This story was written by Nicholas Siegler, Chief Technologist for NASA’s Exoplanet Exploration Program at the Jet Propulsion Laboratory with the help of doctoral student Markus Gogouvitis, at the University of New South Wales, Australia and Georg-August-University in Gottingen, Germany.
This past July I joined a group of geologists, geochemists, microbiologists, and fellow astronomers on a tour of some of the best-preserved evidence for early life.
Entitled the Astrobiology Grand Tour, it was a trip led by Dr. Martin Van Kranendonk, a structural geologist from the University of New South Wales, who had spent more than 25 years surveying Australia’s Pilbara region. Along with his graduate students he had organized a ten-day excursion deep into the outback of Western Australia to visit some of astrobiology’s most renowned sites.
The trip would entail long, hot days of hiking through unmaintained trails on loose surface rocks covered by barb-like bushes called spinifex. As I was to find out, nature was not going to give up its secrets easily. And there were no special privileges allocated to astrophysicists from New Jersey.
The state of Western Australia, almost four times the size of the American state of Texas but with less than a tenth of the population (2.6 million), is the site of many of astrobiology’s most heralded sites. For more than three billion years, it has been one of the most stable geologic regions in the world.
It has been ideal for geological preservation due to its arid conditions, lack of tectonic movement, and remoteness. The rock records have in many places survived and are now able to tell their stories (to those who know how to listen).
Our trip began with what felt like a pilgrimage. We left Western Australia’s largest city Perth and headed north for Shark bsy. It felt a bit like a pilgrimage because the next morning we visited one of modern astrobiology’s highlights – the living stromatolites of Shark Bay.
Stromatolites literally mean “layered rocks”. It’s not the rocks that are alive but rather the community of microbial mats living on top. They are some of the Earth’s earliest ecosystems.
We gazed over these living microbial communities aloft on their rock perches and marveled at their exceptional longevity — the species has persisted for over three billion years. Their ancestors had survived global mass extinctions, planet-covering ice glaciers, volcanic activity, and all sorts of predators. Once these life forms took hold they were not going to let go.
The photosynthetic bacteria that built ancient stromatolites played a central role of our trip for three reasons:
- Their geological footprints allowed scientists to date the evolution of early life and at times gain insight into the environments in which they grew.
- They eventually harbored the first oxygen-producing bacteria and played a central role in creating our oxygen-rich atmosphere.
- By locating ever-increasingly older microbial fossils we observed a lower limit to the age of the first life forms.. Given photosynthesis is not a simple process, the first life forms must have been simpler. Speculating, perhaps a few hundred million years earlier so that the first life form on Earth may have originated at four billion years ago.
When viewed under a microscope, you can see the mats are made of millions of single cell bacteria and archaea, among the simplest life forms we know. Within these relatively thin regions are multiple layers of specialized microbial communities that live interdependently.
Bacteria in the top layer evolved to harvest sunlight to live and grow via photosynthesis. Their waste products include oxygen as well as important nutrients for many different bacterial species within underlying layers. And this underlying layer’s waste product would do the same for the layer beneath it, perfectly recycling each other’s waste. The oldest forms of life that we know of had learned to co-exist together in a chemically interdependent environment.
We saw ripped up portions of the mats that washed upon the shore at Hamelin pool in Shark Bay. A whole ecosystem held in one’s hand. Thousands of millions of years ago ancient relatives of these microbes thrived in shallow waters all around our planet, and left behind fossilized remains. But due to the evolution of grazing organisms these microbial structures are nowadays constrained to very specific environments. In the case of Shark Bay, the very high salt contents of this inlet have warded off most predators providing the microbes with a safe haven to live.
Ironically, the rocks, which help identify these ancient life forms, at the time were just a nuisance for the living microbes.
Small fine grains of sedimentary rock carried along in the daily tides would occasionally get stuck in the sticky mucus the microbes would secrete. In addition, the photosynthetic bacteria found at Shark Bay may have been inadvertently making their own rock by depleting the carbon dioxide in the surrounding water as part of photosynthesis and precipitating carbonate, adding to the grains of sediment trapped within the sticky top layer.
Over time, the grains from both the sedimentary and precipitated rocks would cover the surface and block the sunlight for which these organisms had evolved to depend on. As an evolutionary tour de force, the photosynthetic microbes learned to migrate upward, leaving the newly formed rock layers behind.
These secondary rock fossils today showcase visually observable crinkly, frequently conical shapes, in stark contrast to abiotic sedimentary rocks. These ancient life forms left behind geological footprints reminding us they were here first.
Now to the most important contribution of stromatolites – terraforming the Earth.
Living in shallow water, the top most layer of the Shark Bay microbial mats are known to host cyanobacteria, photosynthetic bacteria that produce oxygen as a byproduct. Scientists don’t know what the first bacteria produced as they harnessed the energy of the Sun. But they do know that they eventually started producing oxygen.
In the evolution of life that eventually led to all plants and animals, this was one of the great events. More than 2.5 billion years ago, ancient bacteria began diligently producing oxygen in the oceans. Earth’s atmosphere began to irreversibly shift from its original, oxygen-free existence, to an oxic one, initiating the formation of our ozone layer and paving the way for the evolution of more complex life. Our planet has been terraformed by micro-organisms!
