Archaeologist Renee Freidman couldn't believe her eyes.

She was pointing a digital infra-red camera at a 5,500-year-old Egyptian mummy on display at the British Museum.

"I said, 'Mmm, there's a smudge on his right arm, I wonder what that is?'," Dr Freidman said.

"We had believed that only females were tattooed [in predynastic Egypt]. I really didn't expect anything.

"And even through the glass, in the case, these two animals on his right upper arm just jumped out at me … that was a real surprise. A 'jump-up-and-down' moment."

An infra-red image of tattoos on the Gebelein Man mummy. ( Supplied: British Museum )

Wise women and wild bulls

Gebelein Man, or "Ginger" as he's affectionately known for his red hair, has been a gallery favourite for over a century.

Curled up tightly in the fetal position as he was found in the late 1800s, Gebelein Man's skin and physical features remain remarkably intact — naturally mummified by the arid conditions of Egypt's Nile Valley.

Previous CT scans had revealed well-preserved internal organs too — and the fact that he came to a grisly end after being stabbed in the back.

But now, Dr Friedman's camera had also unmasked tattoos of a Barbary sheep and a wild bull on the mummy's arm.

"I was astounded," she said.

Gebelein Woman, found at the same site, has tattoos as well. ( Supplied: British Museum )

Gebelein Woman, a female mummy excavated from the same site, also revealed new tattoos with the help of the camera.

"You couldn't see it with the naked eye. It was just so clear under the infrared," Dr Freidman said.

Gebelein Woman's tattoos included three small s-shaped markings and a music clapper or staff on her right shoulder and arm.

The s-shaped tattoos of Gebelein Woman ( Supplied: British Museum )

"It gives us a whole new medium for exploring the motifs used in the predynastic period," Dr Freidman said.

Archaeologists had long assumed that only dancing girls and concubines bore tattoos in ancient Egyptian society.

Dr Friedman's research has shown both sexes were tattooed.

"It's not the nubile young things, the older women were tattooed," she said.

"They were probably the wise women, and the tattoos were there to show their initiation into cult practices and their knowledge of medicine. It wasn't just meant for the gratification of men."

Digging at the Hierakonpolis Expedition in Egypt. ( Supplied: Courtesy of Hierakonpolis Expedition )

For over two decades, Dr Friedman has directed the Hierakonpolis Expedition in Egypt, at the site of a vast predynastic city under the sand — one of the biggest urban centres of its time, in around 3600 BC.

Now armed with new tools and techniques, her team have been shedding fresh light on the extraordinary findings at the site.

"Infrared radiation is heat radiation, and it's very difficult to properly image the mummies without simultaneously cooking them," said Aaron Deter-Wolf, prehistoric archaeologist and author of Ancient Ink: The Archaeology of Tattooing.

"That's bad science, right? No one wants to cook the mummy!"

However digital infrared cameras and imaging have changed that, allowing archaeologists to see things they couldn't before.

"It's a completely non-destructive process. We have the ability, with a stroke of a button, to change our understanding of tattooing in ancient Egypt," Mr Deter-Wolf said.

Why did ancient humans tattoo themselves?

The latest findings from Dr Friedman and her colleagues, recently published in the Journal of Archaeological Science, represent the oldest evidence so far of tattooing in Egypt.

Remarkably, Gebelein Man and Woman are contemporaries of the oldest tattooed mummy found in the world, from a very different culture and location.

Otzi the Iceman melted out of a glacial ice sheet in the Alps at the Italo-Austrian border, and was found by hikers in 1992. His remains are carbon dated to between 3370-3100 BC.

Otzi has an extraordinary array of tattoos — 61 groups of symbols in total — mainly along his joints, spine and left wrist.

The oldest tattooed mummy ever found, Otzi, melted out of a glacial ice sheet in 1992. ( Supplied: Copyright of South Tyrol Museum of Archaeology/EURAC/Samadelli/Staschitz )

X-rays have revealed his tattoos were placed at locations where he suffered from arthritis and other ailments.

"A number of his marks seem to correspond to traditional Chinese acupuncture meridians," Mr Deter-Wolf said.

"This reinforced the idea that perhaps these tattoos were applied as a medicinal or therapeutic practice."

This possibility is still debated.

Rich in meaning

According to Mr Deter-Wolf, tattoos are potent evidence of what scientists describe as "behaviourally modern" humans.

"The desire to decorate the body implies there is complex symbolism, the presence of language, and a social network in which people have value in appearing other than they are naturally," he said.

"As a species, the reason we would do that, is that it improves our standing — it shows people how we want to be perceived."

