So, millions of years ago, a microbe evolved something extraordinary. They could turn light, water, and carbon dioxide into sugar and oxygen creating massive amounts of energy dwarfing the amounts available to previous lifeforms. With this amazing power they became the dominant species on the planet but also polluted their atmosphere with toxic gas, devasting other life and eventually themselves(sound familiar?). That toxic gas gas was oxygen which is massively reactive, just ask any piece of iron that's slowly turning into rust, tearing though, like a chainsaw through paper, the fragile chemistry of the lifeforms at that time.
Now, living in the modern world, you might guess that some life survived, became immune to oxygen, and even evolved to use it. Yes on the first and thrid point but no on the second point. Oxygen is still massively reactive and thus deadly as ever. Today most species reproduce sexually. Why do they do that when asexual reproduction is much more efficient? Because all DNA is in a constant state of deterioration due to being damaged by oxygen. No matter how many copies a piece of DNA makes of itself(clones though asexual reproduction), all of them will get progressively more degraded each generation eventually leading to the end of its lineage. DNA survives by recombination, mixing itself with other DNA, essentially creating new DNA(children though sexual reproduction).
The gas that you can't live five minutes without also slowly kills you. It's horrifying and you have the cyanobacteria to thank.