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
Because some organisms dna got mutated and it allowed it to be multi? Very small chance for it to be a mutation that actually works, but that’s how evolution works.
99,999 out of 100,000 mutations are completely useless and will probably downgrade the organism.
That 1 other mutation though…
Thus over millions of years, and millions of mutations among millions of organisms, we get humans.
Historians skip over it because it’s biology as there are no historical records of when things became multicellular
Likely through colony behavior. Single cellular organisms divided by mitosis just to reproduce and sometimes there would be some sort of variation to one or more of those organisms that offered a net benefit to the colony. These colonies were more successful than other colonies and evolved to produce a certain percentage of those single cellular organisms which over time developed into specialized cells of a multicellular organism.
We actually have some extant organisms that are similar to the links between single-celled organisms and animals. The closest organisms to animals we know are called choanoflagellata. These organisms sometimes form colonies. Then we have sponges — these are considered animals, their cells have some specialization, but some cells look like choanoflagellata. Also they can survive fragmentation quite well, so you can think of them as borderline between colonial and truly multicellular. Sponges probably evolved from colonial choanoflagellata. Then we have other animals that show more cellular specialization.
So the general way multicellular organisms evolve is this:
1. Single celled organism
2. Cells forming an undiferentiated colony for some common benefit
3. Differentiation of functions in the colony
4. Multi-celled organisms, more differentiation of cell functions, tissues, organs etc.
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.
We don’t *really* know.
There are many theories and proposals of key processes that most likely played a significant role, but we can’t be exactly sure.
One of the big processes proposed is colonization and specialization.
Single-cell organisms most likely started to form colonies and clusters that improved their ability to protect against predators, easier to find resources, and more efficient reproduction. This allowed cells to specialize into specific tasks, such as some cells specifically for defense, some for reproduction etc. At this point, you can basically call it a multi-cell organism, since an incredibly simple description of the colloquial term “organism” is a colony of cells that are connected and working together to keep the organism alive and reproduce. However, much more technically, this is not close to detailed enough as we’re still missing out on the changes in genetic regulation of behavior, how they could communicate specific division of labor and specialization, designate when specific cells with specific specializations should divide, how they communicated to differentiate each other etc., not to mention genetic mechanisms for cell adhesion and apoptosis (programmed, intentional, cell death), which are absolutely critical factors for a multi-celled organism to survive.
There is also a high likelihood that different forms of life had different processes that lead to their current form. Animals most likely evolved from some form of single-celled protozoa and plants most likely evolved from single-celled green algae.
There’s a theory where symbiosis also helped in the formation of multicellular organisms. Single-celled beings who teamed up with other single-celled beings to cover up each other’s vulnerabilities so they could thrive in an unforgiving environment together rather than perish separately. Because of this you had symbiotes living together who flourished together, intermixing and sharing lifespans and genetic make-up till they became completely dependent on each other unable to exist alone, forming the first multicellular organisms.
It’s not skipped over. We have living examples of transitional forms between single-celled and complex multicellular organisms. Some sponges are basically just colonies of undifferentiated cells held together by a collagenous matrix.
my favorite theory is that at one point some cells engulfed others but instead of digesting them, they formed partnerships (endosymbiosis), eventually working as one complex organism.
One of the main things people struggle to get their head around is time. Life on earth was (and mostly still is) basically slime for 95% of the time and multicellular beings are a very recent addition. When you have millions/billions of years that’s a lot of cell division and variation.
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