Can Life Change The Earth?
4.5 billion years ago, when the earth was first formed, there was no oxygen in the air. Our planet’s early atmosphere was an unbreathable mix of volcanic gasses—mostly nitrogen, carbon dioxide, and methane. The lack of oxygen in the atmosphere meant that, for a large part of our planet’s history, only simple organisms like bacteria could survive on earth. But then, 2.4 billion years ago, the amount of oxygen in the atmosphere slowly began to creep up. Over the next two billion years, earth went from having an atmosphere with no oxygen in it, to having and atmosphere that was about 12% oxygen, enough for animals and other complex forms of life to start making their appearance. Earth is huge: 24,000 miles around, its surface covered by 196 million square miles of continents, islands, oceans, and lakes. The forces that shape its surface—wind, rain, rivers, oceans, glaciers, volcanoes, earthquakes—are monumental. In comparison, human beings seem tiny. It’s hard to imagine that our activities can change the way the natural world works. But living things, humans included, are more than just passengers on the earth. They play an important role in shaping its geologic and chemical systems. In the past, they have remade the earth in ways even more dramatic than we are today.
The source of the oxygen was not volcanoes, or the movement of earth’s plates, or any other geologic force. It came from bacteria. Three billion years ago, a kind of photosynthesizing bacteria, known as cyanobacteria, appeared on earth. Like modern plants, cyanobacteria use light from the sun to turn water and carbon dioxide into chemical energy. And, also like modern plants, cyanobacteria excrete or “breathe out” oxygen gas as a waste product. At first, all of the oxygen produced by the cyanobacteria went into the oceans. At the time, the oceans were rich in dissolved iron, which reacts with oxygen to form iron-oxide minerals like magnetite and hematite—rust, more or less. Once all that iron had been used up (when dissolved iron combines with oxygen, the resulting iron-oxides sink to the ocean floor), there was nowhere left for the oxygen to go except into the atmosphere. As the cyanobacteria continued to thrive, each year saw a little more oxygen in the atmosphere than the one before.
Drops Make An Ocean
By itself, a single cyanobacterium would never have had much of an effect on earth’s atmosphere. But over the course of a few billion years, the combined oxygen emission of millions upon millions of cyanobacteria caused perhaps the most important change in atmospheric chemistry in earth’s history. Today, no single human being has the capacity to change the atmosphere. But seven billion people, each emitting a small amount of carbon dioxide, can have a profound effect on the way our planet works. But unlike the cyanobacteria before us, we have the knowledge and the insight to understand what we’re doing, and to do something about it.