Pretty good read I found on Sherdog: Link to original thread
The UFC, as far as combatants go, has essentially trekked the following path.
1. The Gracies-
Thought at one time to be magical and all powerful. This family demonstrated that ground fighting technique in a fight AND good conditioning (as shown in his meeting with Kimo) were the most important elements to victory (at the time and in 1 on 1 combat.) The reasons were: Nobody knew the gracie style of submission fighting, and could not subsequently defend against it, you can equate this to a grappling sucker punch. People never saw things coming. Royce dominated in the cage at a much smaller body weight in what was an open weight fighting competition for the first 4 UFC's. While just about all submission wrestling styles world wide utilized the exact same movements and subs as GJJ, nobody had really put them into a sequential system the way the Gracie family did, so their transitions were flawless and they never seemed to make mistakes. Even when they did (Kimo taking Royce's back UFC 3) the reversals were almost instantaneous and they never seemed to be in any danger.
2. Technique cuts both ways.
Enter UFC 5 and the "Superfight" with Ken Shamrock. Ken brought athleticism, explosive speed and a significant weight and physical strength advantage. Ken had done his homework on Gracie's fighting specialty, most notably the guard that Royce was so fond of employing in his matches. This was really the first time in the UFC that someone knew the submissions that Royce had available from this position, and simply defended against them, while occasionally throwing short choppy shots. Ken survived in this position for 36:01 at which point the fight was declared a draw. Ken opened the door in this match for the evolution of ground and pound, and showed that simply being in a submission specialists guard did not mean you were destined to lose.
3. Size and strength DOES matter (but not as much as conditioning and technique.)
After the fight between Ken and Royce, then owners of the UFC, SEG entertainment had to implement time limits for their fights. The first time the UFC had done this, because of the unpredictable nature of the fights, they ran over their time slots on PPV in UFC 4 and 5. This opened the door (which would've happened eventually anyway) for size and strength to give the contestants significant advantages in the matches. If you were heavier and stronger, it was easier for you to control a smaller fighter and stay out of danger assuming you knew what they were trying to accomplish (armlocks, triangles etc..) for a set period of time.
4. "Knowing" a martial art didn't mean you could fight worth a shit, toughness and conditioning were still paramount to victory.
Enter the original large powerful brawler, David Abbott. Although he had adequate wrestling skills, Dave's strength was just that, strength and weight and a powerful right hand, probably the most physically powerful competitor (in terms of feats involving heavy iron - search for the 600lb press on youtube, and hey, bounce at the bottom or not, that's 600 lbs) that has ever stepped into the UFC. He showed that with size, balls, strength and toughness that you had a much better than average chance of beating all but the most well conditioned and technically expert "martial artists". It was, at least for me, this UFC that opened the doors and removed any mystique that "traditional martial arts" still held. It started to re-prove that hitting like a truck could win fights.
5. Then came the beginning of the complete athlete.
Marco Ruas was probably the first well rounded fighter to fight in the UFC, an adequate submission game, good conditioning and striking were what he brought to the table. He showed us that with a more complete technical package that size was again not the end all be all of fighting and that technique still ruled. Physical tools though were much more necessary at this stage of the MMA game to support those well honed techniques since the time limits had been introduced and had made athleticism more important.
6. The complete fighter evolves.
Following Ruas’s participation in the sport, and Ken showing that you could defend submissions practically indefinitely, the first serious athlete's started to make their presence known in the cage en masse ala Mark Coleman, Don Frye, Mark Kerr, Frank Shamrock, Tito Ortiz, Vitor Belfort etc.. barring Frank Shamrock, who along with Vitor Belfort IMO were the first truly complete fighters (though Frank did not bring his skills to the level we see today, and Vitor was too young and unfocused to bring his very impressive physical prowess and impossible athleticism to bear on his opponents consistently, an issue that many would plagues him even today) most competitors still lacked one dimension of the necessary skill set to be complete. If you were strong and could wrestle, you had no subs and your standup was weak. If you could strike and had good cardio you had no take down defense or subs etc..
7. The seriousness of conditioning.
With Maurice Smith’s come from behind victory over Mark Coleman, and later, Frank Shamrock’s rd 4 stoppage of Tito, conditioning very clearly became one of the main components of victory in the cage, even with the rest periods and introduction of rounds.
