(CNN) Our ancestors learned to respect the auroch, and immortalized them in pre-historic cave paintings.

The earliest cows were mighty beasts that stood almost as tall as elephants, with lean, powerful frames and fearsome horns that would make a hunter think twice.

For thousands of years the aurochs were the largest land mammals in Europe, until the rise of human civilization decimated their numbers, and the last of the species died in Poland in 1627 -- one of the first recorded cases of extinction.

Conservationists now believe the loss of the keystone herbivore was tragic for biodiversity in Europe, arguing that the aurochs' huge appetite for grazing provided a natural "gardening service" that maintained landscapes and created the conditions for other species to thrive.

The theory is now being put to the test, as a "near 100% substitute" of the beast is returned to the forests.

Cave painting in Lascaux, France dating back around 17,000 years which is believed to depict an auroch.

Speed breeding

Ecologist Ronald Goderie launched the Tauros programme in 2008, seeking to address failing ecosystems. The most powerful herbivore in European history seemed to offer a solution.

"We thought we needed a grazer that is fully self-sufficient in case of big predators...and could do the job of grazing big wild areas," says Goderie. "We reasoned that this animal would have to resemble an auroch."

Rather than attempt the type of gene editing or high-tech de-extinction approaches being employed for species from woolly mammoths to passenger pigeons , Goderie chose a method known as back-breeding to create a substitute bovine he named "Tauros."

Auroch genes remain present in various breeds of cattle around the continent, and the team identified descendants in Spain, Portugal, Italy and the Balkans. Geneticists advised breeding certain species together to produce offspring closer to the qualities of an auroch, and then breed the offspring.

Maronesa cow, a close descendent of the auroch.

The animals get closer with each generation, and the team have the advantage of being able to test the offspring's DNA against the complete genome of an auroch, which was successfully sequenced at University College Dublin.

"You could see from the first generation that apart from the horn size, there was enough wild in the breed to produce animals far closer to the auroch than we would have expected," says Goderie.

The ecologist had predicted that seven generations would be necessary for the desired outcome, which might be achieved by 2025.

The program is now in its fourth generation, and pilot schemes across Europe are offering encouragement.

'Manolo Uno' - one of the first cross-breeds of the Tauros programme.

In the wild

The Tauros programme connected with Rewilding Europe early on, a group that supports the restoration of natural processes through projects that range from rebuilding rivers to introducing apex predators.

Rewilding Europe was able to provide protected land across the continent; in Croatia, Spain, Portugal, the Czech Republic, and Romania, where Goderie's cross-breeds could test themselves in the wild. In many cases they rose to the challenge.

"We see progress not only in looks and behavior but also in de-domestication of the animals," he says. "This is a challenging process as they have to adapt to the presence of large packs of wolves."

Herds of herbivores are habitually decimated by local wolves at the Croatian site, says Goderie. By contrast, the Tauros learned to defend themselves and suffered few losses.

Selective breeding will continue toward creating the ideal animal. But the current crop is already serving a function.

Small Tauros herd in the Czech Republic.

Shades of green

For Goderie's partners, the auroch project offers an opportunity to restore natural order.

"We support the come-back of large herbivores as they are key species for the specific role of grazing and browsing," says Frans Schepers, managing director of Rewilding Europe.

"Bovines can shape habitats and facilitate other species because of their behavior, and the more primitive and close to the wild the better because it means that eventually they can become part of the natural system."

Many European landscapes are in dire need of grazing animals, which can otherwise become uninhabitable for other species.

"Without grazing everything becomes forest, or barren land when there is agriculture," says Schepers. "The gradients in between are so important for biodiversity, from open soil to grassland and 'mosaic landscapes.'"

"The Rhodope Mountains of Bulgaria are among the richest areas for reptiles such as tortoises, snakes and lizards. But they need open spaces or they lose their habitat."

Aurochs were formidable grazers and are expected to restore dysfunctional ecosystems.

Targeted intervention

Critics of rewilding initiatives have suggested that introducing new species could have unintended consequences.

The International Union for the Conservation of Nature (IUCN) has issued new guidelines in an effort to manage such impacts.

"Bringing back close approximations of original species may help to reverse losses from emotional to ecological grounds, but often the devil lies in the detail," says Dr. Axel Moehrenschlager, IUCN Chair of the Reintroduction Specialist Group.

"Typically emphasis is placed on technical feasibility, without sufficient emphasis on risks. Questions abound whether primarily wetland forests like the aurochs used to inhabit still exist, whether it could negatively impact wild or domestic plants or animals, and if it might endanger people."

The successful introduction of bison in the US shows that such initiatives can have a positive impact, says Dr. Eric Dinerstein of conservation group Resolve, but he adds that one intervention can lead to another.

"If an ecosystem evolved with large herbivores...there is not an alternative and you need something in its functional role," he says. "But to introduce aurochs, you may need predators as well."