…a world without whales. It’s too terrible to imagine.Pierce Brosnan
As a marine biologist I am always spellbound watching whales on the open sea. Witnessing their majestic movements, they are the most marine of all creatures, but I never fully appreciated their origins until recently. But prior to their rapid ascent during the last 50 million years (mya), the ocean was filled with monster aquatic reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs plus ammonites. However, after these amazing animals went extinct at the end of the Mesozoic period (65 mya), the oceans were devoid of large aquatic land animals for million of years. Enter the cetaceans.
So imagine an ocean with no whales, no dolphins, and no porpoises. If you could travel through time and swim in the warm Tethys Sea of 50 mya you’d by startled by the early ancestors of whales — the walking whales.
Whales and their relatives, the cetaceans, are related to terrestrial (even-toed) ungulates like the hippopotamus. They are all descendents of semi-aquatic animals that invaded the empty sea in Eocene time’s to prey on rich marine resources. To truly understand the story of cetaceans we need to place their ancestors in the context of continental drift and the formation and fate of the Tethys, and (natural) long-term climate change.
Continental Drift and Climate
During the warm Eocene period global volcanism shifted the continents towards their current positions and caused abrupt shifts in climate. While the continent that would become India rafted towards Asia, it trapped remnants of the Cimmeria continent and created the warm, shallow, island-rich Tethys Sea which persisted for millions of years. It was the perfect setting for a mammals to invade the sea.
Today, the fossils of early cetaceans are found in the continental shelves of the former Tethys, which are exposed in modern day Pakistan, Afghanistan, and in the Himalaya mountains which were created by India’s collision with Asia.
Simulation of India rafting into Cimmeria and the Asian continent and forming the Tethys sea before uplifting the Himalayas.
Ancestral cetaceans, the Archaeoceti, evolved in the Tethys’ swamp-like seas then spread through what would become the Mediterranean and Caribbean and eventually to the Pacific coasts and worldwide oceans. Much of their success was driven by climate.
In the beginning, when the ancestors of whales invaded the sea, it was super warm –the warmest seen in the last 65 million year. But 16 million years later, the planet shifted to an ocean-rich ice age. It was the perfect climatic driver and resulted in three adaptive radiations among the early cetaceans:
- First (49 mya): warm climates help spread the early ancestors of cetaceans into the coastal seas and across the Tethys (47-39 mya);
- Second (33-28 mya): cold climates created a rich, productive ocean where the ancestors of the modern toothed and baleen whales diverged and spread across the planet’s oceans.
In a recent scientific paper, Coombs et al. (2022) identified a final episode:
- Third (18-10 mya): the diversification of toothed whales, particularly sperm whales.
Today, the cetaceans — dolphins and whales — are composed of 89 species and include the blue whale, the largest animals on earth, ever. Here’s a brief illustrated description of their journey to global prominence.
Pakicetus — the first cetacean (49 mya)
Pakicetus was a dog-sized, mostly terrestrial mammal that occasionally hunted fish in the shallow Tethys sea. It had several unique characteristics adapted to a partial aquatic existence: upward looking eyes, thick bones (which assist in floating), and a thickened skull bone to improve underwater hearing.
Ambulocetus — the walking whale (48 mya)
Fossils of Ambulocetus and were first found in Pakistan in 1994 and made headlines as the first “walking whale” due to its combined aquatic and terrestrial features. Individuals were up to 10 feet long and it’s crocodile-like shape included an elongated snout with upward-facing eyes. Studies suggest it was mostly aquatic, using it’s front and hind legs and tail to swim, but occasionally walked on land to drink freshwater and give birth. It was a big change from Pakicetus and one that supported the eventual spread of its descendants (e.g., Remingtonocetus and Kutchicetus) out of the Tethys corridor.
