Once in a Billion Years: Two Organisms Become One
For only the fourth time in Earth’s history, two species have merged to become a single life form.
Researchers found a bacterium embedded in the cells of Braarudosphaera bigelowii, an alga, in 1998 and thought they were looking at a symbiotic relationship between two life forms. The bacterium helps its host by “fixing” atmospheric nitrogen, processing the gas into useful compounds, but the relationship is actually much closer. The bacterium has become a part of the algae cell, an organelle. Two life forms merging to become a single organism.
Organisms swallowing another to become a new, single life form has only happened three other times in Earth’s 4.5-billion-year history. The first, 2.2 billion years ago, when a single-celled organism swallowed a bacterium that would become mitochondria and give rise to complex life. Then, 1.6 billion years ago, a cell engulfed a cyanobacteria that’d become chloroplast, giving us all plants. The third is chromatophore, embedded in the skin of squids and octopuses, allowing cephalopods to change color.
Organisms merging in this way is called primary endosymbiosis. It differs from a mere symbiotic relationship because the absorbed life form ditches parts of its genetic material and relies on the host for missing products. Not only is the alga supplying proteins to the bacterium-turned-organelle, indicating lost genetic material, the two also grow in synch and divide with one another.
Organisms merging through primary endosymbiosis is exceedingly rare, only occurring four times in Earth's history, but results in fantastic new forms of life. Previous mergers gave us complex life forms and all plants. What will Braarudosphaera bigelowii with its new nitrogen-fixing organelle produce in a billion years' time?
