Steward: Cain was big part of title runs.

Just for fun, I went back and looked at the box score from Matt Cain’s first major league game on Aug. 29, 2005. The “Horse” was just a frisky colt then, a raw-boned, 20-year-old out of Tennessee, surrounded by bunch of worn-out veteran strangers gasping their way to the finish line of a 75-87 season.

Moises Alou in left. Randy Winn in center. Todd Linden in right. Mike Matheny, now the St. Louis Cardinals manager, behind the plate. J.T Snow at first. Ray Durham at second. Omar Vizquel at short. Pedro Feliz at third. Linden was the only guy under 30.

And Felipe Alou in the dugout, at age 70, still managing. Yikes. That’s how long ago it was. Cain even pre-dated Bruce Bochy, which is like pre-dating Moses. Yes, it’s perfectly OK to retire if you’ve been around that long, and Cain’s getting out while the getting’s good, while there are at least a few fumes of the World Series glory era still wafting about AT&T Park.

Cain, 32, saw all of it. He not only predates every player on the current Giants roster by several years, he pre-dated every player who wound up getting a ring on the first World Series title team in 2010. That’s right. He’s the only guy from the 2005 team that even made it to 2010, which tells you just how old and dismal that team was.

In that regard, Cain will always have a special place in the Giants’ three-championship era. He was the first cornerstone, the first significant building block of the post-Barry Bonds era, and definitely not a bad pair of broad shoulders upon which to start building.

Cain started the impressive run of prospects from the Giants’ minor-league system. Fans embraced the stocky right-hander before anyone ever really knew there would be a parade of farm system favorites to follow in the form of Tim Lincecum, Brian Wilson, Sergio Romo, Pablo Sandoval, Madison Bumgarner, Buster Posey, Brandon Crawford, Brandon Belt and Joe Panik.

So, with Cain announcing Wednesday that he’ll be calling it quits at season’s end — a rare career Giant covering 13 seasons — it really does feel like the end of a fabulous era. That’s because he started this fabulous era. When he arrived, the Giants were in a losing rut, and as he steps away, they’re firmly entrenched in another one. But in between … oh, what a memorable ride.

We’ll remember Cain for a lot of reasons. We’ll remember the perfect game, for sure. We’ll remember his incredible record of durability over the first 7½ seasons of his career. We’ll remember all those hardluck games in which he pitched beautifully, but the Giants couldn’t get him any runs. Team guy, we’ll remember him never once complaining about not getting enough run support.

But more than anything, we should remember his 2010 postseason, because it was so crucial to the Giants’ first title and this whole era — three starts, 21 innings, zero earned runs allowed. When the Giants beat Philadelphia in the NLCS, Cain pitched the most pivotal game — Game 3 at AT&T with the series tied 1-1. He outdueled Cole Hamels, allowing just two hits over seven shutout innings, arguably the most important performance he ever turned in for the Giants.

If the Giants hadn’t won that game, they probably wouldn’t have won that series, and of course, they wouldn’t have won that 2010 World Series. Who knows how that might have impacted 2012 and 2014? The fact is, Cain not only was the Daniel Boone of the era from 2006 to 2009, when the most important first Series flag was planted after 52 years of trying, he was at the center of the achievement.

Things could have worked out better since, of course. Cain’s 104-118 career record doesn’t shout greatness. He’ll never get a statue, at least one of those big ones. He never won a major award as a pitcher, although he did make three All-Star teams. And as cruel baseball fate would have it, once he finally got rewarded with a significant contract for his first eight years of stellar work — five years, $112 million — he was never the same pitcher physically through the life of it.

Those six consecutive years of 200-plus innings ultimately took their toll before he signed his career deal. Under that rich new contract, the most he ever threw in a season was 184 in 2013. He made just 30 starts once, in that same year. Since then, much the same as it was with Lincecum in his final seasons with the Giants, it was painful to watch him knowing what had come before.

The funny thing about Cain is that at least until some point last season when it became completely obvious the life in his wellused right arm had faded, many still expected a breakthrough back to the old days. That’s because the pitcher’s demeanor and expectations of himself never seemed to waver, even when things were turning their most sour.

As a man, Cain is pretty much the same person now as the kid who walked into the Giants clubhouse at a very mature 20. He was as normal and understated as Lincecum was freakish and flamboyant. His appearance alone reflected his low-key consistency of character. You can look at photos 10 years apart and it’s hard to tell before and after — he looks almost exactly the same.

With that in mind, it does seem a little bit premature that Cain’s calling it quits. He’ll make the last start of his career Saturday, and celebrate his 33rd birthday Sunday. He still has a lot of life to live, even if he has no more fastballs to give.

At the same time, what does Cain have left to accomplish? He has his fortune in both achievements and earnings. He has more prime of his life to spend with his young daughters Hartley and Everly, who are 6 and 3. He can walk away as part of a rare fraternity in any professional sport these days — a player who only really knew one team from the point he was drafted in June 2002.

Most of all, during his tenure, Cain was the pioneer who provided the first critical steps to the Giants’ most successful period in San Francisco. It’s a large, impressive hook to hang one’s cap on.