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The rhino’s story—a 55 million-year journey through time.

By May 11, 2022Editorial

 

One of the rhino’s ancestors was the hornless Paraceratherium—quite possibly the biggest terrestrial mammal never to have existed. It would have been at least three times heavier than even the largest African bull elephant. Image: Shutterstock

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 emerged not long (in geological terms at least) after the demise of the dinosaurs. These were the perissodactyls, odd-toed browsing mammals that loped onto the evolutionary stage about 55 million years ago. Quite when our human line and rhinos first intersected is uncertain but archaeological finds—in Britain of all places—suggest that it happened at least some 700,000 years ago. This short account follows the rhino’s long, eventful journey through numerous climatic changes and evolutionary adventures to face the challenge of all—surviving the Anthropocene, the age of Homo sapiens.

The decline and fall of the dinosaurs

About 100 million years ago, the age of the dinosaurs was in full swing. But their days were numbered, for they were entering a 40-million-year period of slow decline as a changing and cooling planet did not suit their reptilian needs. On the other hand, mammals were responding well to the new regime and were beginning to put their heads more confidently above the parapet.

Some 66 million years ago, a massive asteroid, or maybe a comet, careened into the Earth and the reign of the great lizards was pretty well over. Image: Alamy

Then, 66 million years ago, came the straw that broke the dinosaurs’ back: a massive asteroid, or maybe a comet, careened into the Earth near Mexico’s Yucatan Peninsula, and the reign of the great lizards was pretty well over as the world cooled rapidly under a shroud of dust and debris. It is still uncertain whether this event alone wiped out the dinosaurs or a chain of calamities, but a new stage opened to the survivors, amongst them the still tiny, warm-blooded mammals.

 

The rise of the mammals

The predatory Arctocyon or “bear dog.”Image: Daniel Eskridge/Stocktrek Images

Temperatures soon rose, and subtropical vegetation once again reached as far south as Patagonia in South America and northwards to Greenland. The fossil record shows evidence of crocodiles having lived there.

During this time, the supercontinents’ slow, inexorable break-up continued. India was still a mid-ocean island, and the Arabian Plate remained attached to northeastern Africa. Eurasia and North America were all relatively fragmented.

The mammals literally exploded in number and diversity into the gap left by the dinosaurs. Although the fossil record is sparse, in just 10 million years, some 4,000 mammal species evolved across 130 genera. These included bats, rodents, whales, the first primates, and creatures such as the predatory Arctocyon or “bear dog” shown here.

A hot, hot world

This was the Eocene Epoch, stretching from 56–to 33.9 million years ago. In the early Eocene, 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 90° F (30° C), the scene was set for the arrival in the northern hemisphere of the odd-toed herbivores, the evolutionary line from which the horses, tapirs, and rhinos of today are all descended.

One of the earliest of the perissodactyls (from which all horses, tapirs and rhinos are descended) was Hydrachyus eximus. Image: Wikimedia Commons

The earliest of the odd-toed grass and browse feeding creatures that we know of was Hydrachyus eximus. It entered a world where ice had retreated completely and hot, steamy jungles, complete with crocodile-infested waterways, extended well into the polar regions. Hydrachyus was nothing like the great beasts we know as rhinos. It was only about 5 ft (1.5 m) long and had no horn. Perhaps, with some imagination, there was something of a horse or tapir in its bearing. Nevertheless, genetics and morphology show that this modest mammal was indeed a common ancestor of all rhinos.

Enter the rhinos

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.

By the mid-Eocene, the planet, though still much hotter than it is today, had been cooling steadily, and the break-up of the supercontinents of Gondawa and Laurasia had progressed to the point that the shape, size, and position of the continents had begun to look more familiar. Mammals were thriving and diversifying to take advantage of every Eocene niche available.

