According to a book of mine on how to write a paper, there's no agreed order. So sometimes it's alphabetical or they take turns. But the first authors tends to be one that done most of the research. I think the last author tends to be the person who funded the project or was in charge of the lab. The one in-between assisted. It becoming more and more common for everyone (even technicians who just washed equipment) to be listed so the number of authors listed are increasing.