Let's go back in time 30 million years, long before modern humans appeared. Tropical forests were shrinking and grassy savannahs were spreading. These lush grasslands were home to creatures long since lost: giant rhinoceroses.

Standing 5m tall at the shoulder and weighing up to 20 tonnes, the colossal Paraceratherium was the largest land mammal to ever live. Its skull alone was over 1m long and it had a much longer neck than today's rhinos, which helped the animal browse for leaves on tall trees. This monstrous creature roamed the open plains stretching from eastern Europe to what is now China.

Paraceratherium, with its enormous body and vast range, illustrates how rhinos lived when they were at their peak. It is the high point of a rarely-told story that spans 50 million years. During that time rhinos have migrated across continents, faced prehistoric hyenas and giant crocodiles, and endured the frigid wilderness of the ice age. But their story begins soon after the extinction of the dinosaurs, in roasting tropical heat.

Imagine a time when most of what is now Asia, Europe and North America was covered in dense forest. It was a very warm period in Earth's history, known as the Eocene. This biological epoch began 55 million years ago and ended 34 million years ago. It was then that rhinos first emerged.

Rhinos belong to a group of animals called perissodactyls. These animals all have hooves, and crucially, they have an odd number of toes on their rear feet.

Nobody is quite sure how perissodactyls evolved. A study published in 2014 suggested that they first appeared 55 million years ago in India, which at the time was not attached to Asia.

What is clear is that the early perissodactyls were the ancestors of rhinos, as well as all modern horses, zebras and tapirs.

The early rhinos that lived in the Eocene were quite different to today's.

The Asian hyracodonts would eventually evolve into giants

For example, amynodonts did not have horns and looked rather like modern tapirs – which look like pigs with unusually long limbs. One group of these, the metamynodons, were rather like modern hippos and spent a lot of time in water.

Then there were the hyracodonts of North America, Europe and Asia. To our eyes they would barely look like rhinos at all, and instead seem to be bulky little horses.

The Asian hyracodonts would eventually evolve into giants such as Paraceratherium. That evolutionary growth spurt took place in the next period of Earth's history, the Oligocene.

It's not clear why Oligocene rhinos got so big. But it may have been a way of coping with the more open habitat, says Jerry Hooker of the Natural History Museum in London, UK.

Despite being so large, Paraceratherium wasn't safe from predators

As grasslands replaced forests, the rhinos had to travel further to find trees to feed on. They also had to make the most of food at the tops of trees, as there wasn't always much vegetation lower down.

"Giraffes today are pretty successful in Africa, as are elephants which also can high-browse because of their size and their trunks," says Hooker. "They often travel huge distances to find food."

For all its size, Paraceratherium had relatively slender legs and wasn't as bulky as a modern rhino. It also didn't have a horn, along with many Oligocene rhinos.

What's more, despite being so large, Paraceratherium wasn't safe from predators. It and other huge prehistoric rhinos were hunted by gigantic crocodiles.

Palaeontologist Pierre-Olivier Antoine of the University of Montpellier in France has found evidence of 10m-long crocodiles eating large rhinos. "In Pakistan we found many, many specimens," he says. "Bones of huge rhinos with the conical tooth prints of giant crocs."

There are no giant rhinos today

One such species, Crocodylus bugtiensis, is known from fossils found in Pakistan, where Paraceratherium once lived.

It's not clear that the crocodiles would have been able to regularly prey on healthy adult rhinos. But they might have snatched young or ill prey when they ventured into water.

Antoine has also found tooth prints, which he thinks were made by a Hemicyon: an extinct predator that looked like a cross between a dog and a bear. Clearly, even giant rhinos had plenty of predators to be wary of.

There are no giant rhinos today. It's not clear why they disappeared, but they may have been out-competed by a newly-evolved rival: elephants.

Elephants were "totally bad news," says Mikael Fortelius of the University of Helsinki in Finland. "They were just so much better at being super-large herbivores on land. They were more versatile and adaptable. The trunk is just such a marvel."

