It's fitting that the best pure pop single of the decade came slap bang in the centre - the noughties had been around long enough for its music to get a general idea of it was about, but just ready to start getting restless. We already knew that Girls Aloud (and, of course, their writing/production gurus Xenomania) were capable of instant classics, but Biology was the first of their songs to start trying to push envelopes as well as all the right buttons - a three-piece suite taking in fifties riffs, sixties strut, seventies swagger and pretty much every manufactured pop trope since the eighties. Sure, the lyrics are borderline unintelligible and downright odd ("You fall on your knees and the geek at your feet says you're neat and the beat gets closer"), but once the celestial synths show up in the proper chorus - this being a Girls Aloud song, everything sounds like a chorus - and the irresistible "way that we walk/talk" hook rears its catchy head, you're too busy dancing to notice, let alone care.
To think that this was the sound of the overground at the start of the twenty-first century. It's almost worth forgiving Simon Cowell for.