The way the startup has evolved is a story of its own. I echo Simhan; please share what the purpose is, as your story will depend on that. Suppose you need a press release to highlight your products/services. In that case, your story needs to remain focused on its evolution and the solution that it provides. A PR initiative to bring the founders to the limelight will need the details of the founders and what inspired them to start the firm. Weaving a story is easy when you know the message you wish to send across.
Looking forward to hearing from you.
Regards