celled entities both small and perishable. Indeed, one might suspect that finding remnants of such fragile creatures would be as likely as discovering fleas from the age of the dinosaurs.

However, we occasionally do find Jurassic insects, inadvertently mummified in hardened tree resin turned to amber. There was no tree sap 3.5 billion years ago – there were no trees. But geologists have another, older preservative: a fine-crystallized rock called chert.

Chert forms when silicon dioxide, a sandy slurry weathered from rocks by the ocean’s slight acidity, accumulates on the sea floor and hardens over time. If there is life in the oceans, even delicate life, it can be fossilized and preserved in the resulting sedimentary layer.

“Chert is the amber of the Precambrian,” says Arizona geologist Paul Knauth. It is chert that holds clues to the small and cryptic life that silently swirled in warm seas for a trillion days before the Cambrian explosion detonated, 540 million years ago. It was only after the Cambrian explosion that multicellular organisms, the kind that can turn into unambiguous fossils, made the scene.

What could chert tell us about this early, fragile life? In the late 1980s, various researchers, including Walter, found what they believed to be actual fossilized microbes – individual organisms – dating from 3.5 billion years ago in the chert beds of the Pilbara. These would have set the goal post of life back nearly a billion years from its previous “first appearance” prior to this work. If these were truly tiny remnants of life, then biology had begun only a couple of hundred million years after that time when the terrestrial landscape became relatively benign, when the devastating rain of celestial rocks ended. But the claimed microfossils garnered a skeptical reaction in the professional community (although Walter remains convinced they are real). Were they fossils or something else? The contention began anew a few years later when Bonnie Packer, a student of palaeontologist William Schopf of the University of California, Los Angeles, made another discovery in the Pilbara, once again of individual, fossilized microbes. Seen under the microscope, these putative relics of our most distant ancestors appeared as carbonaceous threads, somewhat reminiscent of Thomas Edison’s early light bulb filaments.

Such “squiggly wrinkles” were hard to find, and hard to confirm. In addition, many researchers weren’t convinced that they were really microbial fossils at all: perhaps the twisted threads were simply non-living, or abiotic, structures, produced as the result of natural geologic processes. Another worry was that the chert sample from which Schopf’s student had taken her fossils might have been fractured and infused with water that flushed younger remains into older rock.

Although Schopf remained steadfast, some geologists and biologists felt uneasy. Was there another way to prove that life really was around 3.5 billion years ago – assuming it was?

The researchers were aware that, while small, fragile individual organisms will only occasionally be entombed, larger items have a better chance. But in a world where all life is microscopic, how large can any fossil be?

As it turns out, it can be as large as a watermelon, or larger. Such an out-sized remnant of ancient life is known as a stromatolite, a conical or dome-shaped structure built by bacteria as a home in shallow waters. Stromatolites are analogous to coral reefs – big, non-living structures patiently constructed by tiny creatures.

We know about stromatolites because they’re still around today; the best examples are in the tidal waters of Australia’s Shark Bay and the steamy pools of Wyoming’s Yellowstone Park. “Stroms” range in size from small mushrooms to tuffets large enough to sit on. But while these macroscopic artifacts of microscopic life are relatively rare now, they were breathlessly common billions of years ago, before single-celled grazers evolved that could feed on the tiny builders who made them. The pre-Cambrian fossil record, delicately inscribed in chert, is replete with stroms.

Frankly, the existence of stromatolite fossils is hardly controversial. But what is controversial – what raises the blood pressure of the researchers who try to nail down the era in which life began – is which are the oldest stromatolites?

You might think that the problem here is dating the rock layer in which the stroms are found. But that’s not the case. Using straightforward radioactive dating, scientists are able to peg the age of a sedimentary layer to a precision of roughly two million years, or an impressively accurate 0.06% for the older rocks. The problem is not with the clocks in the rocks, but with the fossils themselves. At some point as we go back in time, the fossil evidence becomes ambiguous. In rocks that are 2.7 billion years old, nearly all experts agree that fossilized stromatolites are present. But what about the tantalizing, but less-clear stromatolite-like forms that appear in rocks 3.5 billion years old?

Some say that these could just be the hardened remains of natural, and therefore abiotic, irregularities that built up on ancient sea floors. These skeptics hurry to point out that inferring the presence of life on the basis of these shadowy shapes is dicey. There are many ways to produce strange structures in sedimentary rock, after all, and not all involve biology. The fossilized skull of a Triceratops is pretty unambiguous; not so with the thirty-times older remains of a bacterial colony.

So the situation is this: Schopf claimed to have fossilized microbes 3.5 billion years old, but not everyone was convinced. If some other, corroborating evidence of life, equally ancient, could be found, then we would know something very important – namely, that life began early. Stromatolites are the obvious thing to look for. But the existence of stromatolites in the older Archaean layers has been contentious, too. Walter and others had described what looked like stromatolites in the Pilbara’s oldest rock beds as early as 1980. Once again, the professional community had raised a collective eyebrow.

In the face of this long-standing controversy, Malcolm Walter decided to organize a field trip for about two dozen geologists and biologists to look at the evidence first-hand and together.

“The claims and counter-claims have produced a lot of polarization,” Walter told these scientists as they assembled in Western Australia. “So I thought we’d bring this group to the Pilbara, and get a lot of eyes on the rocks.” Maybe consensus would emerge. As an astronomer, I didn’t have a dog in this fight; this was a matter for the geologists. But the matter of early earthlings was clearly important to my work in searching for extraterrestrial life, as it might give some clue as to how difficult it is for life to begin in the first place. Consequently, I looked forward to setting my own gaze to the ground.


In a World of Chert

The Pilbara hills are not easy. They’re far from just about everything: the closest air field, Port Hedland, is an industrial clot on Australia’s forlorn northwest coast just inside the Tropic of Capricorn. But even this modest metropolis is still 90 miles from the chert beds.

As a consequence, expedition members faced a long commute to work. In the cool of the mornings, two dozen of us would pour out of our economy-priced (read: spartan) Port Hedland motel rooms into four-wheel-drive vehicles for the long and bumpy ride inland. For the first hour, we quietly traversed a monotonous coastal plain, strewn with dead cattle and punctuated by termite mounds taller than a man. Heading south, we watched as the hills of the Pilbara crept slowly over the horizon. They were low, red, and invariably disfigured with pustules of spinifex. Before long we were on dirt tracks, bouncing through uninhabited country at less-than-walking speed. Eventually, we arrived at some “locality,” where the old rocks slice through hills like the plates of a Stegosaurus. Grabbing our cameras and water bottles, we stepped into the landscape.

It was climb and inspect, climb and inspect. By midday, the exertion in the heat, the maddening annoyance of the flies, and the passive-aggressive spinifex would inevitably make me ache for another line of work. Nonetheless, I noticed that the professional geologists hardly flinched. They were surprisingly at ease in the field, scampering atop the hills as casually as squirrels up a tree, decked out in the practical tools of their trade: a holstered rock hammer, a double-lens loupe on a lanyard, gloves to cushion a slip that might force their hand onto either sharp rock or a spinifex clump, and a saucer-brim hat to shield neck and face. As I gasped for breath upon reaching the summit of yet another rumpled hill, the geologists, magnifiers in hand, were inevitably already there, manhandling the outcrops and looking for tell-tale signs of long-dead life.

Those signs were in the shape of things. The normally flat bedded layers, or laminae, of exposed rock from 3.5 billion years ago were occasionally interrupted by bumps a few inches high – miniature hills. Were these simply a deformity built up with sediment because of some nugget of hard rock that had landed on the sea floor, much in the way a pearl accretes around a grain of sand? Or were these lumpy ripples the fossilized shells of structures built by some of Earth’s first inhabitants? To my own untrained eyes, they looked biological. And the more of them I saw, the more I felt convinced. But I’m an astronomer, not a palaeontologist, and I had to remind myself of how often an “obvious reading” of a strangely shaped galaxy turned out to be wrong. Reading shapes for understanding is, indeed, dicey.

