a once-in-a-billon merger of two organisms proves an "impossible" feat of evolution
A new discovery by scientists shows an organism becoming part of another, proving an idea that creationists insisted was impossible.
Mitochondria — say it with me now — is the powerhouse of the cell. Without it, life as we know it today, including us, would be impossible since we couldn’t synthesize the quantities of adenosine triphosphate, or ATP, that we need to do literally anything. It’s also a very weird organelle. It looks kind of like a bacteria, or a very chubby worm. And it also has a circular chromosome inside one of its internal chambers containing DNA unique to itself rather than related to the cell’s genome.
How could that happen? Well, the current hypothesis is that the mitochondria was an independent living thing, a bacteria that found a way to generate lots of ATP. One day, it was captured by a much larger cell and instead of escaping or being digested, it just stuck around and started producing an enormous amount of energy for its new host cell, giving it the equivalent of superpowers in the primordial biome.
It’s kind of like the plot of a sci-fi movie. A person accidentally swallows an alien worm or is infected with an alien virus, and can suddenly regenerate from damage before an enemy’s eyes, punch through concrete and steel walls, and outrun a sports car at full speed. Nick Fury arrives, asks if this person has heard of the Avenger’s Initiative in the post-credits scene, you know the rest. So, what are the odds this really happened? It makes sense as a sound hypothesis, but if it happened once, shouldn’t we see what’s called an endosymbiotic relationship by biologists happen again?
Back in the day, when creationists tried to call themselves “proponents of intelligent design” in a bid to undermine the teaching of evolution in schools, they’d jump on the hypothesis about the mitochondria as an example of “evolutionists just making it up” and calling it a just-so story. Which obviously meant that the magic guy in sandals… err… an unknown force of design, waved a magic wand… err… did something that we don’t know and placed a mitochondria into eukaryotic and plant cells to allow for the kind of multicellular life we see today.
Rewind to 1998, when marine biologists at the University of California at Santa Cruz were analyzing a soccer ball shaped algae called Braarudosphaera bigelowii. (First of all, good luck with pronouncing that name, secondly, no, it was not named in honor of the cinematic turd Duce Bigelow, but Norwegian botanist Trygve Braarud.) Instead of just finding algae DNA, they found genes for a distinct bacteria that could fix nitrogen from the air, making B. bigelowii more efficient in feeding itself.
For a while, the thought was that this cyanobacteria and the algae lived in symbiosis, sharing nutrients like roommates with an open fridge policy. But a further analysis of the flow of nutrient exchange and genetic drift during reproduction didn’t match the hypothesis. Instead, it matched what we saw with mitochondria, meaning this was no bacteria, but an organelle, a part of the cellular machinery.
As far as we’re aware, this happened twice before. First, the mitochondria opened the door to multicellular life about 1.5 billion years ago. Then, 500 million years later came chloroplasts, giving rise to plants. As a fun aside, about 100 to 200 million years after the rise of the plants, an organism branched off a predecessor to animals and went on to become a fun guy who had mushroom to grow. Yes, yes, by which I mean it evolved into fungi, please put down the anti-dad joke pitchforks.
So, in short, we can now safely say that yes, it’s entirely possible for captured bacteria to become parts of cells in organisms where they and their hosts provide so much of a benefit to each that the arrangement becomes permanent, and that this is not only possible in the future, but almost certain to happen again at some point in the roughly billion years life has left on this planet.
It also proves that evolutionary theories very much do make predictions which can be tested and confirmed through observation, so the next time a group of activists raids a school board meeting demanding that science classes stop teaching evolution, you should ignore them because chances are, whatever they point out as a major flaw, or a “convenient story” in the theory is either correct and they don’t know it and refuse to accept it, or we’ll find a better explanation, one that doesn’t rely on literal magic to make ourselves feel special, and amend the textbooks accordingly.
See: Cornejo-Castillo, F., et al (2024) Metabolic trade-offs constrain the cell size ratio in a nitrogen-fixing symbiosis, Cell 187, 1762–1768, DOI: 10.1016/j.cell.2024.02.016