I know this is a question that many of you not be able to answer, but for any players of this game... I'm interested to know when exactly the Lanes and Roles became so defined as they are today; I would think that when the game started, people probably went wherever they please. I'm just curious if there's any sort of story behind the current Lane and Role definitions, that anyone knows of.
ADCs have the weakest laning phases in general, so you want them to be with a support in order to make them safer to lane.
You want your duo lane bot lane so that you can more easily control and contest dragons, where as if they were top you would be unable to.
Mages tend to be squishy and immobile, they are sent mid because the shorter lane makes it much safer for them
Asssassins work best in 1v1s against squishies such as mages, so they go mid.
Mages and Assassins roam well, so they go mid in order to have access to both side lanes.
Top laners tend to be tankier or safer, which is required as they are more isolated and solo in a long lane.
The reason that mages go mid is because the shorter lane means that waves reach there slightly faster. Generally, mid lane is where you get more XP more quickly, and mages and assassins scale incredibly hard with levels (they have high base damages and high base damage growth on their abilities).
Tanks, bruisers, and carries generally scale with gold, as most innate tankiness in kits requires item scalings (Garen's W, Malphite's passive, Vi's passive) and carries don't care about their abilities, they care about auto attacking.
Early Season 1: Anything goes. Most champions couldn't viably jungle (jungle was actually hard then), so the few that could did, but there was even a lot of 2-1-2 back then. People picked what they wanted. In many ways, it was when LoL was the most fun.
Late Season 1: Two metas began to evolve in NA and EU. They varied in some ways, but the biggest difference was the presence of an AD mid in EU and an AP carry mid in NA. (Ashe was probably the favorite mid laner in the game at that point because of the pressure of her ultimate.) Lots of roaming supports.
End Season 1: You pretty much have the meta that we are in now. 1-1-1-2. Although back then it was normally a tanky top laner, sustain jungler, apc mid, adc/support bot.
Season 2 Pre-M5: Pretty much the same as the end of season 1, but jungling got more figured out and fast clear junglers started to get viable.
Season 2 Post-M5: Very different meta overall when M5 showed up in terms of what type of champs were played where. All of the sudden early jungle invades were a thing (that hadn't been much of a thing before outside of nunu). Carry top laners started to show up, along with way more assassins and supportive mid laners. Protect the kog comps actually kinda started around now because of Genja's kog'maw build that covered some of his early game weaknesses.
Season 3: A lot of the stuff from season 2 remains true here, although at this point roaming supports largely fell out of favor towards the end of season 2 and were more or less the ADCs ward bro/early cover.
Season 4: Things again stay mostly the same with the addition of WAY more frequent lane swaps (which is a lot bigger a strat in NA/EU than anywhere else), and roaming supports start to be a thing again because of this.
Season 5: I'm assuming you've been here for a larger chunk of season 5, but it is rather similar to season 4 overall. Different champions go into and out of vogue, but the overall lane setup remains the same.
Well, for a while Riot maintained the stance that they would never do anything to enforce a meta or specific champions to a certain role/lane.
However, when their primary focus shifted to LCS I think they realized in order for people to think of League as a legitimate competitive sport, they needed to define some roles for the game. What kind of sport doesn't have positions/roles, after all?
This is when we started seeing players hard cast into the role of "support" or "top" and the meta was laid down solid.
The game actually started out really fluid and only crystallized into a positional meta towards the end of Season One, iirc. There was initially no defined jungler position, but some champions were just really good at roaming, clearing jungle camps and setting up kills when visiting lanes (
was an absolute terror), all without sapping valuable resources from lanes, and so junglers were born. Mid lane is the shortest, and therefore the safest, which really benefited mages, who both relied heavily on an experience lead to remain relevant and lack the mobility or survivability to escape in longer lanes (at least, that was a defining weakness for pretty much every older mage). Some level-dependent marksmen also became popular in mid lane, and sometimes resurface (mid lane 
, for example), but ultimately mid is always good for mages, due to the safety and faster leveling, and assassins, due to their burst allowing them to kill mages before they could retreat and mid lane allowing them to gank any other lane as quickly as possible. Marksmen and supports coalesced into a single lane, due to supports being gold-independent (far more back in Season One, anyways) and both classes being relatively experience-independent to remain powerful, with marksmen also really needing assistance to cover their weak early game. Bot lane ended up working best as a duo lane, since it meant teams had more people around Dragon, and therefore better control over an objective that could be captured early in the game. Because tanks and fighters could always operate relatively independently in lane and didn't bring the biggest damage contribution to the table, top lane ended up working best for them too, since they'd only need to migrate come the mid game at the earliest, if they were needed in teamfights, and at worst they could just splitpush independently very well.
LCS Is the basic answer. the best teams tried different things and this was what worked best.
As far as when they became defined I don't have that answer, but how is largely because of competitive play.
Teams analyzed what was most effective to put where, you ended up with the Duo lane being bot, because you get solid control over the Dragon, which other than towers is the only real objective on the map until 20 minutes when Baron spawns.
You ended up with Top lane taking tankier champions or champions with lots of mobility, often times anyway, because the longer lane with only a single person you needed more defensive stats or built in escapes to survive a gank if one did happen to come your way.
Mid lane was carry based champions that either have mobility or don't, but are typically squishy champions, so they have the shorter lane and several places to run in the event of a gank for increased safety, because in a longer lane they're typically sitting ducks.
