The richest person ever to walk the planet was not, as it turns out, an Astor, a Carnegie or even a Windsor. Rather it was a fellow named Mansa Musa I, who, if you haven't heard of him, ruled what used to the vast Malian Empire from 1280 to 1337, a terrain now encompassing Mali and Ghana that was blessed with oodles of salt and gold.

That you have to travel to Timbuktu and back seven centuries to find the man with the most wealth – ever – is only one of the surprises of a list of the top 25 richest people in the history of mankind compiled and just released by the website celebritynetworth.com.

The authors drew it up by establishing the peak worth of each entrant during their lifetime (some guesswork here perhaps) and adjusting for inflation to find the equivalent in 2012 US dollars.

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Today's super rich are mere tiddlers. Carlos Slim Helu, currently pegged by Forbes as the world's richest person, comes in only at No 22 on this list with $68bn to his name. He is beaten by Bill Gates, the founder of Microsoft, who comes in at No 12 because at his richest his fortune allegedly touched the equivalent of $136bn in today's dollars.

There are no contemporary celebrities in the usual sense of the word on the list and no women. Not surprising, perhaps, is the dominance of Americans, 14 in all. They include the old barons of steel, railways, cars and fur. In third and fourth place respectively are Andrew Carnegie and John D Rockefeller, whose names still adorn upstanding civic institutions across the US. Cornelius Vanderbilt is at No 10 and John Jacob Astor at 14.

Mansa Musa is not the only despot to earn a place in this rarefied club. Look with envy at the gallery of portraits of this moneyed bunch and one face is suddenly more familiar than almost any, partly because he is only so recently dispatched and by violent means too. That would be Muammar Gaddafi of Libya who liked to sleep in tents even though he could afford to buy a whole hotel chain with $200bn stashed in his personal piggy bank.

Lurking at No 5 is the obligatory oligarch, but of the strictly old-fashioned kind. Tsar Nicholas II's net worth seemingly touched the equivalent of $300bn in 1916. We all know what happened in 1917. And there are some kings of retail in there too, including Americans Sam Walton just making it in at 25 and Marshall Field at 23.

A rich history yields some rich names and so it is with Britain. Our stinkingly rich qualified not so much because of the cash in their pockets but their vast portfolios of land. John of Gaunt was land-rich and in the 14th century so was Henry, Duke of Lancaster. They take 16th and 20th places respectively. One man who pretty much claimed the entire country for himself was a certain William the Conqueror. The authors of the list calculate with surely a degree of speculative inventiveness that when he died he left his sons $229.5bn in today's money. But wait, no living Britons? Actually, yes, though there is some fudging here. The No 2 slot below the mysterious Mansa Musa is a whole family, some branches of which extend deep into British society. This, of course, would be the Rothschild clan.

"The Rothschild family are the richest people on earth today with assets that total at least $350bn," the site claims. "Their net worth is difficult to peg because their holdings are so vast but without question they are the most powerful family in the world. Many people believe they control over $1 trillion in real estate and banking assets." If they say so.