If you think evolution is just selection and mutations, I can see how you'd think it's not enough (even though selection is usually a very strong force that "lo

If you think evolution is just selection and mutations, I can see how you'd think it's not enough (even though selection is usually a very strong force that "locks good mutations in place", which can create a compounding effect in chances of an organism's fitness, so a long time of mutations + locking good ones in place should be almost enough to convince you if you do the math). It's been some time since I studied this so I'm just going to write whatever comes to mind. Maybe a key part of what's missing in your understanding is not so much "micro" genetics but biogeography and population genetics. You'd also want to check simulations and comparisons with real-world data on some models to see how a population evolves to see that it "really works". It's important to understand that there are different models, and for each one there's a set of "forces" and important parameters. The bigger picture is, it's the whole interplay between mutation, selection, genetic drift, gene flow; things like differences in population size over time and space, migrations, isolation and reconnection, etc. that makes it work. You might also want to take a look into genetic/functional/morphological modularity. I've just skimmed these articles, but they seem relevant: https://www.nature.com/articles/hdy2010136 https://www.nature.com/articles/hdy2010136 https://www.nature.com/scitable/knowledge/library/natural-selection-genetic-drift-and-gene-flow-15186648/ https://www.nature.com/scitable/knowledge/library/natural-se... There's much more but my memory is murky. Ideally you would want to take a course or read a textbook on evolution. A few popsci books are ok (Dawkins, E. O. Wilson). TLDR: What the other commenter said -- 4 billion years is a very, very, very long time.