The Earth is 4.6 billion years old, but a human lifetime often lasts for less than 100 years. So why care about the history of our planet when the distant past seems so inconsequential to everyday life? You see, as far as we can tell, Earth is the only planet in our solar system known to have sparked life, and the only system able to provide life support for human beings.
So why Earth? We know Earth is unique for having plate tectonics, liquid water on its surface and an oxygen-rich atmosphere. But this has not always been the case, and we know this because ancient rocks have recorded the pivotal moments in Earth's planetary evolution. And one of the best places to observe those ancient rocks is in the Pilbara of Western Australia. The rocks here are 3.5 billion years old, and they contain some of the oldest evidence for life on the planet.
Now, often when we think of early life, we might imagine a stegosaurus or maybe a fish crawling onto land. But the early life that I'm talking about is simple microscopic life, like bacteria. And their fossils are often preserved as layered rock structures, called stromatolites. This simple form of life is almost all we see in the fossil record for the first three billion years of life on Earth. Our species can only be traced back in the fossil record to a few hundred thousand years ago. We know from the fossil record, bacteria life had grabbed a strong foothold by about 3.5 to four billion years ago. The rocks older than this have been either destroyed or highly deformed through plate tectonics. So what remains a missing piece of the puzzle is exactly when and how life on Earth began. Here again is that ancient volcanic landscape in the Pilbara. Little did I know that our research here would provide another clue to that origin-of-life puzzle.
It was on my first field trip here, toward the end of a full, long week mapping project, that I came across something rather special. Now, what probably looks like a bunch of wrinkly old rocks are actually stromatolites. And at the center of this mound was a small, peculiar rock about the size of a child's hand. It took six months before we inspected this rock under a microscope, when one of my mentors at the time, Malcolm Walter, suggested the rock resembled geyserite. Geyserite is a rock type that only forms in and around the edges of hot spring pools.
Now, in order for you to understand the significance of geyserite, I need to take you back a couple of centuries. In 1871, in a letter to his friend Joseph Hooker, Charles Darwin suggested: "What if life started in some warm little pond with all sort of chemicals still ready to undergo more complex changes?"
Well, we know of warm little ponds. We call them "hot springs." In these environments, you have hot water dissolving minerals from the underlying rocks. This solution mixes with organic compounds and results in a kind of chemical factory, which researchers have shown can manufacture simple cellular structures that are the first steps toward life.
But 100 years after Darwin's letter, deep-sea hydrothermal vents, or hot vents, were discovered in the ocean. And these are also chemical factories. This one is located along the Tonga volcanic arc, 1,100 meters below sea level in the Pacific Ocean. The black smoke that you see billowing out of these chimneylike structures is also mineral-rich fluid, which is being fed off by bacteria. And since the discovery of these deep-sea vents, the favored scenario for an origin of life has been in the ocean. And this is for good reason: deep-sea vents are well-known in the ancient rock record, and it's thought that the early Earth had a global ocean and very little land surface. So the probability that deep-sea vents were abundant on the very early Earth fits well with an origin of life in the ocean.
However ... our research in the Pilbara provides and supports an alternative perspective. After three years, finally, we were able to show that, in fact, our little rock was geyserite. So this conclusion suggested not only did hot springs exist in our 3.5 billion-year-old volcano in the Pilbara, but it pushed back evidence for life living on land in hot springs in the geological record of Earth by three billion years. And so, from a geological perspective, Darwin's warm little pond is a reasonable origin-of-life candidate.
Of course, it's still debatable how life began on Earth, and it probably always will be. But it is clear that it's flourished; it has diversified, and it has become ever more complex. Eventually, it reached the age of the human, a species that has begun to question its own existence and the existence of life elsewhere: Is there a cosmic community waiting to connect with us, or are we all there is?
A clue to this puzzle again comes from the ancient rock record. At about 2.5 billion years ago, there is evidence that bacteria had begun to produce oxygen, kind of like plants do today. Geologists refer to the period that followed as the Great Oxidation Event. It is implied from rocks called banded iron formations, many of which can be observed as hundreds-of-meter-thick packages of rock which are exposed in gorges that carve their way through the Karijini National Park in Western Australia. The arrival of free oxygen allowed two major changes to occur on our planet.
First, it allowed complex life to evolve. You see, life needs oxygen to get big and complex. And it produced the ozone layer, which protects modern life from the harmful effects of the sun's UVB radiation. So in an ironic twist, microbial life made way for complex life, and in essence, relinquished its three-billion-year reign over the planet. Today, we humans dig up fossilized complex life and burn it for fuel. This practice pumps vast amounts of carbon dioxide into the atmosphere, and like our microbial predecessors, we have begun to make substantial changes to our planet. And the effects of those are encompassed by global warming.
Unfortunately, the ironic twist here could see the demise of humanity. And so maybe the reason we aren't connecting with life elsewhere, intelligent life elsewhere, is that once it evolves, it extinguishes itself quickly.
If the rocks could talk, I suspect they might say this: life on Earth is precious. It is the product of four or so billion years of a delicate and complex co-evolution between life and Earth, of which humans only represent the very last speck of time. You can use this information as a guide or a forecast — or an explanation as to why it seems so lonely in this part of the galaxy. But use it to gain some perspective about the legacy that you want to leave behind on the planet that you call home.