Science is an attempt to explain nature. Evolutionary theory is a scientific explanation of what we observe in nature. A scientific theory is well supported by facts and it has not been falsified, despite numerous attempts to do so.
Some of the facts that support evolutionary theory as the most likely, logical and rational explanation for these facts are listed below.
1. Fossils. Human fossils are unknown until about 200,000 years ago, for example, but fossils that have characteristics of both apes and humans (e.g. ape like skull but leg bones that are similar to those of humans that allow walking on 2 legs) can be found all the way back in time to about 4.5 million years ago. The logical conclusion is that humans evolved from an ape, and these fossils with both human and ape features are intermediate species between human and ape. The younger the fossil, the more they resemble humans instead of apes.
2. embryology. The early embryos of a fish, salamander, lizard, mouse, pig, and human look so similar that scientists conclude that it is probably because these organisms share a common ancestor.
3. DNA. All organisms on earth use DNA to encode their genes. They all use the same 3 nucleotide sequences to code for the same 20 amino acids. Bacteria, fungi, plants, and animals all use DNA and the same genetic code. A sequence of AAA codes for the same amino acid in a bacterium, fungus, animal and plant. A sequence of CGA (cystosine, guanine and adenine) codes for the same amino acid in all living organisms. The most logical explanation is that all living things on earth shared a common ancestor that used that particular genetic code.
4. comparative anatomy. All fish, frogs, lizards, snakes, birds, and mammals have eyes that are similar anatomically. Their eyes have the nerves locate in front of the eye, and in order for the nerves to send a signal to the brain, there needs to be a hole in the eye for the nerve to pass through. Again the most logical and likely explanation is that fish, frogs, lizards, snakes, birds and mammals share a common ancestor that evolved an eye with a hole in the middle and they inherited this imperfect eye from this common ancestor. In contrast, the octopus, squid, and cuttlefish have eyes that do not have a hole in the middle because the nerves are located at the back of they eye. The most logical conclusion is that octopus, squid and cuttlefish evolved their eyes from their common ancestor, independently of vertebrates.
5. the dog. It shows that with selective breeding the dog can change drastically, showing us that living organisms can change and that these changes can be passed along to future generations.
Despite such strong supporting evidence, scientists are open to having their theory challenged by falsification. For example, one can falsify evolution by finding evidence that all living organisms can be found in the oldest rock layers, and that they did not change through time. For example, evolution is falsified if anyone can find a bullfrog, a rhino, a Komodo dragon, a human, a tiger salamander, a great white shark, and/or a pigeon in rocks that are 3 billion years old, 2.5 billion years old, 1 billion years old, half a billion years old, 250 million years old, 100 million years old, 50 million years old, 25 million years old, 10 million years old, 1 million years old and 10,000 years old. That means the bullfrog, human, rhino and/or pigeon have been around for 3 billion years and they have not changed one tiny bit. If anyone can do that, evolution will be falsified and will be abandoned. So far, no one has found anything to show that evolution is false. Different layers of rocks representing different periods in earth's history instead show different kinds of fossils, although the fossils of adjacent layers of rock are more similar to each other. That suggest that evolution has been happening throughout the history of the earth, with newer species replacing older ones. People who claim that evolution is not proven either do not know the facts supporting it or they ignore those facts.