Evolution is not a guess but one of the best-supported ideas in science, confirmed by independent lines of evidence that all point the same way. Any one would be suggestive; together they are overwhelming.
Fossils
The fossil record preserves a sequence of life through time, and it tells a story of change. Older rocks hold simpler forms; younger rocks hold forms closer to those alive today. Crucially, it contains transitional fossils that bridge major groups: creatures like Tiktaalik, part fish and part four-limbed animal, or Archaeopteryx, part dinosaur and part bird. These intermediates are exactly what descent with modification predicts, and exactly what a world of fixed, separately created kinds would not contain.
Shared anatomy
Compare the skeletons of a human arm, a whale flipper, a bat wing, and a horse leg: beneath very different jobs lies the same bone arrangement, inherited and then modified. Such homology (the same underlying plan reworked for different uses) makes sense only as a shared inheritance from a common ancestor. So do vestigial structures and the awkward, history-bound quirks of anatomy.
DNA
The deepest evidence is molecular. All life uses the same genetic code, itself a sign of common origin. And the degree of similarity in DNA tracks relatedness precisely: humans and chimpanzees share the vast majority of their genes, humans and mice rather less, humans and yeast less still, matching, independently, the tree drawn from anatomy and fossils. Two entirely different kinds of evidence, molecules and bones, reconstruct the same family tree.
Evolution watched live
Finally, evolution can be observed happening. Bacteria evolve antibiotic resistance in hospitals; insects evolve resistance to pesticides in fields; Darwin's finches have been recorded shifting beak size within a few dry years. These are not reconstructions of the deep past but measurements of selection in real time.
The verdict
Fossils, anatomy, DNA, and direct observation are independent witnesses, and they agree. That convergence is why evolution stands as settled science: the framework that makes sense of all of biology, from the smallest microbe to the whole branching tree of life.