Procetids spread across the Tethys (47-39 mya)
Procetids were a big step towards a more aquatic existence but with their hind legs they occasionally still walked (and gave birth) on land. But over 12 million years multiple adaptations arose to a more-marine existence with some walking on land and others being fully aquatic. Major changes include their eyes shifting to the side for better aquatic vision, their nasal openings moved closer to their eyes, and their ears became more adapted to underwater hearing. These major adaptations were key to the success of these cetaceans.
Geographically, protocetids were the first cetaceans to leave their origins in the seas of the Indian subcontinent and disperse to the shallow subtropical oceans of the world. Fossils are found in Pakistan, SE Asia, the Middle East, Africa, Georgia, and Texas, showing they crossed the Atlantic Ocean to the eastern reaches of the Tethys, the Suwannee Current of the US SW (e.g, Georgiacetus), and Peru on the Pacific coast (Peregocetus pacificus).
Basilosaurids lose their legs (41-34 mya)
The basilosaurids, such as Dourodon, were fully aquatic and distributed throughout the tropical and subtropical seas of the world. Their teeth and fossils indicate they ate fish and looked like modern cetaceans, with an elongated tail and terminal fluke. Some reached lengths of 60 feet. Their nasal opening moved even higher up the snout, closer to the position of the blowhole in modern cetaceans. Their ear structures were similar to modern whales and were acoustically isolated by air-filled sinuses between the ear and the skull which allowed them to hear underwater. With their fully aquatic adaptations, they were a very successful group and reached all the world’s continents, including Antarctica. Some were the ancestors of modern whales, the next two groups.
Modern whales emerge — Mysticetes and Odontocetes (33-28 mya)
The ancestors of toothed and baleen whales diverged as the world’s climate rapidly cooled and opened up new opportunities for basilosaurid diets. Shifting continents 34 mya created large-scale changes in ocean currents and temperatures that coincided with this diversification. Principle among them, the isolation of Antarctica and the openings of the Tasmanian Seaway and the Drake Passage resulted in global cooling and the Antarctic circumpolar current that strongly enriched marine resources.
Baleen whales tap the world’s plankton (36 mya-present)
The evolutionary patterns of modern and fossil species indicate baleen whales (Mysticetes) went through three major phases:
- An early adaptive radiation (36–30 mya);
- A shift towards bulk filter-feeding (30–23 mya); and,
- A climate-driven diversity loss around 3 Ma. (Marx and Fordyce, 2016).
The emergence of crown-shaped teeth 30 mya show an early transition from teeth to baleen, the filter-feeding system inside the mouths of all modern baleen whales. Filter feeding is beneficial and allowed baleen whales to tap huge planktonic energy resources, such as Antarctic krill, which eventually resulted in the large body size of modern species.
Early species were suction feeders, and may have used their serrated teeth to feed on plankton. As the planet cooled, baleen, sheets of fingernail-like teeth hanging from the roofs of their mouths, evolved and baleen whales diversified into many species, including the modern day skimmers (e.g., right whales), bottom feeders (e.g., Gray whales), and the roqual whales, which are lunge feeders (e.g., humpback and blue whales). Their hearing organs became adapted to send and receive long-range sounds, which became the basis for the melodic songs of modern species used for communication.
Toothed whales exploit the deep sea (34 mya-present)
As baleen whales evolved ways to tap into the ocean’s abundant plankton, the ancestors of toothed whales (Odontocetes) developed sonar (echolocation) and became the largest predators on the planet. Echolocation involves emitting a series of clicks at varying frequency using an expansion of the head to send sound waves, bounce them off potential prey or surroundings, and receive the signals with their elongated lower jaw. This key adaptation mad them more efficient and allowed them to dive deeper in search of food which opened up the rich resources of the deep sea (e.g, squid).
Toothed whales diversify (18 mya-present)
The success of these early species eventually gave rise to dolphins and porpoises, sperm whales, killer whales, and beaked whales. Interestingly, early sperm whales, such as Livyatan, hunted other whales with monster teeth.
