From its pony-sized, hornless ancient ancestor, the many iterations of the rhino have included Paraceratherium, quite possibly the biggest and heaviest mammal ever, and Elasmotherium – a giant with a single, massive horn. At one time more than 50 rhino genera roamed the northern hemisphere – today only four remain. And Africa’s rhinos were comparative late comers, only permanently resident on the continent when the land bridge between Africa and the middle east formed some 10 million years ago. But when did the story of rhinos begin?
Broadly speaking, the evolution of rhinos began, as with all life forms, soon after our planet was born. But it took an awfully long time before anything like the horned giants of today began to roam the plains and forests of the world. In fact Earth was already some 99 percent along its 4.5 billion year path to the present before the first creatures we can definitely place as the ancestors of rhinos loped onto the evolutionary stage.
A world of steamy jungles and crocodile infested polar regions
Some 56–33.9 million years ago the world was hotter than at any time since the demise of the dinosaurs and there was little seasonal variation. In this steamy, humid place where even at high latitudes temperatures reached 30° C, the scene was set for the arrival in the northern hemisphere of the odd-toed herbivores, the Perissodactyla, the evolutionary line from which the horses, tapirs and rhinos of today are all descended.
The earliest of the odd-toed grass and browse feeding creatures that we know of is Hydrachyus eximus (seen here) which entered a world were ice had retreated completely and hot, steamy jungles, complete with crocodile-infested waterways, extended well into the polar regions. Hydrachyus, however, was nothing like the great beasts we know as rhinos. It was only about 1.5 m long and had no horn, but perhaps with some imagination there was something of a horse or tapir in its bearing.
Within a few million years the perissodactyls had split into two distinct lines: one embracing the horses and their relatives and the other including the forbears of tapirs and rhinos. As time marched on and the planet became somewhat cooler, the mammals were thriving and diversifying. And the rhino line was no exception. Three rhino families emerged. The “running rhinos,” hornless and adapted for speed across open landscapes, were the first to appear, and then came the “swimming rhinos”, also hornless and adapted for a semi-aquatic life. Finally came the Rhinocerotidae, the ancestral line that includes the five species of today, and the horn finally became a defining rhino characteristic.
Megacerops – the two horned horse
For a while the perissodactyls were the most diverse and abundant herbivores in North America, Asia and Europe. Among them was Megacerops, literally the “two-horned face”, one of the rhino-like Brontotheriums that appeared in North America some 38 million years ago. Despite its appearance it was related to the horse evolutionary line, not the rhinos.
About 34 million years ago, the Circumpolar-Antarctic Current formed, and global cooling accelerated as the flow of other ocean currents were affected. The Antarctic ice sheets began to expand rapidly and the seasons returned. Rainfall lessened and the jungles retreated while grasslands spread. Powerful running creatures evolved to fit the open vistas – as did the predators to catch them. For about 10 million years, from about 33.9 million years ago, the world was a relatively temperate place with patterns of seasonal rainfall. Many new mammals evolved – elephants, cats, dogs and marsupials amongst them.
Paraceratherium – a rhino that would have dwarfed an elephant
This time was much to the liking of the rhinos as well. Highly adaptable, they grew in abundance and rapidly diversified, spreading from Asia into Europe and, thanks to low sea levels and a resultant land bridge, across from Asia into North America. Initially these rhinos were small, like stocky little horses, but gradually they grew in height and mass.
One – a hornless giant named Paraceratherium – was quite possibly the biggest terrestrial mammal never to have existed. While estimates of its true size have probably been exaggerated, at some 7 metres tall and weighing 15–20 tonnes, it was taller than a giraffe and three times heavier than an African elephant.
The rhino’s Golden Age
Around 23 million years ago the planet’s climate remained temperate and relatively cool compared with the period soon after the demise of the dinosaurs; the continents continued to assemble into the positions, shapes and sizes that are familiar to this day. The horses and rhinos also diversified rapidly to exploit the new niches as they radiated across Asia, Europe and North America. This period also marked the first arrival of the Rhinocerotidae in Africa: short, stocky grazers that, in contrast with modern rhinos, were hornless. They had tusk-like forward pointing lower incisors, a characteristic that persists in the Asian rhinos of today. These early “African” rhinos were not, however, the ancestors of the living African rhinos of today. This was arguably the rhino’s golden age – the fossil record shows that some 50 rhino genera lived at some stage or other in Miocene times.
