“It felt like we were all at the centre of the world for a minute,” says Geri Horner (nee Halliwell) as she recalls the heady days of the 1990s. “There was Britpop happening, ‘Cool Britannia’ and it really was a melting pot of excitement.” Horner is on the phone, days after the Spice Girls have announced their latest reunion, the precise details of which remain hazy.

Obviously the world has moved on since the 90s, but the emotional impact that the Spice Girls made on a generation has left its mark. Some of the group’s fans who grew up listening to the band have now taken centre stage: Katy Perry, Little Mix and Adele, who met Melanie Chisholm (Mel C) at a Coldplay show at the Albert Hall. “We were in a box and a friend introduced us: ‘This is Adele,’” Mel C tells me later on the phone from London, “and she was actually in tears. My friend said to me: ‘Why’s Adele crying?’ ‘Cos she just met me!’”

Even now, the Spice Girls stats are enough to make your eyes water. Still the biggest girl group ever, they have sold in the order of 85m albums worldwide. But the musical influence and cultural impact, particularly of their first two albums, Spice and Spiceworld, have been somewhat overlooked. How did they do it?

“We were writing before we collaborated with anybody else, always putting our spin on things that were going on,” Horner says now. “To begin with there was just the five of us on a wing and a prayer. We had to persuade these different musicians to work with us and give us their studio time. We’d go in and say: ‘Look, give us the belief – write with us.’”

“When we first started [with the name Touch], we were pretty bland. We felt like we had to fit into a mould,” says Mel C. “And then we realised that we were quite different personalities, different to each other and to all the female groups in the past. We also realised there was a lot of strength in that.”

This was before they had hired their manager Simon Fuller, let alone secured a recording contract. Even allowing for their lack of industry muscle, they were swimming against the tide in the sense that this was an era when boybands ruled, notably Take That and Boyzone in the UK and the Backstreet Boys in the US. There had never been a group of girls who were addressing themselves specifically to a female audience in such a brash, upfront way.

But the Girls’ sense of mission swept all before them. The first to take the leap of faith was Eliot Kennedy, with whom they wrote Say You’ll Be There and Love Thing at his studio in Sheffield.

“There was no reason I should do it,” says Kennedy, who had previously co-written Everything Changes with Gary Barlow for Take That. “The chances of it coming out were pretty slim. They didn’t have anywhere to stay, so I had to put them up. But they were so dedicated and confident in what they were doing.”

At the start of their next collaboration, with Richard “Biff” Stannard and Matt Rowe, who had worked with the boyband East 17, they hit the jackpot. Rowe set up a drum loop on his MPC 3000 drum machine and the girls started throwing their contributions into the mix.

“They made all these different bits up,” Rowe told me for my 2004 Spice Girls biography Wannabe. “Not thinking in terms of chorus, bridge or what was going to go where; just coming up with all these sections of chanting and rapping and singing. And then we just sewed it together. Kind of a cut-and-paste method.” That song became their first single Wannabe, which sold more than 5m copies worldwide.

Incredible as it seems with the benefit of hindsight, Virgin Records didn’t want to release the song as the band’s debut. “There was about six guys from the record company in the room insisting that we should release an R&B song,” Horner says, “and I was going ‘No’.” According to Stannard, the issue was never seriously in doubt. “Geri’s spirit was in the band; all five of them became like her,” he told Beat magazine last year. “The people from the label didn’t stand a chance.”

Instead of writing a lead line for one of the voices and arranging the others in a supporting role in the traditional way of vocal groups, the Girls maintained a strictly equal, five-way split, constantly passing the baton from one to another.

“Sometimes you’d go in with an idea and they would build on it,” Horner says. “We’d go through old albums from the past, from different countries, and say: ‘Listen to this drum beat or this melody here,’ and build up a track from that.”

“We all had different artists that we loved,” Mel C says. “Madonna was a big influence and TLC; we watched a lot of their videos.”

Horner was a fount of ideas for songs, arriving at sessions with her book of jottings, notes and scribblings, which often produced a starting point of a lyric or a song. “They’d sing melodies,” says Andy Watkins, who together with Paul Wilson was one half of production duo Absolute, who co-wrote four of the songs on Spice and five on Spiceworld. “Typically, she’d sing one line and the girls would pick up on it and construct around it.”

“When they wrote, they were also writing the dance routine, constructing the video, all at the same time as writing the song,” adds Wilson.

“They did what they did,” Watkins says. “They knew what they wanted to write about from day one. You couldn’t force your musical ideas upon them. We had no control over what they did or what they wanted to say. We just hemmed it in and tried to make songs out of it.”

Spiceworld was written and recorded during the height of Spicemania, and when they were making their film of the same name. Rowe remembered them coming in to record Spice Up Your Life. “There was going to be an MTV crew filming them as they did this. People in offices all around the studio were bombarding them, throwing things through the window, getting into the building. There were big crowds in the street outside.” Watkins and Wilson, meanwhile, followed them around in a mobile studio on a lorry and grabbed the girls, often one or two at a time between their appearances on film sets.

“I remember writing Too Much in the back of a car,” Horner says of this time. “Considering the insane pressure we were under, it was quite a feat to get anything written. It’s about that craving we all feel; the emptiness we feel as a human race.”

Despite all the pressure and time constraints, the songs on both those first two albums shine brightly to this day. As well as the major evergreen hits, there are tracks that retain a cool mystery that goes way beyond the usual teenybop pabulum. If U Can’t Dance with its ultra-hip Digital Underground bass sample and risque Spanish-language rap; the slow-burning, sexually compromised tension of Naked, with Emma Bunton talking on the phone to her mystery man (“I’d rather be hated than pitied”); and the sassy, George Michael-lite sloganeering of Do It (“Don’t just do the right thing to be pleasing”).

“As a writer, 2 Become 1 is one of my favourite moments,” says Mel C. “It’s a beautiful song and it had an important message. I expect it went over the heads of a lot of our young fans. But to be a young girl band mostly shouting and being a bit chaotic, and then to have a safe-sex message in your song, I feel very proud of that now.”

On many of the songs there is a powerful streak of assertiveness and sense of personal awareness that was ahead of its time: “What part of ‘No’ don’t you understand?” (Too Much); “God help the mister who comes between me and my sisters” (Love Thing). Rather like The Simpsons – another pre-teen phenomenon of the 1990s – the songs of the Spice Girls resonated on many different levels. “We didn’t realise the impact we were having on people and what Girl Power meant to people,” says Mel C. “We were just expressing the things that we were feeling because of our experiences and because we did encounter some sexism within the industry. We were just getting on with it, and now I see how that has affected people.”

Horner adds: “The music was predominantly for a younger age group and mostly for girls. We put our fingers in the wind and caught the zeitgeist.”