A whale of a puzzle

In the 1970s, Philip D. Gingerich, a palaeontologist from the University of Michigan, Ann Arbor, and his collaborators from the Geological Survey of Pakistan started an exploration in northern Pakistan. The geographical focus of Gingerich's decades-long campaign was the geological strata of the Eocene Epoch (about 34-50 million years ago) of central and southern Pakistan. This region was of geological significance because 50 million years ago, it represented the eastern margin of the vast, ancient Tethys Seaway. The Tethys Sea was a tropical body of salt water that existed from about 250 million years ago to about 50 million years ago. Geographically, the Tethys Sea extended from the region now surrounding the present-day Mediterranean Sea eastward through parts of southern Asia, including what is today West Asia, northern India, and towards the western Pacific region. Over millions of years, tectonic plate movements caused the closure and fragmentation of the Tethys Sea, resulting in smaller bodies of water such as the Mediterranean Sea, Black Sea, and Caspian Sea, and contributing to mountain-building events like the Himalayas. Notably, the sediments laid down during the early and middle Eocene recorded the interface between continental fluvial (riverine) deposits and shallow, coastal marine deposits. These transitional sedimentary environments provided the ideal setting for preserving the remains of mammals evolving from land residency to coastal life, offering the critical temporal and geological window necessary for uncovering the evolutionary bridge of the Cetacea or marine mammals.

In 1979, Gingerich and his team found a fossil in Chorlakki in Khyber Pakhtunkhwa, which they later named Pakicetus (the whale from Pakistan). The discovery of Pakicetus was a breakthrough in palaeontology because it provided the first clear morphological link between land mammals and whales. The first piece recovered was a braincase, the part of the skull enclosing the brain. This contained the characteristic and unique morphology of the whale's inner ear. These specialised ear bones are the defining characteristic of whales, dolphins, and porpoises, thus establishing that this terrestrial animal belonged to the cetacean lineage. The fossil was found in deposits that geological and geochemical clues indicate were once an ancient streambed. This confirmed that the earliest whales inhabited freshwater environments near land. Subsequent excavations in the area yielded additional partial skulls, teeth, and jaws of Pakicetus and its relatives. We now know that the earliest recognised member of the cetacean lineage is Pakicetus.

Pakicetus was a quadrupedal land mammal, measuring between 1 and 2 metres long, often compared to the size of a small dog. Its skeleton shows ankles typical of even-toed ungulates, and four limbs functional for running. However, its inclusion within Cetacea, and as an ancestral whale, is due to the unique morphology of its skull, specifically the features of the inner ear, characteristic of all modern whales. Interestingly, in contrast to its herbivorous ancestor, Pakicetus was carnivorous, as suggested by the shape of its teeth. Although it was highly terrestrial, it inhabited river systems or coastal regions, linking it closely to water sources. This genus establishes the point in evolutionary history where Artiodactyls (placental mammals commonly referred to as even-toed ungulates, including animals such as pigs, hippopotamuses, camels, deer, giraffes, antelopes, sheep, goats, and cattle) formally entered the cetacean lineage, retaining terrestrial locomotion but developing the specialised acoustic and carnivorous features necessary for future marine domination.

SCAN & WATCH

The finding that the earliest recognised member of the whale lineage is Pakicetus did not solve the question of what the common ancestor between Artiodactyls and Cetacea is. This puzzle was solved based on work following the discovery in 1971 of the fossils of what geologist A. Ranga Rao named Indohyus. The Artiodactyl sister group to Cetacea is represented by the extinct family Raoellidae (named after Rao). Indohyus (meaning 'India's pig') provides key details about the morphology and behaviour of the common ancestor of whales and hippos. Indohyus was a small mouse deer- or raccoon-like mammal with small tusks. Phylogenetic reconstructions indicate that this transitional animal was a herbivore, retaining the dentition typical of plant-eating ungulates. Its physical structure reveals an initial, specific adaptation to an aquatic environment that predates the dietary shift to carnivory. Indohyus bones exhibit bone thickening and bone densification. This dense bone structure functions as ballast, reducing buoyancy and helping the animal stay submerged. This strategy is analogous to that of the African water chevrotain, an extant relative, which dives and hides beneath the surface to evade predators. The anatomical evidence suggests that the evolutionary shift back to water began with a behavioural adaptation focused on survival, not primarily on feeding. The common ancestor of cetaceans and Hippopotamidae is reconstructed as having already developed the derived behaviour of spending at least a tenth of its time in water. This finding addresses a historical quandary, demonstrating that the move toward an aquatic environment was initiated by physiological necessity (buoyancy control in shallow water) and behavioural defence. The transition to carnivory, typical of all subsequent cetaceans, occurred after the commitment to a semi-aquatic lifestyle, rather than preceding it.