8. Final evolution and beyond.
The last thing to really enter into the fray, outside of conditioning and all the continuing development of skill sets and physical attributes was the game plan. While Maurice Smith was probably the first to bring this over Mark Coleman, nobody imo has implemented this better than the current HvyWt Champion Randy Couture. Many times in his career he was outmatched physically (Randleman, Belfort), sometimes technically (late striking with Rizzo), but with proper foresight, studying his opponents and planning for the fight, more often than not Randy time and again was able to stand as the victor. I feel that each of these:
Heart, Conditioning, Strength (both explosive and enduring), cardio, technique (stand up and ground striking, wrestling, submissions, as well as defense against all of these.) Will simply continue to evolve. In closing, if you've made it this far remember that when two people walk into that cage, it's a match up of styles, but there's things that cannot be measured. Watch Hughes vs. StPierre 1 and 2. A perfectly conditioned StPierre had to contend with the human element of his own fear and respect for Hughes, that when not there, allowed him to dominate what was the most dominant champion in the UFC at the time. Remember that being great at X may leave you open to Y and Z and a sound ass whipping when someone comes in with the right gameplan to run right through your weaknesses and avoid your strengths.
In summary quickly, here are some of the game changers that pushed the envelope and forced MMA athletes to rethink the game at the time. (Again, UFC only.)
Royce Gracie - The grandaddy of them all, beating bigger, stronger, more athletic fighters. Showed that fighting is as much more mental than it is physical.
Ken Shamrock - Opened the door to ground and pound by sitting without any real danger in the best guard fighter of the time's guard for over half and hour.
Dave Abbott - Being a mean bastard with experience and straight up power can get you through alot of tough spots even if you don't have a magic guard.
Marco Ruas - The evolution began with Marco imo, he was one of the first technical multidimensional fighters in the sport.
Don Frye - Probably the first fighter to force BJJ to start evolving into a more aggressive than passive style in the cage. His destruction of Aumary Bitetti showed you were going to have to bring more than pulling guard and hugging to not get a serious beat down.
Mark Coleman - One of the sports first people to raise the bar and probably the best natural athlete of the time, he had fantastic wrestling, coupled with a body capable of some serious horsepower. He burned the term ground and pound into the sport and brought the practice to a whole new level. At this time headbutts were still legal and he used them better than anyone else in the sport. In his fight with Dan Severn, Mark demonstrated that when two fairly equally skilled athletes fought, the more athletic and stronger athlete was likely to win.
Vitor Belfort - The first serious athlete to bring straight boxing back to the game. Vitor was a mess of speed, power and his most important contribution to the game at the time was the demonstration of his accuracy in his strikes. Showing that with accuracy you could overcome a size and power disadvantage (Abbott, Ferrozzo) and that superior striking could be just as effective as having an advanced ground game. He and Ruas truly returned stand up striking to the sport. Because of his early performances, many thought with BJJ and Western boxing, you were unstoppable only to be disproven later (this thought process is something that we've seen repeated since MMA began, and probably will for sometime to come.)
Maurice Smith - While never really becoming a superstar on par with people like Royce, Ken, Vitor, Tito, Coleman etc., Maurice made the fight cerebral again with the first introduction of a well thought out game plan and guts. He knew that Coleman would gas because of his size and muscular load and sucked it up knowing that he was going to have to survive the storm to come out on top. He worked this to a T with the defeat of the then impossibly dominant Mark Coleman at UFC 14.
Kazushi Sakuraba - Even though "Sak" mostly competed in Pride and in Japan, he was one of the first to dispel the BJJ myth in showing that not only could you beat a BJJ "specialist", but that you could even beat a BRAZILIAN BJJ specialist that outweighed you. To me Sak was to grappling, what Tank Abbott was to the whole sport. He mixed a little of this and that and showed that just because you didn't have a BJJ black belt didn't mean that you couldn't submit someone that did. This to me was where BJJ really started to lose some of it's steam and you have to go back to one of the early UFC's when Jeff Blatnick said, "Hey, you either punch, kick, or seize." That's really all there is.
Frank Shamrock - Upped the ante on being a complete fighter when you walked in the ring, really pushed the conditioning aspect of the sport by showing that you can take out someone bigger and stronger and equally skilled if you can last longer than they can. Fatigue makes cowards of us all was displayed to perfection in his 4th round win over Tito. This fight forever changed Tito's outlook on preparation for his fights and really influenced everyone to some degree from there on out.
Randy Couture - Brought the aspect of studying your opponent and developing a solid game plan to perfection. Never being one to be of the mindset "Hey, I just go in and whatever happens happens." He time and again has used conditioning and a great plan of attack to beat those that on paper there's no way he should have. Chuck Liddell the first time, Belfort, Randleman, recently Sylvia.