Figuring out how this happened a billion years ago is a daunting process for any scientist, but a recent study published in Current Biology shows that a special amoeba may hold some of these answers to how early life was able to become so intimately connected with the photosynthetic bacteria, which now provide food that nourishes animals like us. Unlike algae and plants, the amoeba acquired its bacteria much more recently – about 90 to 140 million years ago instead of a billion.

For a bacterium, becoming trapped inside of a host isn’t common. Bacteria, like all organisms, are interested in growing and reproducing as much as they can in order to pass their genes on to the next generation. Some bacteria are successful because they are parasites that infect animals and plants, exploiting their host’s resources. Most bacteria don’t actually harm their hosts and are housed inside of specific organs like the gut, providing benefits to their host in exchange for protection against other bacteria that compete with them, or viruses that destroy them.

For photosynthetic bacteria to be harnessed by an amoeba the way they were harnessed a billion years ago, they must completely lose their freedom to live as independent organisms.

In fact, the photosynthetic bacteria inside of amoebas have shed two-thirds of their genome – the blueprint that defined them before they entered the host. More than 70 genes associated with the bacteria have been transferred from the bacterium’s genome into the amoeba’s genome.

When researchers explored the proteins being shuffled between the amoeba and the bacterium, they found that control over the machinery responsible for photosynthesis had been taken over by the amoeba itself. In other words, the amoeba is cranking out proteins encoded by the genes it got from the bacterium and then transferring those proteins into the bacterium to complete the process of photosynthesis.

As extraordinary as this sounds, it still begs the question of whether the same thing happened a billion years ago when the ancestors of algae and plants acquired photosynthesis. Researchers, surprisingly, showed that it could have. When they put the same amoeba’s proteins inside of a plant cell, they got a surprising result. The plant’s chloroplasts (the one billion-year-old bacteria that now provide photosynthesis inside of plants) started to migrate toward the amoeba proteins that the researchers were injecting.

That response is really good evidence that plants may have also harnessed control over photosynthetic bacteria in much the same way as the amoeba.

The melting together of bacteria and hosts has driven the diversity of life across our planet. Without photosynthesis, plants and the animals that eat them wouldn’t have been so successful. The control of bacteria may just be one of the biggest innovations that spawned all the animals and plants we see today.