I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

The Earth is 4.54 billion years old. A span of time that big is too mind-boggling to comprehend, so let’s collapse the planet’s entire history into a single calendar year.1 Right now, as you’re reading this page, it is 31st December, just before the stroke of midnight. (Thankfully, fireworks were invented nine seconds ago.) Humans have only existed for the last 30 minutes or fewer. The dinosaurs ruled the world until the evening of 26th December, when an asteroid hit the planet and wiped them out (except for the birds). Flowers and mammals evolved earlier in December. In November, plants invaded the land and most of the major animal groups appeared in the seas. Plants and animals are all made up of many cells, and similar multicellular organisms had certainly evolved by the start of October. They may have appeared before that – the fossils are ambiguous and open to interpretation – but they would have been rare. Before October, almost every living thing on the planet consisted of single cells. They would have been invisible to the naked eye, had eyes existed. They had been that way ever since life first emerged, some time in March.