The order of authors is non-standard and sometimes controversial. There have been differing traditions, spoken or implied, over time, but ultimately, it’s up to individual journals to set guidelines and authors to decide on the order. One way that some journals are now handling this issue is to increase transparency by including a section detailing the contribution of each listed author.
The book How to write and publish a scientific paper (Greenwood Press, 2006; WZ 345 D274h 2006 Ref) has a chapter on “how to list the authors and addresses”:
If you have co-authors, problems about authorship can range from the trivial to the catastrophic.
What is the right order? Unfortunately, there are no agreed-upon rules or generally accepted conventions. Some authors, perhaps to avoid arguments among themselves, agree to list their names alphabetically…. In the past, there was a tendency to list the head of the laboratory as an author whether or not he or she actively participated in the research. Often, the “head” was placed last…. As a result, the terminal spot seemed to acquire prestige…. A countervailing and more modern tendency has been to define the first author as the senior author and primary progenitor of the work being reported. Even when the first author is a graduate student and the second (third, fourth) author is head of the laboratory, perhaps even a Nobel Laureate, it is now accepted form to refer to the first author as the “senior author” and to assume that he or she did the largest part of the research. (p.46)
The International Committee of Medical Journal Editors, in their “Uniform Requirements for Manuscripts Submitted to Biomedical Journals: Writing and Editing for Biomedical Publication” (http://www.icmje.org/#author) reads: “The order of authorship on the byline should be a joint decision of the co-authors. Authors should be prepared to explain the order in which authors are listed.”
Here’s a paper that discusses the problems and proposes an alternative:
Sergei A Grando and Jeffrey D Bernhard. “First Author, Second Author, et Int,and Last Author”: A Proposed Citation System for Biomedical Papers. Science Editor. July – August 2003. Vol 26. No 4. p.122-123. from http://www.councilscienceeditors.org/files/scienceeditor/v26n4p122-123.pdf
For the searcher
Pubmed users can target first or last authors using the tags [first author] or [last author] (e.g., gartland jj[first author]) or the checkboxes on the Single Citation Matcher: