Found!

Take a look at this river in northwestern Montana.

The Kootenai River is a tributary to the mighty Columbia River, which in turn empties into the Pacific Ocean near the Oregon-Washington border. The point where this photo was taken is at an elevation of about 600 meters (1969 feet).

Now imagine this area 1.4 billion years ago (BYA, or Ga). It was part of a shallow soft-bottom sea, part of the formation known to geologists as the Belt Supergroup. This large formation extends from southern British Columbia (and the southwest-most tip of Alberta) through western Montana and northern Idaho. It is a sedimentary formation, meaning that over hundreds of millions of years many layers of sediment accumulated on the sea floor and eventually hardened into rock. The timing of the formation, from 1.47 to 1.38 BYA, makes it almost a billion years older than the fossils of the Cambrian period that represent the first appearance of complex animal bodies in the fossil record.

It’s hard for us to imagine the immensity of geologic time. Organisms like us—macroscopic, multicellular, eukaryotic cells, etc.—have been around for only about 500 million years (0.5 BYA). The first organisms were, for the most part, either unicellular or perhaps simple colonies of cells. Yet some of these “simple” organisms greatly changed living conditions on Earth. They literally broke the atmosphere and made it what we have today.

These superb organisms are cyanobacteria, formerly called blue-green algae. They are indeed a special group of prokaryotic organisms that still live today. They are bacteria, so their cells are simple and lack a nucleus, mitochondria, and chloroplasts. Despite this simplicity, the ecological importance of cyanobacteria is due to two major feats of biochemistry that make the world as we know it, habitable.

Firstly, the cyanobacteria are oxygenic photoautotrophs—that is, they use the energy in sunlight to fix inorganic carbon (CO₂) into organic molecules. In other words, they make their own food (fixed carbon) from inorganic precursors. The first reactions of photosynthesis are called the light-dependent reactions and produce oxygen (O₂) as a byproduct. The earliest photoautotrophs lived in the oceans and the oxygen produced by their photosynthetic activities dissolved into seawater and the rocks of the seafloor. It wasn’t until about a billion years ago that oxygen began to accumulate in the atmosphere.

The Great Oxidation Event (a.k.a. Great Oxygenation Event) was a period of time when oxygen levels in Earth’s atmosphere began to rise. It lasted from about 2.4 to 2.0 BYA. Before the Great Oxidation Event, Earth’s atmosphere was anaerobic and consisted mostly of nitrogen and carbon dioxide, with small amounts of water, methane, and carbon monoxide. We humans and most multicellular life would not be able to survive in such an atmosphere. However, thanks to the cumulative photosynthetic activity of cyanobacteria over many millions of years, oxygen began to accumulate in the atmosphere and eventually changed the geology of the earth itself. And, of course, most (but absolutely not all) extant life forms are obligate aerobes and require oxygen to survive. So thank you, cyanobacteria!

The second biochemical trick that cyanobacteria have is the ability to fix nitrogen. This is a biochemical process distinct from photosynthesis, and one that only some prokaryotes can do. It involves converting nitrogen gas (N₂), which is the most abundant gas in the atmosphere but is biologically unavailable to almost every living thing, into usable forms such as ammonia and nitrate. My students often find it a challenge to wrap their brains around the fact that there’s more nitrogen in the air we breathe than any other gas, but we just exhale it right back out again because our bodies can’t use it. And it is a strange thing.

Stromatolites

Which brings us to the topic of this post. Prehistoric cyanobacteria lived in the ancient sea that eventually became the basin for the Kootenai River. Recall that this was a sedimentary seafloor. Cyanobacteria formed films on the sediment, then eventually died and were covered with more sediment, then another layer of cyanobacteria grew there, and so on. It’s like the world’s longest-baking lasagna. Another analogy that might be more helpful is that of a head of cabbage: If you take a head of cabbage and cut it through just about any axis, you’re going to end up with layers of cabbage leaves even if the angle is a bit strange. Over geologic time, these alternating layers of cyanobacteria and sediment fossilized and formed structures called stromatolites.

Living stromatolites are mounded structures found in—you guessed it—shallow seas. You can find them in a couple of places in Australia and the Bahamas. Fossilized stromatolites are much more common. Because they are actual rock at this point in time, their visibility depends on what has happened to the rock formation over the intervening millions of years since they were alive. Along the Kootenai River at this particular spot in northwestern Montana, you can hike down to some stromatolites. You can definitely see the layered structure of the forms.

You can see those alternating light and dark bands in the roughly round-ish formations to the left of center in the photo. Those are stromatolites!

It’s easier to envision the mound-like structure from a more bird’s-eye view:

But it’s easier to see the layers up close:

The stromatolites in this formation were about 1 meter across and smaller. The smallest ones were about 1.5 times the length of my boot.

You can, of course, find and purchase stromatolites and other fossils from rock shops. If you’re lucky the fossil you buy will be identified and aged. And that’s fine. But to see these myself, and to step on them—I know, I know, but at this point they are rocks, after all—was very special. 1.4 billion years ago these were living cyanobacteria, each cell farting out a tiny spurt of oxygen that, along with the oxygen farts of their neighbors, over millennia destroyed the existing atmosphere and created the one that you and I breathe.

I mean, how cool is that?