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First one for me would be carbon dating. I feel that everything that isn't hermetically preserved and even that which is; is contaminated evidence. Carbon from millions of years ago is floating all around us, from rocks, creek beds being eroded and releasing ancient soil (go look for fossils at your local creeks; you can find lots of shark teeth). Stuff like bones, wood etc is porous, it absorbs everything around it, especially if it's exposed to water.
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