How did the tradition of _____ anniversaries get started for weddings?

Well, I'm not sure why each year doesn't have a gift...maybe people thought that after 20 years they wouldn't give gifts as often? :-) The really obscure ones are very dated. For instance I know one year is tin, of all things. We don't think of it as a gift now, but 100 years or more ago, people used things such as tin but those items weren't always readily available, or affordable.