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One of the world's oldest complete human skeletons has been found in a cave in Mexico - and sheds new light on who the first Americans were.

Named 'Naia', the remains belong to a 15-16 girl who went underground to seek water 13,000 years ago during the last ice age.

She plunged to her death in a large pit known as 'Hoyo Negro', Spanish for 'black hole', in Mexico's Yucatan Peninsula.

Diver Susan Bird working at the bottom of Hoyo Negro, a large dome-shaped underwater cave on Mexico's Yucatan Peninsula. She carefully brushes the human skull found at the site while her team members take detailed photographs of the skull of ¿Naia¿, the remains belong to a 15-16 girl who went underground to seek water 13,000 years ago during the last ice age.

She roamed Earth up to 13,000 years ago when the now flooded cave systems in Mexico's Yucatan Peninsula were much the same, apart from the water level being much lower than it is now.

Her almost complete remains, including an intact skull and preserved DNA, were lying 130 feet below sea level near a variety of extinct animals, such as an elephant like creature called a gomphothere.

These helped scientists establish the age of the bones as between 12,000 and 13,000 years old.

Her pristine preservation enabled the researchers to extract enough DNA to establish the prehistoric girl was an ancestor of the earliest Americans, who arrived from north east Asia between 15,000 and 20,000 years ago, and modern Native Americans.

The ancestry of the earliest Americans is still debated because the facial features of the oldest American skeletons don't look much like those of modern Native Americans.

'Modern Native Americans closely resemble people of China, Korea, and Japan,' James Chatters, lead author on the study, said, 'but the oldest American skeletons do not.'

They have longer, narrower crania than later Native Americans, and smaller, shorter faces, too - more closely resembling modern peoples of Africa, Australia, and the Southern Pacific Rim.

'This has led to speculation that perhaps the first Americans and Native Americans came from different homelands,' Chatters continued, 'or migrated from Asia at different stages in their evolution.'

Naia's skull as it was found (left) and the tooth that researchers analysed to find out about her life

The new find suggests that America was not colonized by separate migration events from different parts of Eurasia.

Rather, the earliest Americans represent an early population expansion out of Beringia.

On the basis of genetics, modern Native Americans are thought to descend from Siberians who moved into eastern Beringia (the landmass connecting Asia and North America) between 26,000 and 18,000 years ago.

THE 'BLACK HOLE' CAVE WHERE NAIA WAS FOUND

 About 12,000 years ago Earth experienced a great climactic change, when the melting of the ice caps caused a dramatic rise in global sea levels, which flooded low lying coastal landscapes and cave systems.

Many of the subterranean spaces that once provided people and animals with water and shelter became inundated and lost until the advent of cave diving.

The Sac Actun cave system on Mexico's Eastern Yucatán Peninsula.

'Hoyo Negro is a more than 100-foot-deep, bell-shaped, water-filled void about the size of a professional basketball arena deep inside a drowned cave system,,' said James Chatters, lead author on the study.

'Only technical cave divers can reach the bottom.

The Sac Actun cave system on Mexico's Eastern Yucatán Peninsula.

'First they must climb down a 30-foot ladder in a nearby sinkhole; then they swim along 200 feet of tunnel to the pit rim before making a final 100-foot drop.

'The divers are the astronauts of this project; we scientists are their mission control.'

Like nearby caves, Hoyo Negro was accessible only via sinkhole; people and animals fell in and were trapped.

Then, starting about 10,000 years ago, global glaciers melted, filling the caves with water. In addition to the near-complete human skeleton, the researchers found the remains of 26 large mammals, including extinct taxa such as sabertooths and gomphotheres.

These people, the earliest Americans, then spread southward.

Chatters and colleagues were delighted with their find: 'This project is exciting on so many fronts: the beautiful cave, the incredibly well-preserved animal skeletons, the completeness of the human skeleton, the success of our innovative dating approach,' he said.

'But for me," he said, 'what is most exciting is that we finally have an answer, after 20 years, to a question that has plagued me since my first look at Kennewick Man: 'Who were the first Americans?''

Complicating the puzzle, it's been very difficult to find intact Paleoamerican skeletons for study.

'Paleoamerican skeletons are rare for several reasons,' Chatters explained.

'The people themselves were few; they were highly nomadic and seem to have buried or cremated the dead where they fell, making the locations of graves unpredictable; also, geologic processes have destroyed or deeply buried their graves.'

Divers Alberto Nava and Susan Bird transport the Hoyo Negro skull to an underwater turntable so that it can be photographed in order to create a 3-D model.

The analysis of Naia analysis revealed a haplotype common to modern Native Americans, subhaplogroup D1.

This genetic signature occurs only in the Americas, likely having developed in Beringia after populations there split from other Asians.

The sample shows that individuals of the Pleistocene era with Beringian-derived mtDNA traveled far and wide through the Americas, all the way down to Mexico, for example.

Critically, it shows that despite differences in craniofacial form, this early American woman was related to modern Native Americans; the differences in craniofacial form are probably best explained as evolutionary changes that happened after the divergence of Beringians from their Siberian ancestors, the authors say.

Their work suggests that America was not colonized by separate migration events from different parts of Eurasia.

Divers Susan Bird and Alberto Nava search the walls of Hoyo Negro, an underwater cave on Mexico¿s Yucatán Peninsula where the remains of Naia were found.

Rather, the earliest Americans represent an early population expansion out of Beringia.

This aligns with the hypothesis that both Paleoamericans and Native Americans derive from a single source population.

The caves of the Yucatan became a sacred place for the Mayan civilisation that established itself about 10,000 years later.

Cave diving scientist Dr Patricia Beddows, of Northwestern University in the United States, said: 'The preservation of all the bones in this deep water filled cave is amazing.

'The bones are beautifully laid out.

'The girl's skeleton is exceptionally complete because of the environment in which she died. She ended up in the right water and in a quiet place without any soil.'

Dr Beddows, whose findings were published in the journal Science, has hovered underwater above the skeleton's site since its discovery in 2011, and prospected in the area.

She said: 'Hoyo Negro is a very complex site.

'By understanding the formation of the shallow caves and the shaft into which the girl fell, we know the girl and the animals visited a site that looks almost like it does today, except the water level was down in the bottom of the shaft.'

The skeletons in Hoyo Negro have valuable rock crystals lying on them, including a new form Dr Beddows calls 'florets' in recognition of their bushy nature and one inch size.

Hoyo Negro is a black abyss in the state of Quintana Roo that is completely filled with water.

It can only be reached by divers travelling more than 4,000 feet through underwater passages using underwater propulsion vehicles, or scooters, which enable them to cover long distances.

It is about 200 feet deep and 120 feet wide and is located inside the Aktun-Hu cave system.

The skull and bones are clearly visible on the floor of the underwater cave

Once you enter the limestone pit you cannot see the floor below, and all that can be seen in front is a black void.

Dr Beddows added: 'Research in flooded caves is much like space exploration, with divers similar to astronauts reporting back to 'mission control', a much larger scientific team at the surface.

'It all has to be done on SCUBA, which is our life support system.

'Our science team has been supported by a great number of dedicated non-science cave divers who have committed hundreds of hours at very dangerous depths to complete this exploration."

Deciphering the ancestry of the first people to populate the Americas has been a challenge, researchers say.

The earliest modern human bones unearthed to date are pieces of human skull from 'the Cave of the Monkeys' in Laos that are up to 63,000-years-old.