Archaeologist Aaron Deter-Wolf in the field. ( Supplied: Aaron Deter-Wolf )

Archaeologists believe tattoos carried deep spiritual and ritual significance in ancient societies.

"When you consider that it might have also been a death sentence —there were no antibiotics if you got an infection — it was a big deal to be tattooed then," Dr Friedman said.

"It wasn't just what you did after a few beers and before a curry."

In addition to tattoos on preserved mummies, the evidence for ancient tattoos is found in the imagery on pottery and other objects.

But there's a third source of evidence that has gone mostly unnoticed — until now.

Tools for tattoos

Mr Deter-Wolf lives in the country music capital of the world: Nashville, Tennessee. But his own tattoos are inspired by another tribe.

One of them he's even inscribed on himself, using a needle handmade out of a deer bone.

Archaeologists have recreated the tools ancient humans used to tattoo their skin. ( Supplied )

But he's not a tattooist or DIY body modification maverick — this is experimental archaeology at work.

Mr Deter-Wolf studies Native American sites with the Tennessee Division of Archaeology, and has a special interest in the origins of ancient tattoos.

And where there are tattoos, there are also the tools used to make them — or so you'd think.

"Globally we have this great historical record of tattooing, but there are relatively few identifications of tattoo implements in the archaeological record — that has really bothered me," he said.

"It means we are overlooking an entire category of material culture.

"Is it because it's not there, is it because people don't know what to look for, or because people deliberately weren't looking for it in the past?"

Mr Deter-Wolf and a colleague have just identified what may be one of the world's oldest collections of tattooing implements — including sharpened turkey bone tips and mussel shells containing red pigment residue.

They were originally found at a Native American grave site in Tennessee containing artefacts dated between 1600-3200 BC, and have only been re-analysed now.

These findings provide the first direct evidence that ancient Native Americans practiced tattooing some 5,000 years ago, Mr Deter-Wolf said.

The self-experimenting archaeologist

Determining what tools were used to administer tattoos is a challenge — one that's turned personal for Mr Deter-Wolf.

"When you have a long sharp bone tool … it could be for leather working, making basketry, holding up hair, or it could be tattooing. It could be for anything," he said.

"The problem is once archaeologists call it something, it is likely to be that thing you called it forever.

"It is likely we are biasing the archaeological record with our interpretations."

Archaeologist Aaron Deter-Wolf becomes the subject for his replica of an ancient bone tattoo tool. ( Supplied: Aaron Deter-Wolf )

Mr Deter-Wolf and others have been conducting so-called microscopic "usewear" studies.

"Usewear is the idea that when you use a tool for something it develops certain microscopic traces," he said.

"For example, if you use a stone tool to cut meat it will have different microscopic trace patterns, [from] when you use the same tool to saw wood or cut cloth."

Recent findings confirm the microscopic wear signatures from tattooing skin are identifiable and distinct.

But such tests are usually done on pig skin as a proxy for human skin.

Instead, Mr Deter-Wolf and his team made replica bone tools using technologies used in prehistoric times — splitting deer bones and whittling them down using stone tools.

First, they tattooed the skin of pig carcasses.

But they needed to know whether pig hide and human skin would produce a different microscopic wear pattern.

"There's only one way to test that. I figured I couldn't ask anyone to do it if I wasn't willing to do it myself," Mr Deter-Wolf said.

Mr Deter-Wolf and his intrepid volunteers tattooed directly onto themselves using small, handmade bone implements they made themselves.

Aaron Deter-Wolf hand tattooed a copy of a tattoo found on the oldest tattooed mummy, Otzi the Iceman ( Supplied: Aaron Deter-Wolf )

"It turns out that even dead pig and live humans result in the same use-wear pattern," he said.

On Mr Deter-Wolf's wrist is a replica of the two parallel tattooed lines found on Otzi the Iceman.

"I figured in the name of science — and as a tribute to Otzi — we might as well do that," he said.

Look and you shall see

Until recently, evidence of tattoos in prehistoric records was mostly ignored by archaeologists.

But it was there, staring them in the face.

"We have a very sanitised view of the bodies of people who lived in the past," Mr Deter-Wolf said.

To the prudish, possibly prejudiced eyes of Western archaeologists, tattoos were associated with criminals or people of ill-repute.

"As they say, the past is a foreign country — we all approach it with our own cultural blinders on," Dr Friedman said.

"Many people wanted to see the Egyptians as very refined, and certainly not tattooed.

"I think it's the other way around. Tattooing was a real sign of status, of knowledge, and piety."