Evolution is an incredible story of how life on Earth has grown and changed over billions of years. It’s the idea that every living thing, from the tiniest bug to the largest animal, to the oldest plants, are all connected and have adapted over time to survive. At its core, natural selection is a simple but powerful idea that says creatures best suited to their environment are more likely to survive and pass on their traits.
Over countless generations, these little changes add up and create the amazing diversity of life we see today. You can find proof of this everywhere, from fossils to the DNA we share with other species. So it is safe to say that evolution is the belief that all living organisms are connected and creatures adapt to their environment.
Cetacean on earth
Whales and dolphins belong to the order Cetacea, the marine mammals. Most amazingly, their ancestors once lived on land before they gradually adapted to living in water over millions of years. Fossils found in the present Pakistan and India of those ancient cetaceans are highly useful in understanding their fascinating evolution and the transition from land to sea.
This journey explains the flexibility of cetaceans and the remarkable forms they took to thrive in aquatic habitats, such as streamlined bodies, modified limbs for swimming, and an enhanced respiratory system. All these changes helped them to adapt to oceanic spaces to travel and become a part of an intricate social structure. By observing this we can safely say that nature remains at its best through dramatic changes over long time scales.
Relatives of cetaceans
Today’s cetaceans’ closest living relatives are Artiodactyls which include hippos and cows. Their last common ancestor lived around 55 million to 60 million after that cetaceans became semi-aquatic and artiodactyls stayed on land. Cetaceans took to the sea to look for food and protection.
Pakicetus— the first cetacean
Pakicetus was the first cetacean to start being semi-aquatic around 50 million years ago. Pakicetus was a wolf-like animal, its fossil shows that it was semi-aquatic and it could run on land. At this stage, its diet consisted of fish and small rodents.
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Ambulocetus— the walking whale
About 49 million years ago a cetacean known as Ambulocetus roamed; it was dubbed the “walking whale” because it showed much more aquatic characteristics compared to its previous ancestors. Though it was still land dwelling it showed many characteristics that led it to become fully aquatic, it was about as large as a modern-day lion. It had large paddle-like limbs and a robust body which allowed it to roam on land and swim in water. It also had a vertical tail for propelling itself forward in water. It was a carnivore, its hunting style was similar to a modern-day crocodile, it used to wait in the water and ambush its prey.
Rodhocetus— the biggest transition
About 47 million years ago Rodhocetus became the latest cetacean, it showed a transition towards spending most of its time in water. Its tail evolved vertical fins. It now mainly relied on its tail to swim. A major shift was its nostrils. Its ancestor’s nostrils used to be closer to their snout but now Rodhocetus’s nostrils were closer to the top of the head which would evolve into a blowhole. Its hind limbs were smaller which became a trend in cetacean evolution.
Droudon— the first fully aquatic whale
6 to 7 million years later (40 to 41 million years ago), a cetacean known as Droudon swam in Earth’s oceans. It was one of the first cetaceans fully adapted for marine life. It had the appearance of a small whale. Its hind limbs evolved into flippers, essential for a streamlined body and swimming.
The nostrils had shifted to the top of the head and formed a blowhole to surface for air easily without tilting its head just like modern whales. Some skull features hint at early adaptations for enhanced hearing, which would later develop echolocation – the location of objects by reflected sound, in some cetacean lineages (like dolphins, orcas, and some whales).
Basilosaurus— made for the open ocean
About thirty-five million years ago, an 18-meter cetacean known as Basilosaurus swam. It had an eel-like body so unlike the dorudon the basilosaurus swam in a serpent-like motion. Their diet consisted of fish and other smaller marine mammals. It was the first cetacean whose body was made for the open ocean. They adapted earbones that could sense the water currents. This ability was crucial for sensing prey.
Tooth whales and baleen whales
Finally, some 34 million years ago, basilosaurus split into toothed whales and baleen whales. Toothed whales include dolphins, orcas, and sperm whales. Toothed whales are all carnivores and they mainly hunt in deep dark oceans which is why they all use echolocation. It works when an animal emits clicks or sounds and waits till the sound bounces back to ”see” what is in front of it. Most toothed whales display high levels of intelligence, particularly dolphins who live in pods and coordinate hunting attacks. Toothed whales have been documented to be much smaller than their cousins baleen whales.
Baleen whales are a group which includes almost all types of whales. Baleen whales are filter feeders which is why they have baleen plates to filter large amounts of krill, hence the name. Most baleen whales migrate to maximize food throughout the year. Baleen whales have been documented to be quite large, reaching around a hundred feet and weighing around 190 tons.
River Dolphins
Modern-day cetaceans are still adapting to their environment. Such as both baleen and toothed whales have adapted blubber to keep themselves warm in arctic environments. Cetaceans can also store their oxygen using myoglobin which allows them to dive deep in oceans and it is crucial for sperm whales as they dive to a depth of around 3000 feet. Most whales can slow their heart rate and direct blood to crucial areas only, such as the brain and other crucial organs. This is to conserve oxygen quality which helps baleen whales dive deep.
River dolphins or freshwater dolphins have adopted some very unique features due to their environment, these dolphins have smaller eyes than normal saltwater dolphins, because rivers are quite murky and it is hard to see so this is why river dolphins mostly rely on echolocation. River dolphin’s neck muscles are much more flexible than their ocean counterpart because rivers can be quite narrow at some points so they have flexible neck muscles to maneuver around those narrow points.
References:
- https://www.dolphin-way.com/wp-content
- https://www.thoughtco.com/pakicetus-pakistan-whale-1093256
- https://dinopedia.fandom.com/wiki
- https://eldarzakirov.com/filter/Paleontology/Rodhocetu
- https://newdinosaurs.com/dorudon
- https://www.deviantart.com/the7thsea/art/Basilosaurus-875035699
- http://www.carefordolphins.net/en/index.php
- https://animal-wildlife.blogspot.com/2011/09/river-dolphin.html
Also Read: Adult Fruit Fly Brain Mapped: A Giant Leap to Understand the Human Brain?
Yameen Bin Yahya is an O-Level Biology student and an active member of the school Science Society. He’s passionate about exploring the wonders of zoological life and the universe. His curiosity drives him to dive into the intricate workings of the natural world and the mysteries beyond planet Earth.
