The Hills Named After Shiva
The Siwalik Hills stretch in a long arc from Pakistan through northern India and into Nepal, running along the base of the Himalayan range. The name comes from Shiva, "Siva" in Sanskrit, and the range was once thought to be the matted hair of the god himself, pressed flat by the weight of the mountains above. What lies inside these hills is stranger than any mythology. The Siwalik formation is a geological sequence of sedimentary rock laid down over roughly 18 million years, from the middle Miocene epoch to the late Pleistocene. Bones, teeth, and skulls fell into ancient riverbeds, got buried in silt, and were compressed into stone. Today, they erode back out of the hillsides across Himachal Pradesh, Punjab, and Uttarakhand. The Geological Survey of India and the Yale-Cambridge expeditions of the 19th and early 20th centuries pulled thousands of specimens from these beds. What they found rewrote the story of life on this part of the planet.
The Herbivores That Dwarfed Everything Alive Today
The largest animal the Siwalik fossils produced is Sivatherium giganteum, a giraffe relative that stood close to three metres at the shoulder and carried two pairs of ossicones, the bony protrusions giraffes have instead of horns. The front pair was short and knobbed. The rear pair was broad, branching, and massive. Sivatherium was not a distant cousin of the modern giraffe; it was a member of the same family, Giraffidae, and it shared the floodplain forests of what is now northern India with animals that would be equally unrecognisable today. Bramatherium perimense, another giraffid from the Siwalik beds, was similarly enormous. Then there were the proboscideans. Stegodon, a relative of elephants and mammoths, had teeth with multiple parallel ridges running across each molar, a structure called lophodont dentition, built for processing tough grass and browse. Several Stegodon species appear in the Siwalik sequence, some of them larger than any living elephant. Alongside them lived Hexaprotodon, a hippopotamus relative whose fossils appear across the Siwalik beds and across the rest of Asia. The landscape these animals occupied was not a dry savanna. Pollen records and sediment analysis from the formation suggest a mosaic of forest, floodplain, and grassland, something closer to a wetter, denser version of the Terai than anything in India today.
The Predators
A landscape full of multi-tonne herbivores supports large carnivores. The Siwalik beds contain Megantereon, a sabertooth felid whose upper canines extended well below its lower jaw and whose skull was built to deliver a single killing bite to the throat of large prey. Megantereon appears in fossil records from Africa to Europe to Asia, and the Siwalik specimens place it firmly in the same time and place as the giant herbivores above. Dinofelis, another large cat with semi-sabre teeth, also appears in the formation. These were not the ancestors of tigers or leopards in any direct line, they belonged to the subfamily Machairodontinae, which went extinct without leaving living descendants. The Siwalik also preserves Pachycrocuta brevirostris, a giant hyena roughly the size of a modern lion, whose bone-crushing teeth could process the carcasses the big cats left behind. The predator guild in these hills, across several million years, was as complex as anything alive in Africa today.
The Primates in the Middle of It All
The Siwalik formation's most consequential fossils for understanding human origins are the hominids. Sivapithecus, discovered in the Siwalik beds and named for the hills themselves, lived roughly 12 to 8 million years ago. Its facial features, a narrow inter-orbital distance, oval eye sockets, a strong subnasal region, align it closely with the orangutan lineage. G.E. Pilgrim of the Geological Survey of India described early specimens in the early 20th century, and subsequent analysis by David Pilbeam of Harvard placed Sivapithecus within the great ape family with reasonable confidence. Ramapithecus, once proposed as a direct human ancestor, was later understood to be a female or juvenile Sivapithecus. These animals were not our direct ancestors either, but they were close relatives of the lineage that would eventually produce humans, and they were living in the same ecosystem as Sivatherium, Stegodon, and Megantereon. The Siwalik sequence captures a window of several million years during which the evolutionary pressures that shaped the primate line were playing out against a backdrop of megafauna that no longer exists anywhere on earth.
What the Bones Add Up To
Paleontologists use the term "faunal turnover" for the process by which one set of species replaces another across geological time. The Siwalik sequence records several of these turnovers. The giant giraffids and the sabertooths disappear from the record. Stegodon gives way to true elephants. The hominid line thins out in Asia and expands in Africa. Climate shifts drove much of this, the progressive drying and cooling of the Asian interior as the Himalayan range rose higher and blocked monsoon moisture from reaching the northwest. The fossils do not just tell us what animals once lived here. They record the mechanism by which a world gets replaced. Each bone eroding out of a Siwalik hillside in Himachal Pradesh is a data point in that replacement, a specific animal, at a specific time, in a specific place, whose existence briefly overlapped with something that would eventually become us.
