2.5 billion years ago (approximate but honestly who's counting?), cyanobacteria or blue-green algae began pumping oxygen into the atmosphere.
Earth's atmosphere at the time was rich with sulfur, nitrogen, and carbon containing compounds. Most importantly, there was life in nearly no-oxygen conditions.
This marked a turning point in Earth's history, where the atmosphere suddenly shifted in its composition from anaerobic to aerobic conditions. What this led to was one of the greatest MASS EXTINCTION EVENTS our planet has ever seen. But alas, natural selection leads to the survival of the organisms that consume oxygen in their metabolic processes instead of sulfur or nitrogen (which is great news if you're a mammal).
People seem to think that tinkering with our atmospheric composition is no big deal, but what happened then is really analogous to what's happening now (we just love turning a blind eye to the effects). We've started to change the atmospheric composition to contain more carbon-containing by-products from various sources (granted it's less than 0.1%, it's still significant to climate stability).
We are no different from cyanobacteria or the blue-green algae that selfishly converted the atmosphere to contain more oxygen. Are people really that naive to think that raising carbon dioxide levels in our atmosphere has NO IMPACT on the ecosystems around the world?
What puts things into perspective is when you recognize that over 98% of all life that ever existed on this planet has gone extinct (with more and more extinctions taking place each year). It's global events that change the rate of survival or extinction of life on Earth.
Obviously, cyanobacteria are quite durable and adaptable to have lived for over 2.5 billion years... But I have my doubts about our longevity as a species at this rate of growing ignorance.
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