It was in the Karijini National Park where we went back in time (2.4 billion years) and observed an extraordinary piece of evidence for the early production of oxygen in Earth’s oceans, a time before oxygen made a strong presence in our atmosphere.
We saw a massive gorge with steep vertical walls carved out by flowing water. As oxygen production by early bacteria increased below the water surface it would react with dissolved iron ions (early oceans were iron-rich) causing iron oxides to precipitate and settle to the bottom.
For reasons not entirely understood — perhaps related to seasonal or temperature effects– the amount of new oxygen temporarily decreased and iron ion remained soluble in the oceans and other types of sediments accumulated, carbonates, slate, and shale. And then, just as before, the oxygen reappeared creating a new layer of precipitated iron.
The result was a banded sedimentary rock, a litmus test to a changing world, where oxygen would be the reactive ingredient leading to larger and more complex life forms. As the oxygen production no longer cycled, the oxygen went on to saturate the ocean and then accumulated in the Earth’s atmosphere eventually to the levels we have today.
After a day of looking down at rocks and spinifex it was both a relief and a joy to look up at the glorious Western Australian night sky. Far away from the light pollution of modern cities, each night would greet us with an awe-inspiring starlit sky. It never got old to remember we are part of a vast network of stars suspended in an infinite space.
The nights would start with the appearance of Venus well before sundown followed shortly by the innermost planet Mercury and then Jupiter and Saturn. It didn’t take long after sunset to see the renowned Southern Cross. Mars joined the evening as well, perfectly appearing on the arc called the ecliptic.
But nothing stirred the group more than the emergence of the swath of stars of the Milky Way, the disk of our home galaxy where its spiral arms all lie. The nights would be so clear that one could actually see the dark clouds of gas and dust that block large portions of the galaxy’s stars from shining through. We partook in the well-known tradition connecting individual points of light to form exotic creatures like scorpions and centaurs. But we also we followed the inverted approach of the Aborigines and connected the dark patches. Only then did we see the emu of the Milky Way. I would never have thought of connecting the darkness.
The night sky appeared even more special knowing that each of its stellar members likely host planetary systems like our own. How many of them host life? Maybe even civilizations? The numbers are in their favor.
At the half-way point of our trip we hiked to an ancient granite region in the red rocks of the Pilbara which contain the world’s largest concentration of Pleistocene rock art also known as petroglyphs. These etchings are believed to be 6,000 to 20, 000 years old.
The artists used no pigments, but rather rocks to pound/chisel shapes into the desert varnish, a thin dark film (possibly of microbial origin) that typically covers exposed rock surfaces in hyper arid regions. We came across many stylized male and female figures with highlighted genitalia as well as animals such as emus and kangaroos. Little is known about the people who created these art works. They left no clues to their origin or fate.
Pilbara is also where the oldest mineral on Earth –a zircon dated at 4.4 billion years old — was discovered four years ago in the Jack Hills region. Because of the geological history of the region, it is a frequent (if hardscrabble) site where many geologists and geochemists specializing in ancient Earth do their work.
In the last several days of the tour we encountered ever-increasing older evidence of stromatolites extending out to circa 3.5 billion years, about 75% of the history of the Earth. I expected the quality of the stromatolites to degrade as we went back in time and it looked like I was right until I saw a remarkably large rock in a locality called the Strelley Pool Formation. The rock measuring approximately 1.5 meters in all three directions gave a rare view of ancient stromatolites from all sides and an unequivocal interpretation of past life.
The shapes of the embedded rocks formed by the microbial mats from the top view clearly show the elliptical areas where the bacteria inched upwards to acquire sunlight. Regions between the conical stromatolites were filled in by carbonate sediments in ancient shallow waters. These were later chemically altered to silica-rich rocks through alteration and etching of minerals by fluids. Silicified rocks are very weather-resistant, making them a great medium to preserve fossils for billions of years.
The side views of the stromatolite-laden rock revealed the expected conical layered shapes we saw in younger rocks (and in the living stromatolites of Shark Bay). Everything we had learned about stromatolite structures was clearly visible in this circa 3.43 billion year old example. It is astounding to realize that complex phototrophic (light-eating) organisms, even if not yet oxygen producing, were around during the deposition of the Strelley Pool Formation.
It is not unreasonable to speculate that the earliest life forms are even older by perhaps a few more hundred million years or so. There is evidence for even more ancient stromatolites in Greenland (3.7 billion years old) and isotope carbon evidence, with considerable controversy, in Nuvvuagittuq greenstone belt in northern Quebec, Canada (4.28 billion years old). Hence, life on Earth may have emerged within 500 million years from its formation. That is astonishingly rapid.
Was Earth an exception or the rule? What does that say for possible life on exoplanets?
Our tour came to an end on July 11. We had traveled over 1,600 miles through Australia’s outback, from Western Australia’s biggest city Perth, all the way up to Port Hedland at the north coast. We were privileged to see the country in ways that very few people get a chance to, and to be steeped in the multidisciplinary sciences of astrobiology while seeing some of its iconic ground.
I had seen some of the earliest evidence for life and the pivotal effect it had on our environment. For those 10 days I learned what it was like to be a time traveler.