Edited for a less broad question, I've retained my original question in the event anyone wants to try to answer it
Narrowed down question
According to evolution we should all share common ancestors, straight to the very first cell. I am trying to find evidence for these ancestors. So therefore as a start, what fossil evidence do we have for the first official mammal to roam the earth? What was the defining moment and change from whatever it was before to a mammal?
I'm just doing some research for my biology class and I was wondering what evidence we have for the elusive "change of kinds" question? And for clarification, I define kinds as the branches of life (essentially birds, reptiles, mammals, amphibians, fish, etc etc). Do we have evidence for the common ancestors that link the vastly different species we have? Because obviously a human, tortoise, jellyfish, and say tarantula are vastly different, each their own distinct "kind", but according to evolution there should be common ancestors and many distinct species leading up to them. We all came from one thing. Man and primates came from one thing. Mammals came from one thing. Where do we get that distinct change of a kind, from say a fish to mammal?
There should theoretically be tens of thousands of in between links and species that were what led to everything today, correct? Therefore, we should have mountains of fossil evidence to support it, yet from what I've researched I've found very lacking evidence and generally just see posts insulting creationists and skimping the question. So by asking the question myself hopefully I'll get a legitimate and clear answer that is the truth.