A study by the Royal Veterinary College suggests the odd growth helps support elephants' extreme weight, the BBC reported Thursday.
RVC Professor John Hutchinson said the growth has been the subject of debate for more than 300 years.
"It's a cool mystery that goes back to 1706, when the first elephant was dissected by a Scottish surgeon," he said.
"Anyone who has studied elephants' feet has wondered about it. They've thought: 'Huh, that's weird,' and then moved on."
Close examination showed the bone has an unusual arrangement, similar to one found in the front feet of pandas.
The panda's bone is not quite an extra digit but acts as one, allowing the animal to grip bamboo when feeding.
For elephants, the structure serves a different and simple purpose, researchers said: It helps the hefty animals stand up.
An elephant's five conventional toes point forward, but the extra "toe" points backwards into the heel pad, providing extra support and helping the heaviest land animals support their weight, they said.