Modern science isn't a one-person affair. Few papers have single authorships nowadays because science is done in teams rather than by lone geniuses (and even in the case of Einstein it looks like his first wife, the mathematician Mileva Marić, should probably have been given co-authorships on some of those early papers). While some fields simply have authors listed in alphabetical order, many have special meaning to the first and last authors -- the first author is given to the person who supposedly contributed the most (which can be contentious; typically a grad student or post-doc) and the last author is given to the mentor of the first author.