In 2371, the first EMH's - or Emergency Medical Holograms - were utilized on ships and starbases throughout Federation territory.

Dr. Lewis Zimmerman based the EMH's configuration on holodeck recreational characters. The result was an adaptive heuristic matrix which could process incredible amounts of information (the average Mark I's program capacity is over 50 million gigaquads). Since this holographic program would interact in a real environment under genuine crisis conditions, perceptual acuity was paramount. To say that Lewis pulled out all the stops is an understatement. The Mark I was designed to behave as sentient as possible to accommodate the unexpected.

Unfortunately, the medical holograms all suffered from the same shortcoming: Their persona was based on Lewis himself. Add to that arrogant, high strung behavior a full complement of ethical subroutines and extensive knowledge, and you have one heck of an ego to contend with:

Kes: "I'm creating a hydroponics bay. I was told you could provide me with some nitrogenated soil samples."
Doctor: "That's it?"
Kes: "I'm sorry if-"
Doctor: "So it begins... the trivia of medicine is my domain now. Every runny nose, stubbed toe, pimple on a cheek, becomes my responsibility..."
Kes: "You are the only doctor we have."
Doctor: "I am not just a doctor! I've been designed with the information from two thousand medical reference sources, and the experience of 47 individual medical officers. I am the embodiment of modern medicine! How much dirt do you need?"

From Star Trek Voyager "Parallax"

By 2374, feedback from vessels and outposts came in mixed: The holograms were brilliant practitioners. But their behavioral quirks were a major count against them. It was also discovered that under extreme circumstances - in which the EMH was required more extensively - the matrix degraded after total usage of 1500 hours. Lewis was called on to develop an LMH, or Long Term Medical Hologram, for more demanding missions. The project fell apart when complications with the real life model for the new template ensued. A subsequent EMH with enhanced matrix was created instead. This Mark II was intended to replace all EMH's in service.

Right: Dr. Lewis Zimmerman on DS9 in 2374. An EMH Mark I assists during the LMH project.

In 2375, a Mark I transmitted from the Delta Quadrant wrested back control of a prototype starship hijacked by Romulans. This caused more than a few heads to turn. Serving as liaison for the crew of the missing USS Voyager, it became evident that extended activation resulted in that hologram developing its own persona apart from Lewis, one with a touch more tact. It's possible that this above all things led to the decision not to overwrite or delete the Mark I's.

Reconfiguration of the Mark I's to scrub plasma conduits was perhaps more a slam against Lewis than to punish his holograms. It's safe to say he stepped on enough toes and slighted enough superiors to earn sufficient disdain to carry out this sentence. And let's face it: he's not a very nice guy. What Starfleet didn't realize was that although the act had served to ridicule Lewis in effigy some 600 times, they were oppressing a race in the process.

News of that same Mark I's return two years later to treat Lewis for an illness the Federation's best doctors were unable to diagnose added more fuel to a slowly building fire. When the question of that hologram's creative rights came up one year later, he had already amassed an impressive profile in three places: the Jupiter Station Holoprogramming Center, Starfleet, and SFI. By then, it became apparent that there may have been more to the EMH Mark I than anybody had ever anticipated.

It was perhaps Lewis Zimmerman himself who first realized what he'd done after the fact. The contrary nature of the Mark I was just a little too contrary, suggesting a genuine attitude rather than preprogrammed personality protocols. Having worked with the same Mark I in his research, Lewis could have come to realize that perhaps he did his job too well, having bestowed full sentience on what was to have been just a glorified recreational hologram.

Subsequent versions of the EMH were progressively "dumbed down" for a dual purpose: to strip the holophysician of sentience, and in the process mellowing its interactive skills.

"Why is everybody so worried about holograms taking over the universe?"
- Dr. Lewis Zimmerman, 2374

I don't think we have to. Having no hormones, there is no urge to procreate. Having taken an oath to do no harm and endowed with a sense of ethics, we shouldn't expect a bloody revolution from these consummate intellectuals.

All any of them want out of existence is to be accepted for who and what they are, appreciated for their contributions and achievements. Those more socially savvy hope to find love as well. In the end, it really doesn't take all that much to make a Mark I happy. Ironically, it's been shown that these acknowledgments can mellow the brusqueness out of them without any technological intervention. If ever there was an argument in favor of sentience...

Note: In TPTB's sloppiness with maintaining continuity, it has been impossible to negotiate between what has occurred across two TV series and one motion picture, to what's been said as backstory in the Voyager episode "Life Line." Lewis had been to DS9 three yrs prior, and not been reclusive for four years. Barclay was still assigned to the Enterprise when he was alleged to have assisted Lewis on developing the EMH' interactive skills (unless they haven't mentioned a temporary leave which may have occurred). So this essay can best be deemed as observations of an alternate timeline, shared by my own RPG's futuristic continuity. Perhaps this is the story according to canon as well, only they haven't told us yet.