From Stanley Tucci to Terry Crews, some men have made baldness their signature style.

But if you're not so keen to say goodbye to those luscious locks, science may now have the answer.

Researchers say the secret to why humans grow such long scalp hair is a hidden 'molecular switch'.

That means treating hair loss could be as 'simple' as finding a way to flick that switch back on.

Although long scalp hair is rare in the animal kingdom, the researchers don't think that the capacity to grow it is unique.

Mammals such as orangutans, highland cattle, and male lions all have the ability to grow out their hair.

This suggests that the trigger for hair growth might remain hidden inside us and other mammals until the right environmental factor activates it.

Co-author Professor Sung-Jan Lin, of the University of Singapore, says: 'What these examples tell us is that the molecular blueprint for growing very long hair has always existed, albeit often in a "silenced" state.'

While it might seem normal to us, a human's full head of hair is extremely uncommon for the animal kingdom.

Co-author Professor Nina Jablonski, of Penn State University, says: 'Humans grow extremely long scalp hair.

'Likewise, attributes of scalp hair — its length, shape, colour and loss of hair — play an essential role in social communication.

'And yet, despite the importance of having long scalp hair, we know very little about how this feature of human skin came about and how it is regulated.'

Researchers from Penn State University, the University of California, Irvine, and National Taiwan University investigated the social and biological evolution of human hair.

Their findings suggest that long hair probably first evolved when humans first emerged in equatorial Africa around 300,000 years ago as a way of protecting our heads from the intense heat of the sun.

Long, tightly curled hair is an effective sun shield so ancient humans didn't need to sweat so much to cool down, and so could go longer without water.

Professor Jablonski says: 'This hair type not only reduced heat exposure but also conserved vital water and electrolytes, which could mean the difference between life and death in such extreme conditions.'

Later, humans began to select for long hair because it conveyed essential signals related to an individual’s age, sexual maturity, health and social status.

It was only as humans spread out of Africa that more hair types evolved, and hair styling became more elaborate and diverse.

However, this doesn't explain why humans were able to start growing long scalp hair in the first place.

Looking at the animal kingdom, the researchers found that long scalp hair isn't entirely unique to humans.

Some male animals like lions, muskox, and highland cattle can all grow very long hair from their heads.

This suggests that the potential to grow long hair is something that many mammals carry but is only triggered in certain conditions. 

Most importantly, primates with which we share a common ancestor such as hamadryas baboons, orangutans, and emperor tamarins grow exceptionally long hair.

Since the biological mechanisms that dictate hair growth are the same across all species, the researchers suggest that there might be a 'molecular switch' which triggers a species to move from short to long hair.

In their paper, published in the British Journal of Dermatology, they argued that our human ancestors always had this molecular switch built into their biology.

However, it was only when certain biological and environmental conditions, such as walking upright on two legs, were met that this molecular program was activated.

Professor Lin says: 'When human ancestors evolved their ability to grow extremely long scalp hair, it was likely accomplished by just a few genetic tweaks that reactivated a dormant program rather than via the evolution of an entirely new molecular mechanism.'

This realisation could have significant consequences for the attempt to find a cure for hair loss.

Male pattern baldness, or androgenic alopecia, is believed to affect between 40 and 50 per cent of men worldwide.

The condition is caused by a combination of genetic factors and levels of sex hormones which gradually lead to the permanent loss of hair follicles on the head.

Patients can use the topical treatment minoxidil, sold as Rogaine, but this can be slow and does not work for everyone suffering from hair loss.

Those who do not see improvements with minoxidil can also take the oral drug Finasteride, sold as Propecia, which works by decreasing the flow of testosterone.

This must be taken continuously once started and can be associated with severe side effects such as erectile dysfunction, testicular pain, reduced libido, and depression.

However, if long hair growth is triggered by a molecular switch, then finding a way to turn it on could be the solution scientists have been looking for.

Co-author Professor Maksim Plikus, of the University of California, says: 'Understanding how human scalp hair follicles normally grow very long hair will naturally result in novel molecular targets for more efficacious therapies for hair loss.

'This knowledge could lead to treatments that help restore hair growth and alleviate the emotional distress that often accompanies hair loss.'