Around 14 million years ago, probably triggered by ocean circulation changes, cooling returned. The rate of change was rapid, faster than many creatures were able to cope with. Rhinos were amongst the many creatures that found the transition difficult and their slow decline began.
About 11 million years ago the northeastern parts of Africa split from Arabia as the Red Sea grew. But the land bridge between it and what, some five to six million years later would flood into existence as the Mediterranean Sea, remained. Now the stage was finally set for the horse and rhino lines that persist to the present day, to migrate southwards into Africa, leaving the tapirs to become permanently confined to the tropical forests of South America and southeast Asia.
Into Africa – at last
The precise lineage of the Black and White Rhinos of today remains unresolved however, although they seem to have been present in Africa from about 10 million years ago. One view is that their common ancestor was Ceratotherium neumayri which roamed in the region of Anatolia in present-day Turkey, from about 11 million years ago. It is thought that Ceratotherium simum, the White Rhino of today is the direct descendent of C. mauritanicum which roamed the extensive and comparatively well-watered savanna of North Africa sometime between about three million years ago through to the Middle Stone Age. At that time a drying climate would have wiped out its habitat in North Africa, pushing it southwards. Slightly older fossils have been found in East Africa but it is uncertain if these are also of C. mauritanicum or C. efficax, a more primitive species known from Ethiopia and Tanzania. An alternative view holds that C. efficax is the earliest African
species and that it diversified into C. mauritanicum in North Africa, C. germanoafricanum in East Africa and C. simum, the extant White Rhino.
About four to five million years ago the Black Rhino line, by then specialist browsers, diverged from the White Rhino and Diceros praecox evolved. Its direct descendent, D. bicornis, the Black Rhino of today, emerged about two and a half million years ago. The Indian and Javan Rhinos probably also emerged around the same time, as did Elasmotherium and the woolly rhino. By now the planet was fully in the grip of an Ice Age, a climatic time that continues to the present, notwithstanding many oscillations between periods of glaciation and relative warmth, and of course our current concerns around global warming.
These were tough times for wildlife and for early humans and many species succumbed. The rhinos were now in fairly rapid decline and only eight genera made it through to about two million years ago. The extant rhinos of Africa and Asia probably survived because they ranged across the warmer parts of the planet. The remaining rhinos in Asia and Europe had to adapt to the cold. The members of the large woolly rhino genus (related to the Sumatran Rhino of today) all developed long, shaggy coats well suited to the cold, dry steppes. Their nasal bones were also separated into two parts to warm icy air more easily.
The other species adapted to the cooling climate was Elasmotherium. Could this giant “unicorn” rhinoceros have been the source of the legend, especially given that its long, running legs gave it a horse-like demeanour. It stood 2 metres tall was about five metres long m, and could well have weighed nearly five tonnes. Its outstanding feature though was its single, massive horn. The woolly rhino became extinct around 10,000 years ago, its demise probably aided by human hunting. Elasmotherium suffered similarly and all rhinos became extinct in Europe.
Homo sapiens – the terraforming species
Over the past 11,700 years, especially the most recent 60–70 years of it – the Anthropocene, as it has come to be called – we humans have redesigned the face of our planet. We started modestly with agriculture, forest clearing for timber and the growth of small cities, but then we expanded exponentially in step with our numbers and our consumptive nature. Our accomplishments have been huge, but they have been realized, it must be said, at the expense of all the natural landscapes of the world and the creatures that live in them.
Rhinos have certainly not done well on our watch. We have hunted them relentlessly for the perceived therapeutic and status value of their horns, to the point that over the past 200 years or so we have reduced them to mere remnants of the hundreds of thousands, if not millions that once roamed the forests and plains of Africa and Asia.