When Epic added a battle royale mode to Fortnite in September last year, PlayerUnknown's Battlegrounds publisher Bluehole was pretty upset.

"We are concerned that Fortnite may be replicating the experience for which PUBG is known," said Chang Han Kim, then its executive producer and now CEO of PUBG Corporation, which today runs the breakout shooter. The press release listed concerns over similarities in user interface, gameplay and 'structural replication' between the two games, and made a vague threat about potential legal action, which hasn't apparently gone anywhere.

Fortnite now has 45 million players, which is probably greater than the number which plays PUBG, and Battle Royale mode is what they play. That has to hurt. But it's not to say PUBG has much of a leg to stand on. "Look, I don't claim ownership," Brendan 'PlayerUnknown' Greene told Rock Paper Shotgun last summer. "So, it's a last-man standing deathmatch. That's been around since people could pick up clubs and hit each other. I would never claim ownership over that ... I love to see what the genre has created. It's various versions on something that I guess I popularised, you know? The idea itself is not mine."

He's absolutely right. The battle royale is way bigger than any one company or creator, even PlayerUnknown. And as PUBG began pushing the 'last-man-standing deathmatch' from cult curio to console mainstay, it's become a widely recognised genre of its own. That transformation, in which a new genre has originated, is a fascinating mirror of the wonderful way ideas merge and evolve, spread and multiply, skating through inspiration and invention, copying and stealing.

Genres almost always have muddy origin stories. That's certainly true of the battle royale. Until Fortnite came along, Greene was the creative force behind the biggest battle royale games around: H1Z1: King of the Kill and the original DayZ mod, PlayerUnknown's Battle Royale. But the whole thing is much older. It's very difficult to trace the earliest last-man-standing-style multiplayer game because it's such a universal concept, but today's battle royale has direct thematic roots in Suzanne Collins' The Hunger Games and the Japanese cult movie Battle Royale, which both depict groups of kids being dumped in tracts of land and asked to fight each other until only one remains. The first time this theme was expressed in a big way in an online game was in around 2011, when the Survival Games game-type, otherwise known as Hunger Games, began to take over Minecraft servers. Its popularity was so great that it was added as a permanent multiplayer feature in Minecraft's console versions called Battle Mode.

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From there, the proto-battle royale jumped to a new game, DayZ, when a group of players started holding special invite-only events in 2012 called Survival GameZ, which were streamed over Twitch. Their drama and realism-inflected competitiveness inspired Greene, then a keen DayZ player, to recreate Survival GameZ as a mod and he found he struck gold. There ends the history lesson. The point is that the general concept of the battle royale has grown almost naturally from wider culture, the evolving nature of online tech and modding scenes, and also from the bit of human nature that blinks into primitive life at the idea of desperate survival against all odds.

But that's not to say that PUBG doesn't feature some critical new ideas. And here's where the whiff of 'clone' comes from in Fortnite: BR. One of PUBG's genius features is the way a game begins with a plane flying over the island, and Fortnite, despite adding all kinds of other features of its own, notably building, has taken that idea, along with the broad mechanics by which the playing area constricts, all of which have played a big role in PUBG's success.

Clone is a powerful word. A clone has no creative ideas of its own. It's a copy, and a malign one at that. Ridiculous Fishing was cloned, and so was Threes. These unique and inventive games found themselves gazumped by close copies which found more success than they did. Vlambeer's Ridiculous Fishing, previously a free Flash game called Radical Fishing, was beaten to the App Store by Ninja Fishing. Threes was followed a month after its launch by 2048.

But a clone operates at the scale of the individual. Ridiculous Fishing and Threes were distinctive and unique designs which were co-opted by savvy developers (Gamenauts and Ketchapp) who saw opportunity in swooping quickly to take them as their own. And the evil of the clone - aside from the human cost - is that it crushes evolution, feeding off new ideas and bringing none of its own. By comparison, PUBG isn't built on a unique idea, and Epic took months to turn Fortnite: BR around, adding lots of its own ideas in the process.

