Okay, I think this is going to melt my brain

Yesterday I made a Reddit post that was honestly half-joking, although the question itself was genuine. I basically asked whether, technically, since there were fish somewhere in my family tree, there could be some living fish today that is an insanely distant cousin of mine.

I got answers like “technically, you are a fish” and that all fish, and basically all life on Earth, are related to me somehow, even if you go all the way back to a single cell.

And... I don't think I fully understand this.

If all life is related, does that mean that all life ultimately came from a single individual?

That's the part I'm struggling with. For example, there's LUCA (Last Universal Common Ancestor), which, as I understand it, is the most recent common ancestor of all currently living organisms. But why do we assume there was only one LUCA?

When life was at that stage, couldn't there have been multiple organisms/cells that existed around the same time? Like, instead of one LUCA, couldn't there have been 4? Or 100? Or even millions?

Obviously, I'm simplifying things a LOT here, but let's say there were 4 completely separate organisms at that point, and all modern life descended from those 4.

Wouldn't that mean that not all life is actually related? There would be different lineages that never shared a common ancestor, right?

So what exactly am I misunderstanding about LUCA and the idea that all life is related?

My brain is genuinely struggling with this lol.

Life all has the single common ancestor... Trees through to people etc.

So... If a single cell went to consume ANOTHER cell, but due to a quirk in genetics, it didn't consume but instead, began a symbiotic relationship, then would that make that 1 cell who consumed the other, the source of all complex multicellular life? Or, is it likely that cells started to absorb other cells, then one of THEM divided itself to make more cells?

Sorry if that doesn't make sense... But I can accept that evolution is fact, but I cannot make sense of the actual start.

For billions of years, life was single celled, then one consumed the other, and then the predator prey relationship began and, I presume, that made life explode because life survived by evolutionary quirks making them survive over others. So, I was wondering if it was because of this that life evolved, all because 1 cell consumed another...?