How did single celled organisms evolve into multicellular organisms?

Question

I feel like this part of evolution is kind of skipped over by historians. The more I think about it the more it hurts my brain

In: Biology

Answer 1

Edit: replace archaea with ‘some third single celled thing like bacteria’

Surprised I’m not seeing my fav theory commented yet (I say theory because I don’t think you can really prove biological history at this detail, but it’s pretty widely repeated).

There were two types of single celled organisms (maybe more but what do I know): archaeons and bacteria. They’re not that different at an ELI5 level, just lil blobbers, and the archaea are generally bigger.

Most importantly, both of them generate energy at a rate proportional to their surface area. They have to stay small, because of something called the cube-square law – if they get bigger then their volume energy requirements are too much for their surface area energy production to keep up with, so they die.

One day, a bacteria headbutted an archaeon, and got jammed inside it. Instead of dying, the bacteria continued trying to make energy from the goo inside the archaeon, and managed to survive and multiply. I guess when the archaeon went through mitosis some bacteria ended up in each half or whatever.

Now you’ve got archaea with bacteria throughout their volume, and at some point the bacteria started sharing their energy production with the host archaeon, because it was advantageous for the bacteria’s host to not die. This meant the archaeon now had volumetric energy production that could keep up with it if it evolved to get bigger, so it did – being bigger is great if you can afford it, because then other stuff can’t hunt you. A single archaeon packed with bacteria became what you’d call a cell in a multicellular organism – the bacteria are the ancestors of our mitochondria.

I guess there’s something about being many cells glued together that’s better than being one mega-archaeon single cell filled with many bacteria, maybe because the latter would be like being a balloon and you’d pop easily. But anyway, yeah, bacteria headbutting our ancestors is part of why we evolved into multicellularity, probably.