The hornless Amynodontidae, or “swimming rhinos.” Image: Wikemedia Commons

The rhino line was no exception and by then had diverged from its common ancestral line with tapirs. Three rhino families emerged. The Hyracodontidae, the “running rhinos,” hornless and adapted for speed across open landscapes, appeared first. The Amynodontidae, or “swimming rhinos” (seen here), followed. They were also hornless, some tapir-like and others more like hippos, and adapted for semi-aquatic life. Then came the Rhinocerotidae, the ancestral line that includes today’s five species, and the horn finally became a defining, rhino characteristic, but not entirely so.

The hornless giant

Paraceratherium was quite possibly the biggest terrestrial mammal ever to have existed. Image: Shutterstock.

About 34 million years ago, the Circumpolar-Antarctic current formed, and global cooling accelerated, affecting the flow of other ocean currents. The Antarctic ice sheets expanded rapidly, and the seasons returned. Rainfall lessened, and the jungles retreated while grasslands spread. Powerful running creatures evolved to fill the open vistas—as did the predators to catch them.

The world became a relatively temperate place with patterns of seasonal rainfall returning. This continued for about 10 million years and many new mammals evolved. Amongst them were the “bone-crushing dogs,” their powerful jaws and teeth well-adapted for scavenging the carcasses of large creatures. Early elephants, bats, rodents and marine mammals also proliferated. Rhinos also did well, except for the swimming or swamp rhinos that were in decline. However, the surviving rhinos grew in abundance, rapidly diversifying and spreading from Asia into Europe. Thanks to low sea levels and a resultant land bridge, they also crossed from Asia into North America.

Initially, these rhinos were 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 ever to have existed (a female and calf are shown here). While estimates of its actual size have probably been exaggerated, at some 23 ft (7 m) tall and weighing 16.5–22 tons (15–20 tonnes), it was taller than a giraffe and three times heavier than an African elephant.

The golden age of rhinos

Around 23 million years ago, the planet’s climate remained temperate and the continents continued to assemble into the positions, shapes, and sizes that are familiar to this day.

Mountain building on a grand scale was happening, changing global rainfall patterns, and causing forests to shrink even further. Great grasslands and other open landscapes such as deserts and tundra continued spreading. A period of sharp global warming took hold. Ocean and atmospheric temperatures soared to 8–10° F (4–5° C) higher than we experience now.

Chilotheridium skeleton the tusk-like forward-pointing lower incisors, a characteristic that persists in the Asian rhinos of today. Image: Hong Kong Science Museum/ GNU Free Documentation Licence

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 from Asia with genera such as Brachypotherium and Chilotheridium. These short, stocky grazers were hornless, but they had tusk-like forward-pointing lower incisors, a characteristic that persists in the Asian rhinos of today. However, these early “African” rhinos were not 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 in Miocene times.

The decline begins

Rapid cooling returned around 14 million years ago, probably triggered by ocean circulation changes, and many creatures were unable to cope.

Teleoceros, an Arctic rhino of the late Miocene (11.6 million to 5.3 million years ago). Image: Roman Uchytel/Prehistoric Fauna

Rhinos were amongst those that found the transition difficult, and the majority (including Teleoceros shown here) did not make it through the latter part of the Miocene. The most archaic of today’s rhinos—the Sumatran Rhino—possibly evolved during this period. 

Then, 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. The stage was finally set for the horse and rhino lines that persist to the present to migrate southwards into Africa.

Although the precise lineage of the Black and White Rhinos of today remains unresolved, however, they seem to have been present in Africa from about 10 million years ago.

The skull of Ceratotherium neumayri possibly the direct ancestor of the White and Black Rhinos. Image: Wikimedia Commons

One view is that their common ancestor was Ceratotherium neumayri (shown here) 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. 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  was the earliest African species and that it diversified into C. mauritanicumin North Africa, C. germanoafricanum in East Africa and C. simum, the extant White Rhino.

The Ice Age cometh

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 (shown here) and the woolly rhino.