This enclosed habitat might not have suited the large rhinos

If elephants thrived and hampered rhinos' access to key food sources, that may well have spelled trouble for the giants, which needed to eat hundreds of kilos of vegetation every day.

Hooker points out that the giants also never made it to Europe, which was more densely forested than the other continents. This enclosed habitat might not have suited the large rhinos, which were used to more open spaces, preventing them from expanding westwards.

Still, even though they had shrunk a bit the rhinos were still very numerous.

From 23 million years ago, Earth entered a new period called the Miocene. The planet cooled by as much as 4 °C.

Rocks laid down in the Miocene contain an astounding range of rhino fossils, says Antoine.

An excavation in Montréal-du-Gers in south-west France uncovered five rhino species all fossilised together "in one pond", he says. In total, the remains of over 100 individuals were recovered. Similarly, in the Bugti Hills of Pakistan he once found up to 9 species.

Clearly, the planet was practically teeming with rhinos, and they came in all shapes and sizes.

For example, Chilotherium was a truly hippo-like rhino. It had little tusks sticking out from its lower jaw and an outsized head. With a small skeleton to support this heavy head, it seems likely that Chilotherium buoyed itself in water.

"I think there's decent evidence they are doing what hippos do elsewhere," says Fortelius. "They live in water, eating grass and other vegetation on the river banks."

There were also rhinos called Diceratherium that had two horns, but not one in front of the other. Instead, they were beside each other on the rhino's nose.

Furthermore, one of the most iconic of all rhinos has its roots in the Miocene. A group called Elasmotheriines evolved single horns on their heads, and as global temperatures continued to cool over the next few million years, they evolved into Elasmotherium.

It was 3m tall, which is impressive but much smaller than Paraceratherium. However, its most striking feature was its huge horn.

Rhino horns don't fossilise, though they are sometimes preserved in ice. No Elasmotherium horns have ever been discovered, but it is apparent from a base-like recess on the skull that a horn was once attached to it.

Earth finally plunged into a full-scale ice age

It's not clear exactly how large the horn was, and palaeontologists generally detest speculating about its exact length. Most, though, think it was enormous. It may have been more than 1m long.

Elasmotherium appeared on the scene around 2.5 million years ago, at the beginning of the Pleistocene. After millions of years of cooling, Earth finally plunged into a full-scale ice age, and sheets of ice spread from the Arctic to cover much of Europe and North America.

Faced with a frigid climate, rhinos evolved thick woolly coats. It's not clear if Elasmotherium was woolly, but plenty of other species were. The woolly rhinos may have had their origin in Tibet, before the ice age began.

The period between the Miocene and the Pleistocene is known as the Pliocene. It lasted from 5 million years ago until 2.5 million years ago.

At this time most of the world – with the exception of Antarctica – was relatively ice-free. But Tibet, because it is so high, was already iced up.

Woolly rhinos could not cope with deep snow

In 2011, a group of palaeontologists described the fossil of a primitive woolly rhino discovered in Tibet. That suggests woolly rhinos first evolved there, and then dispersed to the west when the Pleistocene ice ages began.

Unlike many prehistoric rhinos, woolly rhinos would be quite recognisable to us. They had a large front horn and second, smaller horn, plus stocky legs and a bulky body.

However, despite their thick coats, woolly rhinos could not have penetrated that deeply into the ice-covered regions. They could not cope with deep snow.

We often picture "woolly" ice age animals surrounded by snow and ice. But they would not have been able to exist in such places, says Danielle Schreve of Royal Holloway, University of London in the UK.

The woolly rhinos had a harder time than their Eocene and Oligocene ancestors

"It's one of the things that may have contributed to their extinction," says Schreve. "Because they've got such a stocky and compact body with relatively short legs, they're not good at moving through deep snow, so they need relatively snow-free areas."

Rather than plodding forlornly across ice sheets, then, woolly rhinos would have lived in an environment known as "mammoth steppe". The climate was cold and dry, but there were plenty of herbs and shrubs for them to eat.