Junglers, basically just had to survive higher damage farming, and be able to run cost effective item builds, due to the decreased income, which is why certain items are phased out of a jungler's interest.
I guess initialy ADC champions were played in mid lane for the same safety purpose, not quite sure why that switched over to bot lane with a lane partner, but yeah.
the current lanes where actually conceived by the EU (first teams to run adc + supp in bot lane, etc) go read the history quite intreaging to be honest with the way it evolved, lolesports have done an article on it too,
Mostly because efficiency, with 3 lanes, one jungle and 5 teammates, players eventually found out what was the best way to optimize the gold income depending on the needs of the team.
Good point, I am all for tactical options and flexibility.
you just have to figure what each champion falls under, and now you want to assemble the best 5 people who are going to get to the enemy nexus. it just so happens to end up in a 5v5 scenario most of the time, so how do you get the best 5v5?
if you pick 5 mages who are really good wave clearers, they will just get dived on by a tank and blown up. everyone would build MR and you guys would probably lose.
over time people now know the best team for taking towers and objectives is the standard 1 mage/ 1 adc/ 1 support/ 1 peeler tank/ 1 wild card as needed.
people send the mage mid lane because they scale off of levels and ability levels and need to get the highest level first. people send their adc and support bot lane because its generally safer than top lane due to jungle camp locations and frequency of jungle presence.
Over time the meta formed from players learning the best combination of offensive and defensive picks, where they go, and what the game allowed. For example top has no influence in the area itself, thus teleport makes a great pick and nearly NECESSARY pick unless you're going full split push snowball. Which wont work in LCS, only solo queue.
Bot lane has the most objective influence thus they need 2 people down there, and a support to help the adc. Supports are generally picks that have strong base stats and utility that don't need a lot of gold to be useful and strong. ADCs are champions that need a ton of gold to be useful, and are not strong in solo situations, thus they need the support. The jungler needs to exist because otherwise no plays would happen if people of equally high skill go against each other.
Mid lane is just... mid lane I guess. There are videos on this out there.
Top lane: Mainly fighters because they are isolated and get the least attention from their jungler due to dragon being at the bottom of the map, so it needs to be a champion who can handle themselves for a good bit of time. Fighters are ideal for this because they usually have well-rounded mix of damage, durability, and sustain, so they can take care of themselves for most of the game.
Middle lane: Mages and assassins. Mages prefer to go mid lane because they are usually squishy, immobile, and mana hungry. Since the middle lane is considerably smaller than the others, they have less distance to cover to get back to the safety of their turret, which is extremely important to survive ganks since they are squishy and immobile. Since the middle outer turrets are the closest to the Nexus, they can afford to return to base if they run out of mana without sacrificing their turret. Assassins like middle lane because it's the easiest lane to roam with, as they are equal distances from top and bot lane and can quickly get to where they are needed to pick up a kill.
**Bottom lane: **ADC and Support. Bottom lane is next to the dragon, so it is a duo lane in order to reduce the time it takes to kill dragon, the early game objective, support is also here because it allows warding of the dragon pit and helps protect the ADC, who usually cannot survive in a solo lane once their opponent has access to all of their abilities.
I feel all areas are ver versatile but the adc role that is write on stone I just wish more comps don't use an adc just look at it many assasins and mages and utility champion on mid even irelia have been seen then mages on the top lane with fighters and tanks many kinds of jungler and supports but at the end the same right clicker adc...
The tl;dw, or if you flat out don't like his voice, is basically this:
Gold.
There are 3 lanes, and with minions, there are 3 consistent streams of gold. Logic dictates that you want at least one champion on any one stream of gold. This creates our 3 laners.
There is a 4th stream of gold however, the Jungle. Along with promoting the Summoner Spell Smite for Epic Monsters, this is why we have a single Jungler, to take advantage of the gold in the Jungle.
The last player however is left without a stream. I forgot if they mention this or not in the video but some important context is necessary here: Because a lot of stats in League are multiplicative, champions scale exponentially into the late game. This means that two equally farmed half developed champions are actually weaker in combined strength relative to one fully farmed fully developed champion and one starved one. This means that the last guy is destined to find a niche without gold: this is our support.
There are basically three spots our Support can go now: Mid lane (roaming), side lane, or Jungle (ganking assist/counter jungle). It's inefficient to leave the support rotating around mid however because, as the shortest lane, the support has the least amount of pressure here because the enemy mid laner is naturally safe (it's literally the shortest distance lane from where the creeps meet to his tower). Due to how jungle experience works, it's hard to make a secondary Jungler out of the support, so we throw them into a side lane. Generally, we like the support in the bot lane because this gives us the best control over Dragon in the early game (more players in that general side of the map).
Now, why do we have a carry mid, carry bot, and tanky top? Remember what I said about mid lane being the naturally most safe lane. This means it's so much easier to afford putting a champ with bad defenses in mid lane, for example a burst carry (Mages and Assassins). Top lane however does not have this safety, so the champions who go top lane are usually chosen to go top because they are good at surviving when things get rough (ergo, tanky tops). Bot lane however is protected by a support. It's harder to put a Burst Carry in bot lane however because Burst also depends on the level of your champion; something inhibited by a support. On the other hand however, DPS Carries mostly only have to worry about gold. Because of this, we tend to put a DPS based carry in bot (ADCs).
I really do suggest watching the video, it really does justice on this explanation.
