But the theory of evolution doesn't assert that a monkey would ever turn into a human, and so the presence or absence of such an observation is irrelevant.
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How do I know for a fact the sun will rise tomorrow if I have not observed it yet? How do I know the sun was rising a 1000 years ago but not 7 billion years ago? All of this works like being a detective looking at the evidence as clues that tell us what actually happened. Science collects factual evidence and pieces these facts together like putting a jigsaw puzzle together. The entire puzzle is what science labels as a scientific theory. The puzzle is not an educated guess, it is absolutely demonstrating how the puzzle pieces (facts) fit together. It would be like checking the four sides of a puzzle piece to make sure each side connects perfectly. When talking science, simply replace "Theory" with "Model". So we are talking about the "Model" of Evolution that demonstrates how all of the facts fit together. Just like putting a puzzle together, you don't have to have every piece in place to know what the big picture is. If anything, knowing the big picture helps to identify where future pieces will go and be a perfect fit. In science, "Theory" is as high as it gets. It means you have enough pieces fitted together that you can see the big picture. The bottom is called an hypothesis where you make an educated guess about something, then think of ways to test the hypothesis. Sometimes we collect facts, but don't yet see the bigger picture, so facts alone are in the middle of the two ends.
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Evolution is a proven fact. Even most Christians now accept that.
But evolution does not say that a monkey turns into a human. Kids can understand this from about the age of ten, so I don't know why you seem to have so much trouble grasping it. Evolution has proved that man and monkeys and apes all evolved from a common ancestor - over millions of years.