The world's largest flower has been revealed by molecular analysis to have evolved almost 80 times in size to become today's stinking, 15lb mega-bloom.

Although this transformation took tens of millions of years, such an evolutionary spurt is still one of the most dramatic size changes ever reported.

	Who ate all the flies? Rafflesia, discovered in Asia, has evolved to 80 times its original size

If humans were to undergo a comparable growth, an average man would end up 146 metres tall, the height of the Great Pyramid of Giza.

Rafflesia got its name because it was first discovered in the Sumatran rain forest 180 years ago by Sir Stamford Raffles.

But American botanists report today in the journal Science that although typically a full metre across, with a bud the size of a basketball, it evolved from a family of plants whose blossoms are tiny.

Rafflesia is unusual in several ways: It has a carcass-like appearance, reeks of decaying flesh, and in some cases emits heat, much like a recently killed animal. These traits help the flower attract the carrion flies which pollinate it. Because rafflesia lacks the genes most commonly used to trace plant ancestry, the scientists had to delve deeper into its genome, looking at some 11,500 ''letters" of DNA.

This determined that the giant flower's closest relatives are in the Euphorbiaceae family, many of which have blossoms just a few millimetres in diameter.

"The massive increase in flower size could never have been deduced by conventional methods," says Charles Davis, the team's leader from Harvard.

"While it's surprising to find this giant plant evolved from a family typified by much smaller blossoms, it's frankly been difficult to imagine it fitting neatly into any plant family.

"Many had refused to even speculate on where this botanical outlier might fit into the tree of life."

As for why these big blooms blossomed, Davis speculated that there was probably a very strong selective pressure to do so at the start of the plant's evolutionary process.

"An increase in surface area would help to radiate the smell further distances, and be a very effective visual 'stop sign' for the carrion in the vicinity."