This is not actually true! We have seen evolution happening, in many different types of animals. My favourite example is superbugs like MRSA, which are antibiotic resistant strains of bacteria. Before the use of antibiotics there was no MRSA, but now there is. The frequent use of antibiotics has forced the bacteria to be able to adapt or die. Some would have had a gene for resistance, and they would have survived the antibiotics, and this would then reproduce to form new resistant bacteria.
Another nice example of evolution in action is the peppered moth, which has been studied for about 100 years, and we have seen new types evolving and spreading.
But usually, evolution takes thousands of years or more to produce large changes, so over the life of a human we don’t really notice is happening. But nevertheless, there is very storng evidence for evolution. Briefly this is:
1) The similarity of the bodies of animals, and plants, from different parts of the world.
2) THe geographical distribution of organisms. Those that are more similar to each other tend to live closer together, suggesting they descended from a common ancestor.
3) Similarity in embryological development between amimals is also evidence that we once shared the same form.
And evem though Darwin didn’t know about it, some of the most powerful evidence for eovlution comes from DNA:
4) The similarity in the DNA of animals. This is extremely powerful evidence that evolution has taken place, because we can actually see how the genetic code has changed. Even organisms that don’t seem very closely related share a lot of DNA in comm
I agree with Steven and Ed. It is like with a murder case. Imagine the site of a murder with fingerprints, footprints, and DNA traces from a suspected murderer all over the crime scene. In the murderer’s home nearby, you find the murder weapon with the victim’s blood on it and also a written plan of attack lying around on the table. I suspect that no court in the world will accept that the suspected murderer is innocent just because no one saw the murder! It’s the same with evolution, there is just so much evidence for it that all other explanations are just not reasonable.
All of these answers are great: 1) people have directly observed evolution happening, and 2) even if they hadn’t the evidence is so strong that it would be ridiculous to not believe in evolution unless a better theory that also explained all the evidence could be conceived. Other explanations for the natural world’s wide variety do not explain the known facts as well.