In some cases, the tops, rather than the sides, of the laminae were exposed, revealing “cabbage heads” that were – possibly – stromatolites unpeeled from above. Day after day, we visited places like the Dresser Formation and the Trendall locality, named after the geologists that first found them. Sometimes we were near the dry, broad Shaw River, a yellow gash running through the Pilbara, and often we were in places where only fossil hunters, and occasional stray cattle, ever bothered to go. For reasons that elude me, it was invariably the case that the most interesting rocks were always at the tops of the hills. But in outcrop after outcrop, we saw things that, even to my untrained eyes, looked like structures that life had made.

The days were long. At 5:00 pm, when the declining tropical sun finally weakened, we would turn our vehicles in the direction of Port Hedland, looking forward to a slump into showers and soft beds. On other days, deep in the Pilbara, we either camped out or stayed in motels that would never make it into the guide books. On one occasion, my accommodation resembled a shipping container, dropped onto the landscape and crudely fitted with a door (but not plumbing). It was a hermetically sealed domicile that kept snakes out, and some mosquitoes in. Worn by the day’s activities, this absence of convenience hardly mattered: falling asleep was quick business.


Is It Life?

For the most part, the Australian experts were already convinced that the rocks of the Pilbara were sending us a clear message: Earth was riotous with life by the early Archaean. Martin van Kranendonk, of the Geological Survey of Western Australia, persuasively argued that the case for stromatolites in the oldest rocks was solid. He pointed out that the thin layers of sedimentary bedding found in these rocks come to an abrupt stop at the edges of the putative fossilized stromatolites, at the edges of the bumps. This seemed to rule out the “pearl” mechanism, where the layers would build over a bit of random rock. “These are not just natural mineral deposits,” he stated. “They’re stromatolites.”

His views were echoed by Abby Allwood, a doctoral researcher who’s spent months camping near the Shaw River. Her conclusions were straightforward: “It’s clear that there was diverse microbial life in the Pilbara by 3.4 billion years ago,” she told us. “There’s simply too much morphological evidence to explain it all away with abiotic causes.”

Nonetheless, and despite the field trip and the conviction of the Australian researchers, not everyone was so sure. Bruce Runnegar, a geologist who heads NASA’s Astrobiology Institute, admitted that the evidence is certainly intriguing, but the case, at least for him, was not yet closed.

“Lots of these people look at these things, and for them the rocks scream stromatolites,” he said with a smile, suggesting that some of his colleagues might be heeding a seductive, but possibly misleading siren call.

Others who were originally skeptical changed their minds. The expedition was making converts. In the end, a straw vote showed that the majority of the participants came away thinking that the structures in the rocks really were the remnants of a time when Earth had just begun down the yellow brick road that would lead, eventually, to us. They seemed convinced that the “frozen cabbages” trapped in the sediments were compelling evidence for life in its earliest stages, when the moon was still a new object, the Sun was only two-thirds as bright as now, and the days were hours shorter. They were willing to admit that the case for life 3.5 billion years ago was looking good.

Of course, questions of science aren’t settled by votes, straw or otherwise. They’re settled by overwhelming evidence. And the evidence for life in the primeval seas of the early Archaean, while strong, is still and simply not yet overwhelming. Getting more “eyes on the rocks”, while useful, wasn’t conclusive. As Paul Knauth repeatedly pointed out, when it comes to the Archaean “we weren’t there.” Indeed, we weren’t. Morphological evidence – based on appearance, rather than the more easily quantified evidence of isotopes and composition – is unavoidably ambiguous. As is so often the case in science, “more research is needed.”

As we winged back to Perth, with the ancient, red landscape sliding quietly behind us, I knew that, for most of my companions, this was not a final trip to this rugged place. The truth was, for many, still tantalizingly out of reach. And if ambiguity was still in abundance, this much was sure: the Pilbara would continue to draw those for whom its worn, scabrous landscape remains the best time machine on Earth.

  • Dr Seth Shostak is Senior Astronomer at the SETI Institute in California

Video - Seth Shostak places the Pilbara in the context of the formation of the universe (22.6 MB). Click here for transcript.



Contents


Introduction

Context

Early Life

Evidence

Acknowledgements

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