If history is written by the victors, the story of life on Earth belongs to blue-green cyanobacteria. Since they first appeared in the oceans perhaps 3 billion years ago, they are thought to have caused mass extinction, triggered climate change so catastrophic that even the equator was covered in glaciers, and taken dominion over the land by embedding themselves into every plant in the world.
That's quite a profile for simple little blue-green bacteria that we usually come across as toxic “scum” on ponds, or as health-promising spirulina.
When cyanobacteria first evolved, there was very little oxygen in the Earth’s atmosphere, and hence no ozone layer to block out the Sun's deadly UV radiation. Primitive life therefore confined itself to the oceans, using chemicals such as sulfur to make food.
Cyanobacteria did things differently. It absorbed the sun’s energy to make food by photosynthesis, creating oxygen as a by-product, just as plants do today. But this was a time in which life had evolved without oxygen. Oxygen was toxic.
For a few hundred million years the cyanobacteria and the oxygen-sensitive anaerobes survived together, because the oxygen reacted harmlessly with iron dissolved in the ocean, eventually creating a band of iron in rocks that can still be seen today. Once all the dissolved iron was gone, however, the poison started wiping out any anaerobes that couldn’t burrow away.
The oxygen had a huge impact on the atmosphere too, where it reacted with the greenhouse gas, methane. Since methane keeps the atmosphere warm, its rapid removal triggered ice formation, which reflected more of the sun’s rays, which caused more cooling, more ice, and before long, even the equator was covered by glaciers. The Earth was one giant snowball, and inhospitable to life.
Somehow cyanobacteria survived the catastrophe of its own making, and in the process it started preparing the Earth for life as we know it today. We are, after all, quite partial to oxygen in our atmosphere. The oxygen also led to the creation of the UV-blocking ozone layer up in the stratosphere (the one we almost destroyed a couple of decades ago). With the ozone layer in place, it was finally safe to live on land.
And that's what cyanobacteria did, in a fashion. At some point, one type of cyanobacteria was engulfed by a very different type of cell and started an alternate existence not as a living thing in its own right, but as a chloroplast, the part of the plant cell responsible for photosynthesis. Plants spread over the land, and the little green energy converters hitched a ride in exchange for providing food. Cyanobacteria's world dominance was complete.
Billions of years later, we find ourselves in the midst of another mass extinction and another episode of rapid climate change. This one, of course, was not caused by brainless cyanobacteria, but by ourselves. The cost to other species is already apparent, with the number of endangered and recently extinct species on the IUCN’s Red List far above the "normal" level.
The cost to humans is also shocking. For example, the garbage truck of plastic we dump into the oceans every minute doesn't just threaten the coral reefs and their inhabitants, but it is starting to come back in our food; if you eat shellfish you might also be consuming 11,000 small pieces of plastic each year.
Meanwhile the effects of climate change - from stronger and more frequent hurricanes to higher sea levels to more frequent droughts and wildfires - are already evident; the number of natural disasters each year has more than tripled in the past 50 years.
Cyanobacteria survived the catastrophe of their own making. The story of how Homo sapiens affected the planet is, as yet, unfinished.
Further Reading:
The Sixth Extinction: An unnatural history by Elizabeth Kolbert, 2014
When did Earth's first whiffs of oxygen emerge? / Live Science
From Sea to Plate: how plastic got into our fish / The Guardian
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| Spirulina /by Will Power/CC BY-ND 2.0 |
When cyanobacteria first evolved, there was very little oxygen in the Earth’s atmosphere, and hence no ozone layer to block out the Sun's deadly UV radiation. Primitive life therefore confined itself to the oceans, using chemicals such as sulfur to make food.
Cyanobacteria did things differently. It absorbed the sun’s energy to make food by photosynthesis, creating oxygen as a by-product, just as plants do today. But this was a time in which life had evolved without oxygen. Oxygen was toxic.
For a few hundred million years the cyanobacteria and the oxygen-sensitive anaerobes survived together, because the oxygen reacted harmlessly with iron dissolved in the ocean, eventually creating a band of iron in rocks that can still be seen today. Once all the dissolved iron was gone, however, the poison started wiping out any anaerobes that couldn’t burrow away.
 |
| Bloom of cyanobacteria/by Christian Fischer/CC BY-SA 3.0 |
The oxygen had a huge impact on the atmosphere too, where it reacted with the greenhouse gas, methane. Since methane keeps the atmosphere warm, its rapid removal triggered ice formation, which reflected more of the sun’s rays, which caused more cooling, more ice, and before long, even the equator was covered by glaciers. The Earth was one giant snowball, and inhospitable to life.
Somehow cyanobacteria survived the catastrophe of its own making, and in the process it started preparing the Earth for life as we know it today. We are, after all, quite partial to oxygen in our atmosphere. The oxygen also led to the creation of the UV-blocking ozone layer up in the stratosphere (the one we almost destroyed a couple of decades ago). With the ozone layer in place, it was finally safe to live on land.
And that's what cyanobacteria did, in a fashion. At some point, one type of cyanobacteria was engulfed by a very different type of cell and started an alternate existence not as a living thing in its own right, but as a chloroplast, the part of the plant cell responsible for photosynthesis. Plants spread over the land, and the little green energy converters hitched a ride in exchange for providing food. Cyanobacteria's world dominance was complete.
Billions of years later, we find ourselves in the midst of another mass extinction and another episode of rapid climate change. This one, of course, was not caused by brainless cyanobacteria, but by ourselves. The cost to other species is already apparent, with the number of endangered and recently extinct species on the IUCN’s Red List far above the "normal" level.
The cost to humans is also shocking. For example, the garbage truck of plastic we dump into the oceans every minute doesn't just threaten the coral reefs and their inhabitants, but it is starting to come back in our food; if you eat shellfish you might also be consuming 11,000 small pieces of plastic each year.
Meanwhile the effects of climate change - from stronger and more frequent hurricanes to higher sea levels to more frequent droughts and wildfires - are already evident; the number of natural disasters each year has more than tripled in the past 50 years.
Cyanobacteria survived the catastrophe of their own making. The story of how Homo sapiens affected the planet is, as yet, unfinished.
Further Reading:
The Sixth Extinction: An unnatural history by Elizabeth Kolbert, 2014
When did Earth's first whiffs of oxygen emerge? / Live Science
From Sea to Plate: how plastic got into our fish / The Guardian
Who knew that pond scum had such an impressive history. Let's hope that we humans are as resilient as the cyanobacteria.
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