About two million years ago, not long before humans first arrived in south-east Europe, a giant was disappearing from the west of the continent. Titanochelon, Europe's last giant tortoise, was on its way to extinction.

The last confirmed sighting comes courtesy of a few newly-described bone fragments found in an ancient hyena den in Spain.

Tortoises are a family of turtles that have evolved to live on land. They are a successful group, with more than 50 recognised species alive today.

Arguably, the most famous are the giant tortoises of the Galápagos. With shells that can grow more than 3ft (1m) in diameter, the Galápagos tortoises are icons of the natural world. There are about a dozen distinct populations, all belonging to the species complex Chelonoidis nigra.

Less well-known, but almost as large, are the giant tortoises living half the world away on the Aldabra Atoll in the Indian Ocean. Again, they all belong to one species.

Given that the only two living populations of giant tortoise are island dwellers, it would be understandable to assume that the reptiles only become whoppers if they are confined to islands for long periods of evolutionary time. After all, plenty of other animals have also grown large while isolated on islands.

But the fossil record tells us otherwise, says Adán Pérez-García at the National University of Distance Education in Madrid, Spain. In the relatively recent past, giant tortoises plodded across almost every large landmass on Earth. The only exceptions were Antarctica and Australia.

Several giant tortoises called Europe home. The last, Titanochelon, was also the largest ever to evolve on the continent. Some Titanochelon tortoises had shells approaching 6.6ft (2m) in length. This makes them considerably larger even than today's Galápagos tortoises.

Until recently, palaeontologists mostly assumed that Titanochelon had disappeared from western Europe about 3.3 million years ago. They may have clung on in a few places – there are remains, perhaps 2.4 million years old, from the Greek island of Lesbos, which was connected to mainland Europe at the time – but this seems to have been the overall trend.

However, the tortoise remains described by Pérez-García and his colleagues in January 2017 suggest a rethink is in order.

At a site called Fonelas P-1 in the Baetic Mountains of south-east Spain, palaeontologists came across a thighbone and toe bone, both of which clearly belonged to Titanochelon.

Predators may well have been one of the factors that drove Titanochelon to evolve such large bodies

The bones are just two million years old. That means giant tortoises survived on the European mainland for 1.3 million years longer than palaeontologists thought.

The bones were unearthed in an ancient hyena den. However, Pérez-García suspects that their presence there reflects scavenging rather than predation.

"Carrion was very abundant in that warm region, with abundant water resources," he says.

In fact, Pérez-García says that the pressure from predators may well have been one of the factors that drove Titanochelon to evolve such large bodies.

There may have been other factors too, says Scott Thomson at the University of São Paulo, Brazil. In particular, a warm climate and a stable, nutrient-packed diet probably helped the tortoises grow.

The colder temperatures force a need to have shelter, which these tortoises cannot do

"The warm climate encourages plant growth, but specifically it encourages plants with high food value," says Thomson. "This allows for faster or greater growth, particularly in reptiles."

As long as the climate remained benign, life was probably relatively easy for Titanochelon. Unfortunately, the giants' luck probably ran out about two million years ago, because Europe's climate began to turn cold.

Cooler conditions are bad news for giant tortoises.

"The colder temperatures force a need to have shelter, which these tortoises cannot do," says Thomson.

It seems likely that Titanochelon's demise had nothing to do with humans

Small tortoises have the option of burrowing to escape the cold, which might explain why small tortoises are still found in Europe to this day. But giant tortoises would find it difficult to dig burrows.

Giant tortoises also need to incubate their eggs for longer than smaller tortoises, says Thomson. Again, this is harder to do in a cooler climate.

But there is another, even more significant problem faced by giant tortoises – and tortoises and turtles more generally. Many were hunted to extinction by humans.

This makes the new remains of Titanochelon significant for another reason.

Although there is still not much evidence to go on, it does appear that Europe's giant tortoises disappeared shortly before ancient humans first arrived on the continent. Given that, it seems likely that Titanochelon's demise had nothing to do with humans.

This serves as a reminder that spectacular beasts can be driven to extinction entirely independently of human activity.

"Our species is certainly responsible for many extinctions, but not all of them," says Thomson.

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