Evolutionary theory doesn't rest on any one single, decisive proof; it rests on several genuinely independent lines of evidence that were gathered through entirely different methods and still converge on the same underlying pattern of common descent. The fossil record shows an ordered succession of forms across geological time, simpler organisms appearing in older rock layers and increasingly complex, more modern-looking forms appearing in progressively younger ones. Comparative anatomy shows shared underlying structures across genuinely different species, a pattern that becomes considerably harder to explain without shared ancestry.
The fossil record shows a genuinely ordered succession, not a random scatter
Fossils found in older, deeper rock layers consistently show simpler, more primitive forms, while fossils in progressively younger, shallower layers show increasingly complex and more modern-looking forms, a pattern that holds up consistently across an enormous number of independently dated rock formations worldwide. This ordering isn't simply that older fossils happen to look different from newer ones; it's a genuinely consistent, predictable directional pattern that a random, unordered distribution of fossils across geological time would be extraordinarily unlikely to produce by chance.
Comparative anatomy reveals shared structures that function completely differently
Comparative anatomy studies the underlying skeletal and structural similarities between different species, finding that a bat's wing, a whale's flipper and a human hand all share the same basic underlying bone arrangement despite serving completely different functions, flight, swimming, grasping. That shared underlying structure, despite radically different current function, is exactly the pattern common descent from a shared ancestor directly predicts, and it's a genuinely independent line of evidence from the fossil record, arrived at purely by comparing living species' anatomy rather than by studying rock layers at all.
What we're still unsure about
That the fossil record shows an ordered succession of forms, and that comparative anatomy reveals shared underlying structures across species with different functions, are extremely well established, extensively documented findings confirmed across an enormous body of independent geological and biological research. What's more genuinely an active area of ongoing scientific work is exactly filling in the specific evolutionary sequence and timing for particular lineages, since the fossil record is genuinely incomplete for many groups, preservation itself being a matter of chance, and palaeontologists and evolutionary biologists continue to fill in and refine specific evolutionary timelines and relationships as new fossil finds and genetic evidence become available, rather than every specific lineage's detailed history already being fully documented.
This sits inside Evidence for Evolution (Fossil Record, Comparative Anatomy), one of seven topics in Evolution, one of six domains in Biology, one of seventeen subjects the app can quiz you on.