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Scientists exclaimed last week that they had solved the mystery as to why humans are the only animals to have chins.

But a new study has been published which calls these claims into question. 

The original reports said the chin developed after our ancestors began eating soft food, but researchers from Iowa now claim there is little evidence to support this theory. 

And they believe chins instead became a 'bony prominence' when our ancestors started being more sociable and stopped fighting over territories.

By studying scans of skulls as people grew from toddlers to adults (pictured), researchers from Iowa said that as our faces changed shape in our evolution from hunter gatherers to more cooperative groups, levels of testosterone dropped. This caused the skull to shrink and the chin to become more prominent

The appendage has mystified scientists for decades because no other species, including our primate and Neanderthal ancestors, possess it.

It was originally believed to have formed to help humans attract a mate, while some have argued the development of the chin could be a purely random example of 'genetic drift', with no evolutionary purpose whatsoever. 

But earlier this month a team of scientists from the University of Florida suggested it formed around two million years ago when humans became more intelligent and began to learn how to make food softer over a fire.

Author James Pampush said the fact that apes and monkeys do not have chins proves they must have evolved after the ancestors of man split off from other branches of the primate family.

The appendage has mystified scientists for decades because no other species, including our primate and Neanderthal ancestors (skull shown right), possess it. It was originally believed to have formed to help humans attract a mate, while some argued it is an example of 'genetic drift', with no evolutionary purpose

Earlier this month a team of Florida scientists suggested the chin formed two million years ago when humans began eating softer, cooked food. They said the fact that apes and monkeys don't have chins proves they must have evolved after the ancestors of man split off from other branches of the primate family (shown)

His research said that because the evolution of the chin happened 77 times faster than the average genetic change, it is highly unlikely to have been random.

In particular, Dr Pampush calculated that the chin began to emerge some time between 6 million and 200,000 years ago, with the most likely estimate being around 2 million years ago.

This would coincide with the enormous leap forward in human intelligence, which led to breakthroughs including the invention of cooking.

The evolution of the chin came about as a result of humans' teeth and jaws shrinking, because they no longer needed to chew through raw meat and plants, Dr Pampush said.  

But the new study from the University of Iowa used advanced facial and cranial biomechanical analyses of nearly 40 people whose measurements were plotted from toddlers to adults.

It found that chewing was incapable of producing the resistance needed for new bone to be created in the jaw area.

'In short, we do not find any evidence that chins are tied to mechanical function and in some cases we find that chins are worse as resisting mechanical forces as we grow,' said co-author Dr Nathan Holton. 

'Overall this suggests chins are unlikely related to the need to dissipate stresses and strains and other explanations are more likely to be correct.' 

Modern humans evolved from isolated hunter gatherer groups to increasingly cooperative social networks.

People became more tranquil during this period and less likely to fight over territory and belongings.

This change in behaviour was linked to reduced hormone levels which resulted in changes to the skull with the face becoming smaller and the chin more prominent, according to the findings published in the Journal of Anatomy.

As modern humans evolved from being isolated hunter gatherer groups to more sociable they were less likely to fight over territory. This change in behaviour was linked to reduced testosterone levels which caused the skull to shrink and the chin to become more prominent. Scroll down to find out the owners of these chins

1) Football manager and commentator Jimmy Hill is known for his large chin; 2) Reese Witherspoon is another star with the prominent facial feature

3) Bruce Forsyth would not be as recognisable without his large chin; 4) The Thinker by Rodin depicts a man resting his head on his chin

Dr Holton continued: 'Primates or the Neanderthals did not have chins.

'In some way, it seems trivial but a reason why chins are so interesting is we are the only ones who have them. It is unique to us.

'As our faces became smaller in our evolution from archaic humans to today - in fact, our faces are roughly 15 per cent shorter than the Neanderthals - the chin became a bony prominence, the adapted, pointy emblem at the bottom of our face.' 

And this evolution quickened when our ancestors migrated from Africa around 60,000 years ago. 

Males in particular became more willing to make alliances, evidenced by exchanging goods and ideas and the change in attitude was tied to reduced testosterone, resulting in noticeable changes to the male skull. 

Anthropologist Professor Robert Franciscus added: 'What we are arguing is modern humans had an advantage at some point to have a well connected social network, they can exchange information, and mates, more readily, there is innovation, and for that to happen, males have to tolerate each other.

'There had to be more curiosity and inquisitiveness than aggression, and the evidence of that lies in facial architecture. 

Witherspoon is another star with the prominent facial feature

5) Buzz Lightyear is a caricature of the square-jawed action hero; 6) Meg Ryan is known for her large chin