Cinema shows us many marvels and impossible events, but perhaps the most implausible of these is a scene in the Danny Boyle directed film Yesterday. This features Lily James playing a real live human adult who hears the song 'Yesterday' for the first time in her life, without any baggage. I can't imagine how, as an actor, she would have prepared for this scene. 'Yesterday' is the most covered song in history, having been recorded by over 3000 artists. It so ubiquitous that by the time children develop their own interest in music, they are already sick of it. When they are old enough to hear it, in other words, they can no longer hear it.

The song 'Yesterday' is overshadowed by its success, but the real reason it's important is its pivotal placing in the middle of the Beatles story. We are possibly still too close to what happened to recognise the enormity of that story, but the further we get from the days of Beatlemania the greater perspective we have on that unprecedented cultural shift. I suspect that it won't be too long until the phrase "The Beatles and the Stones", and the idea that those two bands were in any way equivalent or equal, will start to seem a little quaint.

It is a huge challenge to explain the massive changes in culture and social attitudes that occurred between 1962 and 1970, the period in which the Beatles were active. There were clearly many factors at work, but any explanation needs to factor in how the most successful, best loved and most famous four people in the world soaked up the avant-garde and the counterculture and then used it to hose down the mainstream. In the future, there will be conspiracy theories arguing that it wasn't possible for four relatively uneducated working-class northerners to create a body of work like that over such a brief period. The songs must have been written secretly by a group of people from the Oxbridge establishment, we will be told.

To appreciate the full Lovecraftian horror of 'Yesterday', it is necessary to put it in the context of that extraordinary eight-year recording career of The Beatles. This is a story in which the latter half tends to get more attention than the early part. This is understandable, because those of us born after The Beatles split are likely to have heard later tracks such as 'Tomorrow Never Knows', 'A Day in the Life' or 'Strawberry Fields Forever' before they heard early singles like 'Love Me Do' or 'Please Please Me'. In those circumstances, those joyful early boyband records can appear as lesser. But to grasp the significance of 'Yesterday', you need to wipe the second half of the Beatles story from your mind. It was not just that those later records did not exist then. It was that nobody could even begin to imagine music which sounded like that. The Beatles were a great little rock band, raised on American R&B and early rock & roll, and that was enough. They had become the best and most successful rock band in the world, and there was nowhere else they could go from there. They had achieved everything they set out to do. It was just a question of trying to sustain their career for as long as they could.

On the evidence of 1964's Beatles For Sale, they might not be able to sustain it for much longer. While their previous album, A Hard Day's Night, had been all original material, Beatles For Sale was padded out with quick covers in order to hit the lucrative Christmas market. From the cynical title to the exhaustion visible on the sleeve photo, it looked like a band trying to milk what they could from their fans before it was too late. But then George Harrison's dentist spiked John Lennon's coffee with acid, and a few days later John sat down and wrote the song 'Help!'.

1965s Help! album is often overlooked in The Beatles story. It is seen as the end of their boyband phase, rather than the start of their creative growth. Even the Bootleg Beatles skipped it on their last tour. This is a shame because it is a great little album with much to enjoy, not least relatively obscure gems such as 'It's Only Love' and 'You're Going To Lose That Girl'. It's here we're properly introduced to the idea that George Harrison is a talented songwriter, which for a band that includes Paul McCartney and John Lennon is something of a plot twist. The album is sharp, fast and consistently brilliant. It is the sound of a band operating as single entity.

One of the most important parts of the Beatles story is the extent to which those four men became one. They merged telepathically through 1000 hours of gigs on Hamburg stages, and then were fused into a single atom by being at the centre of the immense pressures of global Beatlemania. The process was alchemical. The four material elements – John (fire), Paul (water), George (air) and Ringo (earth) were fused under unimaginable heat and pressure into the fifth immaterial element. The cover of Help! is an example of this. The four Beatles stand in line with their arms in semaphore shapes. The actual letters they spell out are NUJV, which is a meaningless jumble. But such is our expectation that they are four people functioning as one entity and communicating a single message that we automatically assume that they are spelling 'Help'. The title song, which opens the album, demonstrates how musically synced they are, as the backing vocals playfully skip ahead, stay in time with, or fall after the main vocal. Every musician knows exactly what the other three are about to do and plays accordingly. 'Help!' may not be The Beatles' most original, best loved or most important song, but there is an argument for it being the most perfect thing they ever did.

Help! the album is also that rarest of things - the spark of transition captured.

When inspiration strikes, you know it immediately. Your eyes go wide and your mouth opens, but you don't actually know at that moment what it is that has hit you. It takes seconds for the idea to be translated into English so that your brain can be informed. That period, between inspiration hitting and conscious understanding of the new thing that's arrived, is a wonderful, lovely moment. But it is also so fleeting that it is rarely captured by artists, and this is what makes the Help! album so special.

The second track, Paul's 'The Night Before', establishes this theme. The song is about realising that something has changed in a relationship, but not knowing what. It is preparing us for 'Yesterday', the monster penultimate song at the other end of the album - the darker, wiser mirror to 'The Night Before'. 'Yesterday' is also a song about how something has changed, but this time it understands what the change is, and this knowledge makes it miserable. It has tipped from innocence to experience. It reacts by wanting to somehow retreat to the now-vanished past.

The Beatles always knew that 'Yesterday' was ominous, right from the moment it presented itself to Paul in a dream. It was clearly too good to ignore, but they weren't comfortable enough with it at the time to issue it as a UK single. During Lennon's bitter, Gollum-lost-the-ring early 70s phase, he recorded the anti-McCartney song 'How Do You Sleep?', with the key line "The only thing you done was 'Yesterday'". To attack McCartney on the grounds that his body of work as a songwriter isn't up to much is, it's fair to say, not a convincing diss. But the line does single out 'Yesterday' as important. Even Lennon at his worst couldn't bring himself to discount it.

When as it arrives as the penultimate track on Help!, 'Yesterday' changes everything. All of a sudden, McCartney has gone from being a good little rocker to penning page one of the Twentieth Century songbook. The landscape has changed, and huge new vistas of possibility have opened. It is no longer the case that they have achieved everything imaginable and can only try to sustain. It is now clear that they've only just begun.

That the album follows 'Yesterday' with a wild cover of the old rock & roll stormer 'Dizzy Miss Lizzy' seems to show the band panicking, horrified by what they have done and desperately retreating to the safety of their simpler, Cavern-pleasing old ways. This reaction is understandable, because within that song is the realisation of what that growth will cost.

'Yesterday' was not a Beatles recording. It was a solo Paul McCartney recording, too afraid of the future to announce itself as that and hiding behind the Beatles' name for security. To fulfil their new potential, the four Beatles would have to grow as individuals. In doing so, they will no longer fit together as perfectly as they did. When they grow, the unit will crack. The way ahead requires the single Beatles unit to become four individual men again.

'Yesterday' is a song about loss. The loss of the old is, ultimately, the price you must pay for new growth, although lyrically the song does not recognise this. But in the creative growth that it heralds, in its ability to communicate directly with not just fans of rock music but everybody on the planet, in its context in the greatest cultural story of the twentieth century – we can glimpse in the song the growth that follows loss.

By four men growing as individuals, the whole artistic flowering of the later Beatles albums became possible. For this, we have so much to be thankful for. By the White Album, we would become used to seeing four individual portraits of the four Beatles, instead of the focused group portraits of before.

But like the song, we too are aware of what we have lost. When they were fused into one, the Beatles were the most joyful, positive and miraculous event of the twentieth century. It is possible that we will never see their like again. That is quite a thing to lose. Such is the horror of 'Yesterday'.