Scientists have confirmed that giant insects found on a rocky outcrop off Lord Howe Island are a genetic match for the island’s stick insects that were believed to have gone extinct almost 100 years earlier.

The species were assumed to be one and the same. However significant morphological differences between the Lord Howe Island stick insects collected in the early 1900s and stored in museum collections, and the phasmids discovered in 2001 on Ball’s Pyramid (a remnant volcano about 23km off the main island), created a suspicion the latter could be a related species – rather than the original back from the dead.

That suspicion prompted scientists to map the genome from descendants of the Ball’s Pyramid phasmids, which were bred in captivity at Melbourne zoo. They compared it to DNA extracted from museum specimens held by the CSIRO.

The result was a less than 1% variance, within the range of difference for a single species. Despite their different looks, both are confirmed to be the critically endangered Dryococelus australis.

The results of the study were published in Current Biology on Thursday.

Its lead author, Sasha Mikheyev, said researchers had not been able to make the comparison when the first two breeding pairs were collected off Ball’s Pyramid and taken to Melbourne zoo in 2003 because they did not have the technology to extract and sequence the DNA of long-dead specimens.

“We are now able to get DNA out of things that we considered impossible,” he said. “We took a leg from each of the specimens and we extract the inside part that contains a lot of DNA.”

The stick insects now held in museums were collected on Lord Howe Island (in the Tasman Sea, 600km east of the New South Wales coast) when it was settled as a whaling colony in the 19th century. The animals were abundant on the island until the introduction of black rats, which arrived on ships in 1918 and swiftly hunted the insects.

In 2001 a small colony was found on a rocky ledge of the largely barren Ball’s Pyramid. The isolated outcrop and Lord Howe Island are not believed to have ever been linked by land and it is unclear how the phasmids, which are sometimes called “tree lobsters”, came to be living on the former.

Now that they are confirmed to be the same species, Mikheyev said the animals could potentially be rereleased on Lord Howe Island, provided the rats were eradicated. This year the Australian government approved a mass rat-baiting plan which will begin in mid-2018.

Mikheyev said it was not clear why the descendants of the Ball’s Pyramid colony look so different to the earlier specimens but it could be due the harshness of that habitat compared with the lush surrounds of Lord Howe Island.

“We won’t know for sure until these animals are back in their natural habitat on Lord Howe Island,” he said.

Female Lord Howe Island stick insects grow to about 12cm long, while males mature to just over 10cm. They are not the largest stick insect in Australia – that honour goes to Ctenomorpha gargantua, which can grow to 50cm – but their unlikely survival is a cause for celebration.

“When we think about stories we hear about extinction events, more often than not the history [shows that] things get worse,” Mikheyev said. “Very rarely there’s a chance to go and fix something.”

Kate Pearce, the invertebrates manager at Melbourne zoo, suggested the morphological differences between the four insects that formed the foundation of the captive breeding program and the museum specimens could be a false impression caused by selective collection by the 19th-century zoologists.

“The specimens in the museum collections were always a lot bigger,” Pearce said. “The legs were a lot chunkier and they had much bigger spurs.

“Now that we have answered that question [of whether they are the same species], we think that what happened with the museum specimens is they were collecting the biggest, meatiest specimens that they could find.”

Pearce said some stick insects bred in Melbourne zoo, of which there are now 800, grew to a similar size.

The entire captive population of Lord Howe Island stick insects, which are also kept at Museums Victoria, on Lord Howe Island itself, and in San Diego and Bristol zoos, are descended from the two breeding pairs collected in 2003.

This year another female, named Vanessa, was collected to join Melbourne zoo’s breeding program. Her eggs are due to start hatching this week.

Pearce said the results of the study, which Melbourne zoo requested, would guide the future conservation of the species.

“We can look at pushing for their reintroduction back on to Lord Howe Island without that question of whether we are just introducing a new invasive species,” she said. “We now know for sure that it’s the same species.”