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Another contrasting example with the relationship between PUBG and Fortnite is that of Firaxis' XCOM series with newcomer Mario + Rabbids Kingdom Battle. Mario + Rabbids has none of XCOM's crucial strategy layer but in taking key elements of its tactical game, specifically a tweaked version of its move-and-action mechanic and a camera and cursor which behave in much the same way, the experience of playing it feels very close. This co-opting of a game's play aesthetic is very different to cloning, because while Mario + Rabbids evokes XCOM, you're playing a very different tactics game which places much more emphasis on dynamic movement within its stage than XCOM does.

Mario + Rabbids also seems unlikely to be the stirrings of a new genre of strategy game based on XCOM, because Mario + Rabbids takes mere slivers of its design. A genre is built on a strong conceptual foundation, not little design ideas: Last-man-standing. Pass the winning line first. Destroy your opponent's base. Improvise with what you get to reach the end. Successive games take and rearrange little ideas to make new expressions of that foundation.

Most genres bubble up outside the mainstream industry, built by modders and tinkerers, amateurs and enthusiasts. In these 'folk games' it's sometimes hard to find a single originator or author, instead groups of people feeding from each other, freely copying, rearranging and rebuilding to develop and refine a core concept. The best ones find audiences and rapidly grow, even as they're still evolving. Look at the history of the MOBA, for example, which began with Defense of the Ancients, a mod of Warcraft III originally by Kyle 'Eul' Sommer. Others built on it, notably Steve Feak and Abdul 'Icefrog' Ismail, adding maps, items and characters. Variants splintered from it; arguments spiralled about which direction they should take and what defined them. Feak wound up helping to found League of Legends. Icefrog went to Valve to make Dota 2. Now it's a distinct genre, comprising multifarious expressions of the core idea of opposing teams of heroes pushing into each others' territory to destroy their tower.

The MOBA is nevertheless pretty defined. By comparison there's the Rogue-like, a looser, wilder, less lucrative, but profoundly important genre which has spurred close and highly refined expressions like Brogue and real-time action expressions like Spelunky, which has almost become a kind of sub-genre in itself, the Rogue-like platformer. Aspects of it even appear all the way out in games like Dark Souls. A genre can be amorphous, but it has to have a strong core concept.

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No matter how strong the idea, it generally takes a single game to make it explode. Once that exemplar appears, others rush to replicate it and accusations of cloning abound. For the first-person shooter, it was Doom. The market was awash with 'Doom-clones' during the mid '90s, until the genre became known as the FPS. That's despite the fact that Doom wasn't the first FPS by a long shot, but it was the first to capture a profound sense of being in an all-out action world, using lighting, sound and complexity of geometry to such effect that it's still a delight to play today. Many games followed it to recapture and build on the magic: Dark Forces, Duke Nukem, Chex Quest.

There's a point in the process when the accusations of cloning dissipate. It's interesting to ask when - and I don't know the answer - the Doom-clone ended and the FPS began. Was it in 1995 with first full 3D first-person shooting game, perhaps Descent? If so, does that mean that until that point, the genre was focused on the specific aesthetics and affordances of Doom's engine? Or was it after Quake in 1996? If so, does that mean that the world waited until id, the leader of the genre, had diversified its expression of the FPS into a fresh new game? Or was it when GoldenEye 007 came out in 1997, which was when the genre successfully manifested itself outside PC, the platform on which it originated?

The FPS came from fuzzy roots, in 3D Monster Maze and Battlezone, Dungeon Master and Ultima Underworld, Catacomb 3-D and Wolfenstein 3D, and was then focused and refocused by Doom and what came next. The fact that not being able to discern exactly when the FPS began shows how the whole question of genre is about feel. It's about the point when a body of similar works has mapped out the boundaries of what they're interested in - what they are and what they aren't - and when there's no clear leader any more.

Once that happens and a new genre has surfaced, it tends to flower. The games within it no longer have to circle around the game that got things moving. They don't need to evoke it to attract attention, or to be worried about losing what makes the whole thing tick. They can be themselves. That's what is happening to the battle royale right now. Fortnite has undoubtedly taken some of the unique ideas that have helped PUBG reach such success, but its own success also marks the point when the battle royale is no longer dominated by one game. That means we can expect to see it diversifying fast from here on. SOS, The Darwin Project, Europa, Paladins: Battlegrounds, Islands of Nyne - maybe there's a new classic in there somewhere. Let's play a battle royale.