Chins come in all sizes but why do humans have them?

This uniquely human feature seems about as useful as our appendix, and may be an evolutionary remnant of our big-faced ancestors, according to University of Iowa research published in the Journal of Anatomy.

"It's an interesting study but by no means the last word on this question," says Darren Curnoe, an anthropologist at the University of New South Wales. "The chin is a surprisingly controversial and complex feature."

Some aspects of the human skeleton make perfect evolutionary sense.

When hominids started walking on two legs, their leg bones got longer, lengthening their stride. But as early as the 1800s, anthropologists puzzled over the emergence of the chin.

Our close relatives the Neanderthals didn't have one - it evolved in the modern human. A question mark has hovered over its purpose and opinions are divided.

"There is almost nothing in paleoanthropology that's not controversial," says the study's co-author and anthropologist Robert Franciscus.

"It's like putting together a 3-D jigsaw puzzle with very few pieces."

One idea has been that the chin provides our jaw with structural support, making it more resistant to the stress of chewing.

Young children have little chins, prompting the thinking that as a human chews more food as they age, the chin is placed under greater stress and more bone is laid down to compensate.

But Franciscus, his colleague Nathan Holton and their team were not convinced, and decided to put the idea to the test.

The researchers examined X-rays of the chin and jaws of 37 people who had been regularly X-rayed from the age of three until their mid-20s as part of a longitudinal study of growth. Using mathematical models, they calculated the stress chewing placed on each jaw.

As the team suspected, as the people grew and their chins took shape, their jaw did not become stronger. The authors found the jaws of the three or four-year-olds, with their tiny chins, were more resistant to some chewing stressors.

So your chin doesn't strengthen your jaw. Could it be left over from a time when we had larger heads?

A fossil analysis published last year suggests that small faces are a relatively recent evolutionary quirk.

Our faces today are 15 per cent smaller than the Neanderthals'.

Up to around 80,000 years ago, Homo sapiens also had heads that were larger than ours. But then their skulls became smaller - which coincides with the period around 80,000-60,000 years ago when our ancestors gave up living in small, isolated groups and began forming wider social networks.

The authors' theory is that our ancestors' testosterone levels dropped as they began living side-by-side. High levels of testosterone, associated with aggressive behaviour and handy for solo hunters, would have been less than ideal in social groups.

Animal studies suggest that hormonal adjustments can alter the shape of faces. Changes in skull structure have been shown to coincide with hormonal changes in Russian silver foxes, for example. In a breeding project started in 1959, silver foxes captured from the wild were selectively bred over successive generations for traits of tameness, until they became domesticated. The skulls of the tamed foxes were shorter and wider, and they had lower levels of corticosteroids, hormones involved in stress responses including fight-or-flight.

The same could have happened in humans, with testosterone levels gradually falling over the generations. "Humans are essentially a self-domesticated species," lead author Nathan Holton says. And as testosterone levels shrank, so did the face, which in the process changed shape and developed a chin.

But not all experts agree with the team's theory. Curnoe accepts the study adds weight to the idea chins have no function but isn't convinced testosterone caused the shift: "It's an interesting speculation but I don't see how we could test it."

As we can't replicate human evolution in a lab, animal experiments will try to mimic the missing puzzle pieces. Holton and his team will next investigate whether changes in hormones could lead to different shaped skulls in mice.

But the past may yet shed more light on our distant relatives. DNA from Neanderthal fossils has already given clues to the colour of their skin and hair. Franciscus is confident genomic analysis will one day let us match up hormone levels to changes in their development.

"Exciting days are ahead," he says.