A chrysalis is one of the most familiar, yet cryptic, transformations in nature. We know what goes in. We know what comes out. For a long time, what happened in between was essentially invisible to us. Not because we weren’t curious, but because the mechanism was sealed inside something the size of a thumbnail, and we had no way in.

This same invisibility exists on a much older and much larger scale.

Sometime around two billion years ago, a cell swallowed a bacterium and, instead of digesting it, kept it alive inside itself. This process, called endosymbiosis, is arguably the single most consequential event in the history of complex life. The bacterium became a permanent resident, and over billions of years of co-evolution, it became something else entirely: the mitochondria that power every complex cell on earth. Without it, the living world as we know it doesn’t exist.

Scientists have known for decades that this kind of cellular acquisition had to have occurred. What has proved harder to explain is not that it happened, but how it started. What did the earliest molecular steps actually look like from the inside?

In the ocean, there is a microscopic single-celled organism called Rapaza viridis. It hunts algae by propelling itself through the water on whip-like appendages called flagella. That hunt may be showing us the beginning of a modern endosymbiosis: the same process that gave every complex cell its mitochondria and every plant its chloroplasts.