Imagine if, after the collapse of our civilisation, all that remains of 1980s Britain are the dusty diaries of Alan Clark and some yellowing copies of The Telegraph. Imagine if for 2,000 years, historians take these sources at their word. They write long theses taking it for granted that Britain was besieged by black immigrants who belonged in 'Bongo-Bongo Land', and was menaced by a flame-haired communist named Neil Kinnock who wanted to turn this country into a satellite of the Soviet Union. In 4005, blockbuster movies will laud the heroic Alan Clark, who overcame countless illnesses to play a key role in saving Britain from these converging catastrophes while also finding time to admire the Goddess Margaret's ankles.

Absurd? I hope so " but for two millennia, we have done something very similar when we gaze back at Ancient Rome. Very few scraps of information survive from the Roman Republic, and the skewed histories we have come from a tiny, vicious aristocratic elite. Until fairly recently, most classicists read the aristocrat Cicero's description of ordinary Roman people as 'a starving, contemptible rabble' who must be suppressed, and took it at face value. Distinguished classicists like Christian Meier chastised 'the bilge of the city' for trying to achieve a level of political participation that 'was far beyond their capacity', while Sir Ronald Syme dismissed the slaves and poor people Caesar promoted to the Senate as 'a ghastly and disgusting rabble'.

This view of ancient Rome dominated nostalgia-soaked classics such as I, Claudius and more recent Hollywood hits such as Gladiator, and it still drives the public's picture of that time.

Today, some old-style classicists are bleating about the BBC's new eight- part bonkbuster Rome. The series takes us into the fetid, poverty-scarred slums of Rome. The wealth-soaked palaces are shown to be a tiny sliver of Roman society; the series' heroes are not aristocrats but ordinary proletarian Romans. For people raised on the old, Ciceronian interpretation of classics, this is disorientating. One of the right-wing critics of the show moaned last week that it makes 'ancient Rome look more like Calcutta than the most powerful city in the world " the temples are dirty, the streets full of mud, the walls scribbled with graffiti copied from the brothels of Pompeii'. He pines for the old mythology of a clean, civilised Rome " no unwashed masses here, guv " no matter how preposterous it is in light of the evidence.

To see why we were deluded about Rome, we need to look at one of the few surviving historians of the period " Cicero " and a crime he incited and praised: the assassination of Julius Caesar. For centuries, historians described Cicero much as he described himself: a noble man committed only to the highest virtues. In reality, Cicero was considered at the time to be a callous ultra-conservative facing huge opposition from the people. He said anybody who sought reform on behalf of the poor was suffering from 'a sort of inborn revolutionary madness'. Whenever popular politicians emerged to curb the wild excesses of the Roman rich, he helped ensure they were swiftly killed.

Three decades before Caesar was born, a politician called Tiberius Gracchus called for land seized by the aristocracy to be redistributed to the landless, starving poor " and was hacked to death, an event described with glee in Cicero's histories. A few decades later, Lucius Appuleius Saturninus proposed distributing subsidised grain to the proletariat " and he was soon stabbed, with Cicero applauding in the background.

But for two millennia, Cicero shaped the way we understand one of the most important events in ancient history: the murder of Julius Caesar. We all know his version, not least because it was dramatised so brilliantly by Shakespeare. In this fairytale, the city's decent, disinterested senators (like, hem hem, Cicero) could see Caesar was turning into a tyrant. On behalf of the people and the highest principles of the Republic, they disposed of him. Their sole motive was to restore democracy.

There's only one problem: this is a self-serving lie. As Michael Parenti writes in his Pulitzer-nominated book A People's History of Ancient Rome, Caesar was not killed because he was a dictator. His assassins had embraced dictators before, and they would embrace dictators again. No; he was killed because he was trying to redistribute wealth away from aristocrats like Cicero and towards ordinary Romans. His assassins' motives were as pure as the driven slush....