Elasmotherium herd. Could this giant rhinoceros have been the source of the unicorn legend? Image: Shutterstock

By now, the planet was fully in the grip of an Ice Age 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 early humans, and many species succumbed. The rhinos were now in fairly rapid decline. Only eight genera made it through to about two million years ago.

Archeological hunting site on th island of Luzon in the Philippines showing rhino bones dating back 700,000 years. Image: University of Wollongong, Australia

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 extensive woolly rhino genus (related to the Sumatran Rhino of today) all developed long, shaggy coats well suited to the cold, dry steppes. Also, their nasal bones were separated into two parts, making it easier to warm icy air.

The other species adapted to the cooling climate was Elasmotherium. Could this giant rhinoceros have been the source of the unicorn legend? After all, it had a single, massive horn, and its long, running legs gave it a horse-like demeanor? It stood a majestic 6 ft 6 in tall (2 m), was about 16 ft 3 in long (5 m), and could well have weighed nearly 5.5 tons (5 tonnes). However, Elasmotherium’s days were numbered. Once thought to have died out 200,000 years ago, more recent indications suggest that it survived through to about 39,000 years ago. It seems that the species succumbed to a sudden catastrophic event—it was a specialist grazer, but increased snowfall would have covered its food source, forcing it to browse trees and shrubs to which it was ill-adapted. Examination of fossil teeth corroborates this assumption. Because these creatures survived into the time of modern man, probably hunting also played a role in their demise. We know this from archeological sites as distant from each other as Boxgrove in the UK and the island of Luzon in the Philippines. Both show evidence of rhinos, amongst many other mammals, having been expertly butchered by our human forebears, probably Homo heidelbergensis and Homo erectus respectively.

A world reshaped

About 12,000 years ago, the human population was no more than four million worldwide. Quite probably there were more rhinos on earth than people. Extraordinary to think there was a time when the entire global count of people could have lived in an area no greater that the New York boroughs of Manhattan and Queens. It was a seminal time for us and the 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.

Some 10,000 years later, in 1750 which is generally accepted as the start of the Industrial Revolution, our numbers had grown to just over 800 million. Since then our global population has climbed almost 10 fold and we have redesigned the face of our planet. So much so in the most recent 60–70 years that we have coined a new name for it in geological time—the Anthropocene Epoch

Our accomplishments have been extraordinary. But, it must be said, they have been realized at the expense of the world’s natural landscapes and the creatures that live in them, including ourselves.

Rhinos have certainly not done well on our watch. The woolly rhino became extinct around 10,000 years ago, a victim of climate change and possibly human hunting, and all rhinos disappeared from Europe. But they remained abundant across the more temperate parts of Asia and in Africa.

Reason for hope. On March 24,2022, the Government of Indonesia announced the birth of a female Sumatran rhino at the Sumatran Rhino Sanctuary. Fewer than 80 remain in tiny, remote, and fragmented populations deep in the Indonesian rainforest. Image: International Rhino Foundation.

We have pushed them aside for our farms, mines and towns, and we have hunted them relentlessly for sport and the perceived therapeutic and status value of their horns. Over the past 200 years, we have reduced them to mere remnants of the millions that once roamed the forests and plains of Africa and Asia. Still, there are some reasons for encouragement in the sea of concern. The White Rhino population stands at some 15,500, down from a high of about 21,000 around 2012 because of poaching, but saved from dire straits in the early 1900s. Black Rhinos hit a low point in 1995 when their numbers crashed to a mere 2,410, also as a result of poaching. Today the population has recovered to some 5,600. The Reason to hopeIndian Rhino also stood at a few hundred at the turn of the 19th century, but now stand at more than 3,700 in India and Nepal. At fewer than 80 individuals each, the Javan and Sumatran Rhinos teeter at the brink. Nevertheless, conservationists and NGOs are doing all in their power to save them.

Clearly, there is still much to be done to secure a future for rhinos in the wild, particularly regarding the practicalities and cost of protecting them, and in curbing the greed and senseless demand for their horns in Asia.