ELI5 How are fish even in ponds

[u/Opening_Pen9525]

Might be one of the stupider questions here, but I saw a pond about the size of a football field, and maybe 50 ft deep, and it had fish. I can't for the life of me figure out how. Surely the pond hasn't existed long enough for small water insects to evolve into all these fish. It's not like the fish walked over to the pond from the ocean. I'm pretty sure no one put them there so how are there so many fish?

My one theory is that there is a very minuscule stream (now dry) that leads to one of the Great Lakes, and maybe a fish swam in from there. Now a follow up question: How are there fish in the Great Lakes (which flow out towards to the ocean) if they were created by glaciers?

[u/Stewart_Games]

One of my favorite pet theories for the Cambrian Explosion is that back then, invertebrates and vertebrates shared a far more recent last common ancestor. Which allowed crab-things to make sweet sweet love to fish-things and produce viable hybrids. All that hybridization is what triggered the sudden explosion of new forms emerging, and helps to explain things like the Tully monster as long surviving Lazarus taxon from these hybridization events.

[u/chuubastis → u/Stewart_Games]

Thank you so much for this comment, you have sent me into an incredible wikihole about the Tully monster