Once we thought that ours was the only species to use rituals to mourn the dead.

Then archaeologists discovered hints of pollen from colourful meadow flowers around a Neanderthal skeleton in a remote cave in Iraq.

Now, half a century later, new remains of these human cousins have been unearthed at the same "flower burial" site in Shanidar Cave, about 150 kilometres from Mosul.

Key points: Pollen found around a Neanderthal skeleton 50 years ago led to the controversial idea that they buried their dead with flowers

Pollen found around a Neanderthal skeleton 50 years ago led to the controversial idea that they buried their dead with flowers Now archaeologists have found a new skeleton in the same cave

Now archaeologists have found a new skeleton in the same cave Analysis of the skeleton and soil around it could help settle the flower burial controversy

"It's almost the entire top half of a body from the waist upwards," said Emma Pomeroy, an archaeologist from the University of Cambridge who led the discovery.

"We've got the skull, we've got the left arm and hand fairly complete, the right shoulder and the right hand, then the spinal column down to about the waist level and all of the ribs as well."

Dubbed Shanidar Z, the new Neanderthal skeleton is the most intact discovered anywhere in the world over the past 25 years, the team report in the journal Antiquity.

And it offers scientists a unique opportunity to explore when Neanderthals were buried in this cave and to answer the controversial question of whether or not they used flowers in death rituals.

Shanidar Cave is located in Iraqi Kurdistan. ( Supplied: Antiquity )

"The way we treat the dead is an expression of our mourning and also about our ideas of what happens after we die," Dr Pomeroy said.

"It's exciting and interesting to contemplate were [Neanderthals] thinking in similar ways? Were they experiencing similar emotions? When did those kind of reactions to the dead evolve?"

Returning to the "flower burial" site

In the late 1950s and early '60s the remains of 10 Neanderthal men, women and children were excavated from the Shanidar Cave in the Baradost Mountains of north-east Iraqi Kurdistan.

One of the skeletons, a male known as Shanidar 4, was so fragile it had to be removed in a one metre square block of rock.

Members of the original excavation team with Shanidar 4 encased in block. ( Supplied: Ralph Solecki, with kind permission )

It was not until the fossil was transported back to the Baghdad Museum — strapped to the top of a taxi — that archaeologists, led by the late Ralph Solecki, realised it also contained fragments from three other smaller skeletons.

Traces of pollen from flowers such as yarrow, cornflower, ragwort, grape hyacinth and hollyhock were found around Shanidar 4, which led Professor Solecki to conclude that they may have been picked for burial rites.

But others disputed the flower hypothesis, saying that traces of pollen may have been blown in, brought in on the shoes of the excavators, or introduced by a small burrowing rodent known as the Persian jird.

When Professor Solecki and team originally excavated Shanidar 4 in 1960, they noted some small bones falling out of the rock that suggested there might be more to be discovered, but political instability prevented them from returning to Iraq.

A number of Neanderthal remains have been found in Shanidar Cave in north-east Iraqi Kurdistan. ( Supplied: Graeme Barker )

In 2011, the Kurdish Regional Government invited Dr Pomeroy's colleague Graeme Barker to return to the cave, but an ISIS invasion delayed the dig until 2015.

Over the next four years the team carefully excavated the area near where Shanidar 4 and the other nearby skeletons had been removed.

Dr Pomeroy was removing dirt that had been filled back into the original trench when she noticed a couple of hand bones.

"As we cleaned the walls of the trench back very carefully then the bones started to appear," she said.

The Neanderthal skull was flattened by thousands of years of sediment and rock fall. ( Supplied: Graeme Barker )

"I could see the outline of the ribs and they were in anatomical position, and then there was part of a hand there that was clearly sort of clenched. It was absolutely incredible."

But they weren't expecting to find what came next.

"As we were excavating down from above ... the first thing that came out was the edge of the eye socket.

"I knew exactly what it was. I went 'Oh my God, there's a skull'."

Positioned right next to the block that contained the "flower burial" the new skeleton, Shanidar Z, was lying on its back with its smashed skull lying sideways over its left hand.

Shanidar Z's head was resting on its clenched left hand. It had two pyramid stones behind its head.

What do we know about Shanidar Z?

The team continue to analyse the skeleton and soil around Shanidar Z using dating and soil analysis techniques that weren't available in the 1960s.

Dr Pomeroy said it was likely that Shanidar Z is the top half of one of the other skeletons which became jumbled when they were originally collected.

"We're pretty sure this is part of those same individuals, but they need a thorough reassessment with all the bones in front of us to actually that sort out," she said.

Emma Pomeroy inside Shanidar Cave ( Supplied: Graeme Barker )

Preliminary dating of the sediment in the layers that encompass the new skeleton as well as the other four suggest the group were buried between 60,000 to 70,000 years ago.

"There's lots of questions about how a number of individuals came to be in that exact spot."

The fact that Shanidar Z was largely intact also suggests it was put at that particular site on purpose.

Analysis of soil structure at the site supports the idea that a pit had been deliberately dug for the skeletons and that they were placed in the cave at different times.

The researchers say two mysterious triangular-shaped rocks behind Shanidar Z's head could have helped repeat burials at the same location.

"The rocks seem quite out of place," Dr Pomeroy said.

"There have been periods when it's been colder and there's been substantial rock falling from the roof, but this isn't one of those layers.

"One hypothesis we are trying to test is perhaps they used those stones as a marker of the way to come back to the same spot."

Did they use flowers?

Michelle Langley, an archaeologist at Griffith University who was not involved in the discovery, said Shanidar Cave was such an important site for our understanding of Neanderthal communities.

It is still unclear whether the plant material was blown in from the valley below or was deliberately buried with the skeletons. ( Supplied: Graeme Barker )

"We've got to remember that when Professor Solecki was suggesting Neanderthals placed flowers on graves they were dehumanised. They were seen as little more than animals," Dr Langley said.

Now, she said, most people accepted our human cousins buried their dead.

"It's what goes in the grave, like flowers, that is controversial."

Early analysis of the sediment around the new fossil reveals the presence of plant material

Analysis of the layers so far shows no evidence of burrowing animals near the skeleton, which could have brought the plant material in.

But Dr Pomeroy said it would be important to find out if the plant material was throughout the cave. If it was only in the burial area this would suggest plants were being used symbolically.

The next step would be to analyse the plant samples for traces of pollen.

And Dr Pomeroy is keen to compare their samples with some of the original samples found with Shanidar 4.

"We are going to look at our soil samples and see ... if it bears any resemblance to the species that were with the "flower burial".