All in all, the woolly rhinos had a harder time than their Eocene and Oligocene ancestors. According to Schreve, the Pleistocene is when life became truly difficult for many rhino species.

For one thing, towards the end of the Pleistocene the climate began fluctuating wildly. Temperatures rose and fell as much as 10 °C within a generation. For slow-breeding rhinos, dependent on stable food sources, these changes were disastrous.

All of the bone is scored with tooth picks, scratches and punctures

Predators were also a problem. Giant crocodiles didn't threaten European rhinos, but instead they were attacked by prehistoric hyenas.

Schreve has found evidence of hyenas eating baby rhinos. These dog-like carnivores would even have crunched the bones of their prey to get as much nutrition as possible.

"All of the bone is scored with tooth picks, scratches and punctures, so it was an important resource," says Schreve. "And yes, they seem to be taking and consuming adult rhinos as well."

If that wasn't bad enough, woolly rhinos were probably being hunted by humans as well.

Humans were probably the last straw, says Schreve. "You can probably lay some of the blame for extinction at their door, but really they're the final nail in the coffin," she says. The woolly rhinos had already "gone through millennia of rapid climate change that they were poorly suited to withstand."

Despite all this, woolly rhinos clung on until just 10,000 years ago. In February 2015, it was reported that hunters in Siberia had stumbled upon a preserved baby woolly rhino of about this age.

Unstable climates and human hunting put an end to many rhino species

Oher species were also feeling the brunt of human hunting. A site in Boxgrove in the UK has fossil evidence of early humans butchering rhinos for meat between 90,000 and 700,000 years ago. In some cases, carnivores have bitten through marks in bones already made by human tools, says Schreve. That suggests rhinos were first hunted by humans, and their carcasses then scavenged by other animals.

The combination of unstable climates and human hunting put an end to many rhino species. Until this happened, they were very common in Europe, along with other huge animals like elephants and mammoths. Such animals are now confined to Asia and Africa, if they even exist at all.

Today, all the diverse rhinos have been reduced to just five species. They have all been heavily hunted, and in recent decades poached for their horns, so none of them is in a good way.

Africa's white rhinos are divided into subspecies, northern and southern. While the southern subspecies is in fairly good shape, the northern one has been driven past the point of no return. There are only five left alive, and only one male. He is under constant armed guard to protect him from poachers, and has even had his horn removed to deter them.

The other African species, the black rhinoceros, is critically endangered. There are thought to be seven or eight subspecies, of which three are already extinct and another is nearly gone.

The smallest species is the Sumatran rhino, which unlike the other surviving species is slightly woolly. It is also critically endangered. One subspecies is represented by just three captive individuals. As well as the threat from poachers, rhinos are also hindered by their need to give birth in secluded, shrub-covered areas. Such places are becoming harder to find.

Unlike other rhinos, Javan rhinos are sparing with their horns: only males have them. They are also critically endangered, being confined to a tiny area on the western tip of Java. There may be only 40 left.

It's not all bad news, though. Indian rhinos are considered vulnerable, and while that's not ideal it is far better than critically endangered. They survive in northern India and southern Nepal. A recent count suggested that the Nepalese population had grown by 21% in four years.

At least some of the rhino species could start to recover and grow their populations

All sorts of ideas have been put forward for saving the remaining rhinos, but most experts agree that the best approach is also the hardest: nations working together to protect conservation sites and, crucially, to stop the illegal trade in rhino horns.

That means stopping the poachers who kill the rhinos, but it also means tackling a vast network of organised crime that ships the horns to China and other Asian countries, and sells them at a huge mark-up. It will also be important to end the demand: at the moment, rhino horns are status symbols in China, so people pay lots of money for them.

If this could be achieved, at least some of the rhino species could start to recover and grow their populations. It may well be too late for some of the species and subspecies, whose populations are now so small that they could never recover. But at least the black and Indian rhinos, surely, could be rescued.

Still, we are a long way from the time when many species of rhino roamed the landscapes together, some of them towering over every other land animal. Whatever happens now, the age when rhinos ruled the world is gone.