It's roamed the Earth for perhaps 250 million years, but tuatara have only now yielded their genetic secrets – and they're unlike anything scientists have ever seen.

A world-first study, published today, reveals the rare reptile's entire genetic makeup, or genome, is more than two-thirds bigger than our own – and helps confirm its evolutionary position and unique life history.

Tuatara were once widespread across New Zealand, but humans and pests has squeezed its total population to somewhere between 60,000 and 100,000.

The species have been out on their own branch of the evolutionary tree for a staggering amount of time, with prior estimates of divergence ranging from 150 to 250 million years.

With no close relatives, the position of tuatara on the tree of life has long been contentious.

Some argue tuatara are more closely related to birds, crocodiles and turtles, while others say they stem from a common ancestor shared with lizards and snakes.

The new study, featured in prestigious scientific journal Nature, puts tuatara firmly on the latter branch – but with some twists.

They appear to have split off on their own trajectory for around 250 million years – a massive length of time when we consider primates only originated around 65 million years ago, and hominids, from which humans are descended, originated approximately six million years ago.

"Proving the phylogenetic position of tuatara in a robust way is exciting," said the study's leader, and Otago University geneticist Professor Neil Gemmell.

"But we see the biggest discovery in this research as uncovering the genetic code and beginning to explore aspects of the biology that makes this species so unique, while also developing new information that will help us better conserve this taonga or special treasure."

One area of particular interest was to understand how tuatara, which can live to be more than 100 years of age, achieve such longevity.

Examination of some of the genes implicated in protecting the body from the ravages of age found that tuatara have more of these genes than any other vertebrate species thus far examined.

"Could this be one of the keys to their long lifespan?" Gemmell added.

"Tuatara also don't appear to get many diseases, so looking into what genetic factors might protect them is another point of focus for our study, as too we have also explored genetic aspects that underpin the vision, smell and temperature regulation of tuatara."

With the genome now sequenced, the international science community now had a blueprint through which to examine the many unique features of tuatara biology, aiding our understanding of the evolution of the amniotes, a group that includes birds, reptiles and mammals.

"It is not far-fetched to suggest that through our new understanding of the tuatara genome, there may be novel insights that emerge that will benefit our understanding of our own biology and health."

Another big part of the research was its collaboration with local iwi Ngātiwai, something Gemmell said had proven "incredibly rewarding", while providing an exemplar to future studies.

"Very early on it become clear that a primary goal for us all was to develop new knowledge that would improve the conservation of this species," he said.

"We agreed to partner together to achieve that aim, whilst also looking for opportunities to share other benefits that might derive from the research."

Ngātiwai Trust Board resource management unit manager Alyx Pivac said tuatara were a taonga, and it was pleasing to see the study published.

"Our hope is that this is yet another piece of information that will help us understand tuatara and aid in the conservation of this special species."