A tall, heavy beast with long, forward-curving horns faced down a smaller bull.
Its head was held high as if in challenge. The smaller animal seemed to recoil in submission. Even the cave lion, the largest of predators, looked unlikely to challenge the dominant bull. Behind them a herd of giant deer ran from unseen danger.
This scene was depicted in a painting on a cavern wall at Lascaux, France, 17,000 years ago. Megaloceros, the giant deer in the background, are extinct. The cave lion is extinct. The bulls - a species called aurochs - have disappeared. Their genes, however, are still present in modern cattle, and scientists have been trying to bring them back to life.
The Taurus Programme, a partnership of ecologists, geneticists, historians and cattle breeders backed by Stichting Taurus, a Dutch nonprofit, is seeking to re-create the aurochs by crossbreeding modern cattle in a process known as back breeding. Laboratory-based genetic engineering is not required.
The programme's scientists have identified breeds of cattle that share characteristics with their auroch ancestors: large stature, long legs, a slender and athletic build, horns curving forward, black coats in the males and reddish brown ones in females. The back breeding began in 2008 with seven varieties crossed.
"What's kind of surprised me is the low-hanging fruit that's quite easy to get at," said Ronald Goderie, an ecologist who directs the Taurus project and is co-author of the book The Aurochs: Born to Be Wild.
"What you see already in the second generation is that the coloration of the animal is very aurochs-like. The bulls are black and have an eel stripe [along the spine]. They're already higher on the legs. What's more complicated is the size and shape of the horns. I would say that in some cases you can see an individual animal is 75 per cent of where we need to get at. . . . We think in six, seven generations we will get a stabilised group of Taurus cattle. That will take us another seven to 10 years."
The Taurus project was formed as part of the "rewilding" conservation movement. Rewilding involves the restoration of large tracts as much as possible to their pre-human state. This often entails reintroducing key animals and plants that had disappeared.
Aurochs once ranged across Europe and much of Asia. A combination of hunting and conversion of wild pastures to farmland reduced Europe's wild aurochs to a small remnant population in a Polish forest, where it was protected by royal order until the last one died in 1628. For more than a century, the Polish royal family tried to save the aurochs, giving villagers tax breaks for cutting hay and feeding them in the winter. But political instability, domestic cattle diseases and other threats finally rendered Europe's early attempt at wildlife conservation a failure.
Rewilding Europe, a nonprofit organisation, was formed in 2011 with the goals of creating large natural habitats offering havens to endangered species and, in the case of the aurochs, mimicking the world that they thrived in. The group has selected 10 locations for rewilded parks, with plans to put aurochs in some of them. Work has already begun in seven of the areas.
The case for rewilding parts of Europe - and re-creating the aurochs - is that large tracts are ecologically out of balance. The acreage, much of it former farmland, has become inhospitable to many species of birds, insects, small mammals and predators that evolved in the presence of big animals that compacted and enriched soil while grazing on grass.
The case against both rewilding and making aurochs is that too little is known about how all of these species would interact with each other. The results of releasing aurochs might be difficult to predict, sceptics say.
Chimpanzees and humans have virtually the same genes, but you would hardly issue passports or marriage licenses to chimpanzees
Manuel Lerdau, a University of Virginia ecologist who studies the dynamics of large-scale wild places, questioned the viability of Rewilding Europe's proposals. He said the project should consider how the introduction of the aurochs would affect other plants and animals.
"If we're targeting one particular member" of an ecosystem, Lerdau said, "what will we need to be thinking about with the other members of the community?"
Simply introducing aurochs and hoping for the best may not be enough, according to Lerdau, who says that without proper preparation the animals might only become extinct again.
"The example that I think of in terms of those larger scales [meaning herds of more than just a few dozen animals] is the bison in Yellowstone's reintroduction project," Lerdau said, "where it was based on decades of research on grasslands and bison and what it would take to maintain viable bison populations in the face of the constraints of the modern world. We've learned a lot about grasslands, about the ecology of the West as well as about bison as a result of that effort."
Rewilding Europe has a goal of eventually releasing at least 150 Taurus cattle into each rewilded area. Other animals considered for reintroduction include wild horses, wisents (a distant cousin of the American bison) and ibex.
This is not the first attempt to re-create aurochs. In the 1920s, biologists Heinz and Lutz Heck, two brothers, began crossing domestic breeds of cattle at German zoos in a somewhat similar effort. Heck cattle have been used in some European rewilding projects, but the reviews have been mixed.
Goderie said the animals were too small. Valerius Geist, a professor emeritus of environmental science at the University of Calgary and a former student of Heinz Heck, disagreed. "They were short-legged, all right, but they were not small," Geist wrote.
The Taurus project is using modern genetic technology to establish a benchmark by which to measure success. A complete genome of an aurochs was sequenced in 2014 from a 6700-year-old humerus bone found in an English cave.
"We want to compare this sequence with sequences which we are generating at the moment from primitive breeds," said Richard Crooijmans, a molecular geneticist at Wageningen University in the Netherlands who is guiding the DNA analysis for the Taurus project.
As more Taurus cattle are bred, Crooijmans will compare samples of their DNA to the reconstructed aurochs genome. If the Taurus DNA starts to look more like aurochs DNA, they will know the project is working.
Geist questioned whether similar DNA would mean success. "Chimpanzees and humans have virtually the same genes, but you would hardly issue passports or marriage licenses to chimpanzees."
No matter how closely Taurus cattle come to resemble the aurochs of old, it will be difficult to ever declare victory.
"We're not going to breed back an aurochs," Crooijmans said. "We're going to breed back an aurochs-like animal. The way we're going to do it now is combining animals. We don't know from what time point you want to have an aurochs back. Do you want to have it from the Pleistocene? From the early Holocene? From the Middle Ages?"
With a fossil record stretching back 2 million years, the aurochs had plenty of time to change and adapt. The mild-mannered aurochs of the 1620s may have been different from the Neolithic animal whose bone was used for DNA sequencing.
Commercially raised cattle could be at risk if a nearby wild population of aurochs-like cattle have not been vaccinated. To keep track of the animals and be sure that they have all been vaccinated, they need to be given ear tags. Regular veterinary care and tagging could make it impractical to allow large populations of wild animals to become established.
Descended from livestock, the re-created aurochs would be regulated as domestic animals under European Union law, and it is unclear whether any of them could be declared legally wild. Aurochs will officially be domestic cattle.
But there's another way of thinking about this.
"In principle, the aurochs is not really extinct," Crooijmans said. "It's still living in the current cattle breeds, but it's changed so much."