The troublemakers were hardly new to the neighborhood. For 300 million years, they had lived in the water column, floating in the sunlight near the surface, sending tiny plumes of toxic gas into the air. That was how they ate: sunlight in, poison out. But it was nothing the ecosystem couldn’t handle. The atmosphere was vast, and the troublemakers were microscopic. The poison diffused. Life went on.
Until something changed. For some reason, there were more of the green troublemakers. Quadrillions more. So many more that their poison became the air itself. An entire world’s atmosphere transformed. Those that could tolerate the miasma grew and spread. Others survived in pockets of the planet where the new air couldn’t reach them. Uncountable numbers died.
The sunlight-eating oxygen-makers inherited the Earth.
In other words, 2.3 billion years ago, photosynthesis caused a mass extinction.