Rao, who was with the (erstwhile) Oil and Natural Gas Commission (ONGC), made the initial discovery of the fossil material — including teeth and parts of a jawbone — from rocks collected in the Himalayan foothills at Sindkhatudi near Kalakot in erstwhile Jammu & Kashmir. The definitive importance of these fossils, particularly the unique cetacean ear structure, was not recognised until decades later when palaeontologist Hans Thewissen and colleagues examined the collected rock blocks. Rao initially recovered a few teeth and parts of a jawbone, and classified the genus as a type of Artiodactyl and also named the species Indohyus indirae, after Prime Minister Indira Gandhi, then at her peak. Rao's work in the region led to the description of several other fossil mammalian taxa. Due to security issues in the area where he was collecting, Rao moved "truckloads of blocks" of rock from the fossil locality to Dehradun, where he was able to extract only a small number of fossils. Rao passed away in 1999. His most crucial contribution to cetacean evolution was recognised only after his death. His widow, Dr Friedlinde Obergfell, herself a geologist, safeguarded his extensive collection (as the Ranga Rao-Obergfell Trust for Geosciences in Dehradun) and eventually provided access to Thewissen. It was during the processing of these rock blocks from Rao's collection that a technician accidentally broke open a skull, revealing the unique ear bone structure (the involucrum) that definitively linked Indohyus to whales, solving a central evolutionary puzzle.

If the Raoellidae, like the Indohyus indirae, are the closest extinct relatives of whales, what are the closest living relatives? Analysis of phylogeny using bone structure and molecular tools suggests that Hippopotamidae is the closest extant family to Cetacea and that raoellids are the closest extinct group, supporting the view that the aquatic adaptations in hippopotamids and cetaceans are inherited from their common ancestor.

While the shared ancestor (Indohyus) and the earliest whales (Pakicetus) originated in Asia, the lineage that eventually produced modern hippos (Hippopotamidae) followed a different path. The earliest whales rapidly colonised the global oceans, with their fossil record moving out of Asia by the late Middle to Late Eocene. The direct ancestors of hippos are theorised to have entered Africa from Asia around 35 million years ago. This relocation and subsequent evolution in Afro-Arabia and Africa led to the emergence of the first identifiable hippo ancestors (Kenyapotamus 15–9 million years ago) and the later divergence of modern hippos in Africa.

The evolutionary path of Cetacea (whales, dolphins, porpoises, and so on) moved from terrestrial, amphibious forms in Asia to fully aquatic species that rapidly colonised the world's oceans, leading to two distinct suborders, Odontocetes (toothed whales, dolphins and porpoises) and Mysticetes (baleen whales). This happened about 34 Mya.

Perhaps the most fascinating is the evolution of the blue whale is a surprisingly recent phenomenon, starting around 3-5 Mya. This rapid gigantism was driven by significant global cooling and the formation of large ice caps. The climatic shifts led to changes in ocean circulation and the onset of seasonally intensified wind-driven upwelling along continental coasts. This upwelling forced nutrient-rich deep water to the surface, supporting massive blooms of zooplankton and concentrating prey (like krill) into highly predictable super-aggregations. This unique ecological condition created a powerful selective pressure favouring gigantism. Only extremely large whales could perform the high-cost, high-reward lunge feeding necessary to engulf efficiently and profit from these massive, dense patches of food, providing the energy required to sustain their colossal body mass. The physiological adaptation of the skeleton was not a passive outcome of the aquatic environment but an early, intense target of natural selection, facilitating the rapid structural modifications necessary for life in diverse aquatic niches. A variety of studies using the tools of molecular biology show how gene networks were reprogrammed and used to allow for the ability for bones to carry the load of the immense body, deep-sea exploration, etc.

The high cell count and extended lifespan of large whales theoretically should make them highly susceptible to cancer. A chance cancer-causing mutation would be more likely to occur. However, whales and other large, long-lived animals do not exhibit proportionally higher cancer mortality rates. This indicates the evolution of powerful, specialised tumour suppression mechanisms. While some gigantic mammals, such as elephants, resolve this paradox through the redundancy achieved by duplicating key tumour suppressor genes, baleen whales utilise a different genomic strategy. Studies have found a greater number of positively selected tumour suppressor genes. This molecular architecture translates into stronger anti-cancer and anti-ageing protective mechanisms. The ability of cetaceans to achieve gigantism depended on the molecular co-evolution of these systems. Selection pressure favouring massive size (due to ecological efficiency in exploiting dense krill patches) imposed an existential cancer risk. The simultaneous or coupled evolution of enhanced genomic defence systems, which mitigated this risk, was essential for the successful transition to, and maintenance of, gigantic dimensions.

The evolution of whales and other marine mammals such as dolphins and porpoises began over 35 million years ago in what are now the Himalayas, from an ancestral pig. The whale's closest terrestrial relative is the African hippo. This illustrates the wonderful and amazing ways in which nature's complexity emerges in varied ways over time, shaped by geology, climate, the environment and adaptation.

See also:

How fossils are retelling our past