It is just an accumulation of very small changes over time; given enough time, and we've had plenty of time, many small modifications can eventually result in something dramatically new.. There are no sudden jumps to radically different forms, nor do there need to be any. An analogy: A 4k screen contains about 8.3 million individual pixels. If you change one pixel per year, you can have a totally different picture on the screen after 8 million years or so have gone by. But during one human lifetime, fewer than 100 pixels will change, something you probably won't even notice. Getting back to living things, the fossil record hasn't preserved every step of every evolutionary branch. Far from it. But we know enough to see what's going on, and to know with certainty that supernatural explanations aren't required.