Atheists do not believe in gods. That's it. Atheism has nothing to do with science and vice versa.
Evolution is science and is accepted by theists and atheists alike, including all the mainstream Christian religions, because the evidence for it is overwhelming.
Animals do not evolve into fossils, species evolve into other species. When an animal dies, if the conditions are right, the bone will be replaced by stone which is then a fossil. In rare cases some of the soft tissue, feathers, scales etc are imprinted in the stone and preserved. Fossils show animals that once lived but are now, usually, extinct. Sequences of fossils show the course of evolution.
If fossils were the only evidence for evolution, the evidence would still be very strong. If there were no fossils at all, the evidence for evolution from all other sources would still be very strong. Together the evidence puts the fact of evolution beyond doubt.
Design is a very poor argument. If you were designing the human body, you would not design it as it is. What we have is a compromise, the result of evolution, not something designed. If humans were designed, you would sack the designer.