Without these changes, tomatoes would be little more than berries on a bush.

'The cherry tomato would be considered very large compared to what is found in the wild,' said plant geneticist Steven Tanksley of Cornell University in Ithaca, New York.

Professor Tanksley has been working to understand the genetic changes that allowed humans to transform wild tomatoes - which are naturally about the size of a blueberry - into modern varieties such as the beefsteak tomato, which can weigh half a kilogramme or more.

'Humans began domesticating plants in the last roughly 10,000 years. They had no knowledge of genetics and no knowledge of breeding, but somehow they rendered these changes genetically on plants,' Prof Tanksley said in a telephone interview.

'The question we're asking is how and what did they do?' Prof Tanksley focused on the genetic changes that give rise to a large number of compartments or locules inside the tomato, a plant that originated in the Americas.

'If you take a beefsteak tomato from the supermarket and cut it open inside you'll see these compartments in there that have wells between them. They may have anywhere from 10 to 20 of these compartments,' said Prof Tanksley, whose research appears in the journal Nature Genetics.

A true wild tomato may have only two to four of these.

'Somehow, something made the plant start making these compartments, and by making more compartments, you can get larger fruit.'

Gene troll
To understand this process, Prof Tanksley first mapped the tomato's roughly 30,000 genes, looking for differences between the wild and modern tomato. That turned up about 10 genes, which they winnowed down by checking different gene databases to see if these gene changes were common to other plants.

They were left with a handful of genes. After comparing the sequences of these, they found one called 'fas' with a large mutation. 'It was a smoking gun,' Prof Tanksley said.

None of the wild tomatoes they studied had this mutation.

When they took the 'wild' version of this gene from a wild plant and put it into a modern plant, it started to make tiny tomatoes.

Prof Tanksley said this mutation weakens a signal that tells the plant to stop making compartments.

In prior research, his team found changes in a similar gene that told cells to stop dividing.

'It would tell the developing, very small fruit to stop here. That's enough cells,' Prof Tanksley said. Tomatoes with just this change could reach the size of a cherry tomato, he said.

Prof Tanksley thinks it takes both genetic changes - one controlling structure and another controlling cell division - to explain the vast differences in the size of wild and modern tomatoes.

'What's marvellous is that humans did this with no knowledge of genetics ... yet the impact was so enormous,' he said.

'This knowledge may help us in the future to have a renaissance of plant domestication, because we are basically living off what humans did in pre-history.' -- REUTERS