The residents of Earth live out their lives in the warm light of their yellow Sun, totally unaware of the grave danger they face. For these Earthlings routinely pollute their environment with tons upon tons of a powerful and dangerous waste gas. Slowly, imperceptibly, this gas changes their world, yet the Earthlings plod on, blissfully ignorant of the growing threat.
For a new creature has appeared on this world, crawling on its belly through the muck that lies beneath gently lapping waves. This creature thrives on the Earthlings’ waste gas and, most horribly, fills its belly with the very bodies of those self-same pollutors!
Sound like the beginning of some terrible science fiction story? Maybe, but this story really happened in the shallow seas of Earth, around six hundred million years ago.
The “Earthlings” of the story are ancient life-forms called stromatolites. Some stromatolites look a bit like cauliflower, others more like grasping fingers. They are not plants or animals, but layered colonies of a very simple kind of life called cyanobacteria.
Cyanobacteria are microscopic, one-celled creatures (your body is made of between fifty and one hundred trillion cells). When they come together, these tiny beings can form the colonies we call stromatolites. Building layer upon layer, stromatolites range in size from a football to half a football field.
Around three and a half billion years ago, the cyanobacteria perfected an amazing skill: they learned to mix sunlight, seawater, and a gas called carbon dioxide to make food. We call this process photosynthesis, and it is still the key to life on Earth.
Yet in time, photosynthesis spelled doom for the stromatolites. The waste gas from photosynthesis is oxygen. Over hundreds of millions of years, oxygen slowly built up in the oceans and atmosphere. Finally, around six hundred million years ago, oxygen opened the door to the evolution of a new kind of living thing. We call them animals.
Among these first animals were creatures we would recognize as worms. They gathered the oxygen produced by the stromatolites, and used the oxygen to help digest food. Their favorite food, it turned out, was stromatolites. The digestive fires sparked by the abundant food and the copious oxygen gave the worms insatiable appetites. In about 100 million years, these early predators had virtually wiped stromatolites from the face of the Earth.
Those early predators are with us still. In fact, one of them is reading this very passage. Every time you eat a cheese sandwich, you are combining fuel with the waste gas oxygen, just as those early worms (your own distant ancestors) did so many millions of years ago.
The only stromatolites left from this horror story we call evolution live in extremely salty environments where worms, snails, and other predators cannot survive. When we look at a shallow, salty bay (like Shark Bay in Western Australia) and see a “forest” of living stromatolites, in a sense we are looking back in time, catching a glimpse of our planet’s first polluters, organisms that, through their own lack of insight, spelled their own doom — and helped create the world as we know it today.