So we begin our story in the old days of Starcraft 1 with a custom map known as Aeon of Strife made by someone named Aeon64. The concept of last hitting and attempting to reach the other player's base came from this game and would follow through to the Warcraft 3 mods, eventually leading to Eul making a game called "Defense of the Ancients" in 2002. More modern recognizable changes occurred during this period including the addition of spells, levels, and equipment. I would also like to point out during this time that these DotA versions, and the other many, many adaptations, are not very similar to the one we have today - simply a few concepts are shared.
Eul would eventually leave, letting others fiddle with DotA, and soon we would arrive at DotA Allstars in 2004 thanks to Meian and Ragn0r. Shortly after this version was further refined by a man named Guinsoo who would add many new heroes, leading to the birth of an actual competitive scene. Many consider Guinsoo to be the founder of this version of Dota Allstars. Later in 2004 Guinsoo would receive help from Icefrog and Neichus to keep up with the pace of the growing game and its rapidly growing fanbase. It was during this time that Pendragon would create a popular fan website known as dota-allstars.com
It was in 2005 that Guinsoo would take his leave (what he worked on during this time is up to speculation) and Neichus and Icefrog would take over. More heroes were added under Neichus and Icefrog, but it would not last as Neichus would also resign as developer, leaving Icefrog as the only one remaining between the 3 giants. Many have heard of Icefrog and he remains as the most influential DotA developer to this day. Developments throughout this time period weren't quick or drastic, but they were consistent and loyal to the game.
It would be around 2008-2009 in which the development of more modern versions with modern engines of DotA would be sought out for. The DotA scene still existed and did reasonably well in all parts of the world, but it still remained a Warcraft 3: The Frozen Throne custom map.
Heroes of Newerth would be developed by the company S2, essentially a clone of DotA with a new engine, graphics, and art design. At one point in the beta it would reach upwards of 50k concurrent players online, but dropped off after release. It was also rumored that Icefrog was aiding in the design of the game at one point. That rumor, however, was unconfirmed although sources such as Pendragon and an anonymous Valve employee claimed it to be true. There would also be a blog on the subject of Icefrog called "The Truth About Icefrog" on aiding both S2 games and Riot, but other Valve employees dismissed the blog to be false.
Up until that point it seemed as though Pendragon and Icefrog remained relatively close as well as many other prominent figures. Most of these developers and important community members probably kept contact with each other, but rumors of differences in ideals and how to market the game clashed, leading to these people spreading apart and going in different directions. Pendragon wished to profit off the game; for so long it remained as a free, non-profit game for the players as just a custom map. Icefrog disagreed originally and wanted to keep the game free and open to everyone. Pendragon stated in his (in)famous letter to the dota-allstars community that Icefrog intended to sell rights for DotA to both Riot and S2 games behind his back, while Icefrog... well, said nothing because he's Icefrog. However, in defense of Icefrog, S2 replied "no comment" while Riot claimed they had never worked with Icefrog.
Pendragon and one of the original creators of Dota Allstars, Guinsoo, would then move on to team up with Riot. Back then in 2008 Riot was not a particularly large company, and no one really knew how far the game would go. Pendragon, previously stated to be the creator of the most popular dota fan-based website dota-allstars.com would shut it down for good in 2010 to completely move on to League of Legends. He claimed to archive the site but ultimately never kept his promise. Steve "Guinsoo" Freak made his sudden return to become the lead developer for League of Legends.
In 2009 Icefrog would officially work with Valve on the upcoming game DotA 2 while still remaining anonymous. It was also rumored that Eul, the original creator of DotA, was also part of the development team. This would also explain as to why an item titled "Eul's Sceptor of Divinity" keeps Eul's name in while the DotA 1 name "Guinsoo's Scythe of Vise" has the name Guinsoo removed.
And here we are today. League of Legends remains one of the most popular games in the world, holding millions of players with worldwide attraction including Korea and Taiwan backed by a multi-billion dollar company in Tencent. DotA has since been adopted by Valve, one of the most prominent if not the most prominent video game company in the world, carrying over its professional scene and previous talent in full.
If people were ever wondering why there is constant bickering between these communities, and why there always seems to be some sort of drama forming up behind the scenes then there you have it. League of Legends and DotA were not simply just games of the same genre in the same way that Call of Duty and Halo could both be considered FPS games. I don't have proof, but it is highly likely that relationships between the developers are not so great, and that they most likely never will be.