For more than 300 years, the structure has puzzled researchers, but this study suggests that it helps to support elephants' colossal weight.
Fossils reveal that this "pre-digit" evolved about 40 million years ago, at a point when early elephants became larger and more land-based.
The research is published in the journal Science.
Lead author Professor John Hutchinson, from the UK's structure and motion laboratory at the Royal Veterinary College, said: "It's a cool mystery that goes back to 1706, when the first elephant was dissected by a Scottish surgeon."
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Anyone who has studied elephants' feet has wondered about it. They've thought: 'Huh, that's weird'”
Professor John Hutchinson
Royal Veterinary College
Many people, he said, thought that the structure was a huge lump of cartilage, and over the years its purpose or lack of purpose has been debated.
"Anyone who has studied elephants' feet has wondered about it. They've thought: 'Huh, that's weird,' and then moved on," he added.
But Prof Hutchinson and colleagues used a combination of CT scans, histology, dissection and electron microscopy to solve the puzzle.
The researchers said the structure was made of bone, although bone with a highly irregular and unusual arrangement.
But closer examination also revealed that it showed a strong similarity with an unusual bone that is found in the front feet of pandas.
This bone - which is not quite an extra digit, but does the job of one - helps the panda to grip bamboo, and is called the panda's "thumb" or "sixth finger". Moles too have a bone masquerading as an extra digit, which helps them to dig.
And now, the team says that elephants can be added to the list of species for whom five fingers or toes are not quite enough.
The elephant's five regular toes give it a tip-toed stance
Prof Hutchinson said: "It would have started out as a little nub in the tissue, which may have not even have been bone originally - it could have been cartilage.
"A lot of animals have these structures, cartilaginous lumps, and they sometimes turn to bone with these very different functions in some species."
For elephants, the structure serves a simple purpose: it helps the hefty animals to stand up.
Unlike pandas and moles, which only have the false digit in their front feet, elephants have the bone in all four of their feet.
And although their feet may look like tree trunks, inside the anatomy is more complex.
Their five conventional toes point forwards, giving elephants a tip-toed stance, but the extra "toe" points backwards into the heel pad, adding extra support and helping the heaviest land animal to hold up its bulk.
Evolution example
To find out when and why this strange bone appeared, the researchers examined elephant fossils.
Prof Hutchinson said: "The first elephants appear around 55 million years ago.
The sixth "toe" is an example of how a structure evolved to have a new function
"We looked at early elephants and they had a different kind of foot, which seemed to be quite flat footed and didn't leave much room for this structure underneath.
"The structure seemed to evolved around 40 million years ago, and it seems to have evolved in concert with elephants getting bigger and more terrestrial and having upright feet, with a more tip-toed foot posture."
He added that this was a remarkable case of evolution in action.
He said: "It is a great example of how evolution tinkers and tweaks tissue to provide different functions - in this case to be co-opted to be used like a digit."
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