Status and Conservation
Clearly, cetaceans have a spectacular evolution history of invading the sea and are one of the best examples of gradual evolution. Within 15 million years they went from a terrestrial lifestyle to a fully marine existence and are now the most aquatic and widely distributed of all marine mammals. Similar invasions of the sea by the marine, but coastal, manatees and dugongs (40 mya), the semi-aquatic seals and sea lions (24 mya), and the coastal sea otters (2 mya) and polar bears (130k) occurred but with less success.
Tragically, these magnificent animals, the whales, porpoise, and dolphins, have a dark history of human exploitation and most all are on the UN’s endangered species list. Pre-human global cetacean populations were…well, we don’t know and never truly will. Based on genetics, current populations of the remaining great whales are estimated at >10% of their pre-contact populations sizes in most species. Prior to whaling, Antarctic blue whales were thought to number about 250,000 individuals but were reduced to fewer than 400 animals by 1972 — about 1% of its former populations size (Roman et al., 2014). Researchers estimate that in the 20th century alone, three million whales were killed by the whaling industry (Cressey, 2015).
Without a doubt, these magnificent, intelligent animals with their beautiful songs, amazing sonar capabilities, and role as ecosystem engineers which enhance the productivity of the world’s oceans, deserve our utmost respect and the highest level of protection. Their iconic evolutionary record from land mammals to the walking whales, to the dominant animals in the seas just adds to their amazing story.
References and further reading:
- Coombs, Ellen J. , Ryan N. Felice, Julien Clavel, Travis Park, Rebecca F. Bennion, Morgan Churchill, Jonathan H. Geisler, Brian Beatty and Anjali Goswami. 2022. The tempo of cetacean cranial evolution. Current Biology 32: 1-15.
- Cressey, D. 2015. World’s whaling slaughter tallied. Nature 519: 140-141.
- Gingerich, P. 2012. Evolution of Whales from Land to Sea. Proceedings of the American Philosophical Society. 2012 vol: 156 (3)
- Lambert, O. et al. 2019. An Amphibious Whale from the Middle Eocene of Peru Reveals Early South Pacific Dispersal of Quadrupedal Cetaceans. Current Biology 29, 1–8, https://doi.org/10.1016/j.cub.2019.02.050.
- Lubis, M. Z. 2016. Behavior and echolocation of male Indo-Pacific Bottlenose dolphins. In: Male Info-Pacific Bottlenose Dolphins Captive
- in Indonesia. Chapter: 3, Publisher: Lap Lambert Academic Publishing, Editor: C. Evans.
- Marx F, Fordyce R. 2015. Baleen boom and bust: A synthesis of mysticete phylogeny, diversity and disparity. Royal Society Open Science, 2015 vol: 2 (4)
- Marx F., Hocking D, Park T, Ziegler T, Evans A, Fitzgerald E. 2016. Suction feeding preceded filtering in baleen whale evolution. Memoirs of Museum Victoria vol: 75 pp: 1447-2554.
- Marx, F., O. Lambert, and M.D. Uhen, editors. 2016..Cetacean Paleobiology (TOPA Topics in Paleobiology). Wiley Blackwell. 319 pp.
- Roman et al. 2014 Whales as ecosystem engineers. Front Ecol. Environ. 12(7): 377–385, doi:10.1890/130220.
- Steeman M, Hebsgaard M, Fordyce R, Ho S, Rabosky D, Nielsen R, Rahbek C, Glenner H, Sørensen M, Willerslev E. 2009. Radiation of extant cetaceans driven by restructuring of the oceans. Systematic Biology. vol: 58 (6) pp: 573-585
- Thewissen, J. G. M. 2014. The Walking Whales: from land to sea in eight million years. University of California Press. 245 pp.
- Uhen, M. 2010. The Origin(s) of Whales. Annual Review of Earth and Planetary Sciences, vol: 38 (1) pp: 189-219.