Why Did Whales Return to the Ocean Around 50 Million Years Ago? — Here’s What Evolutionary Clues Reveal

Around 50 million years ago, if you saw a whale, you probably would not even recognize them. It is because they are not yet the large animals that have come to live in the oceans. Instead, they look more like dogs than the marine mammals they are today. They lived and walked on land, but then something happened.

Over millions of years, some of these land-dwelling species slowly moved back into living in the waters. Then, after thousands of generations, they have turned into whales and dolphins that we see now. But what exactly could be the reasons why these ancestors of whales, who were currently adapted to living on land around 50 million years ago, returned to the ocean?

Did the water habitats offer something land environments could not? To answer these questions, let’s dive deeper into the story that evolutionary clues left behind about these huge, magnificent creatures.

From Small Land Mammals to Early “Walking Whales”

Studies suggest that the evolution of whales began in the Indian subcontinent, where four-legged mammals like Indohyus and Pakicetus lived. Around 50 million years ago, these creatures resembled some modern dogs, as they had four legs, long snouts, and a collection of sharp teeth.

Fossils suggest that these ancient whale relatives lived mainly on land and nearby bodies of water. Moreover, it shows that they already have some transitional features that could link them to present-day whales. Example include their inner ear bones, which match whales more than any other group of mammals.

However, Indohyus and Pakicetus, despite being the earliest whale relatives, are not yet capable of living in the water. It is only after a few more million years that Ambulocetus arrived. An amphibious creature that was able to live in water, but still able to retain its walking capabilities, although probably not as good anymore. Then Kutchicetus existed with more specialized aquatic traits, spending most of its time in the water, but was also still capable of walking on land. These species were often referred to as walking whales.

However, what exactly pushed these early creatures to spend more time in the water, so that they fully developed to be marine mammals?

Read more: Berkeley.edu

Why Moving Back to the Ocean Was an Advantage

Whales evolution

It is believed that animals on land primarily originated from the oceans, but around 50 million years ago, fully terrestrial relatives of whales went back to the waters over the course of millions of years. But why? One of the first possible reasons is food. The early fossils of whale relatives like Indohyus and Pakicetus showed that they may have lived near bodies of water, hunting for fish and other types of food supply, like aquatic plants.

The seas, along with the coastlines, were full of prey. So, early whale relative that were capable of swimming or paddling in the shallow waters could’ve been the species that were able to survive and pass on their genes. This is because over time, those who are better at moving in the waters could have hunted more by reaching grounds that other land mammals were not using, and then they were the ones who may have had more offspring, too.

Additionally, the ocean may have provided early whale relatives with safety from land predators. As they spend more time in the waters, they can escape certain land dangers and spread to more areas. For context, around 50 million years ago, the land environment was getting crowded, following the end of non-avian dinosaurs, new groups of mammals, both herbivores and carnivores, were emerging. Predators were competing to hunt the same food.

But this era was also marked by warm climate and high sea levels, which may have created vast, resource-rich coastal areas. So, being able to survive both on land and in water may have increased the chances of survival of the whale lineage.

How Whale Bodies Changed for Life in the Sea

If you think of whales around 50 million years ago, you may accidentally picture a huge creature walking on land. However, that is not the case. It is only after they transitioned to an amphibious creature that lived mostly on water that their bodies developed to be heavier. It is a result of adaptations for buoyancy control and living underwater, where gravity is less of an issue.

First, fossils showed that the earliest known relative of whales that could already live on water, like Ambulocetus, had long bodies, but also strong enough legs to still move on land. They resemble more of a giant otter or crocodile. Then, species like Protocetus and Dorudon, which spent more time in the sea, now had a more streamlined body with smaller hind legs.

Then over millions of years, their noses moved from the front of their faces, to the back of their heads, to help them easily breathe on the surface, becoming blowholes. Moreover, without the need to walk on land as they have not become fully aquatic creatures, their front legs became flippers, their back legs shrank a lot, and their tails became powerful and developed for swimming. These are changes that did not happen all at once, at a short period of time, but actually opposite.

Read more: Natural History Museum

What Fossils and DNA Reveal About Whale Origins

The Evolution of Whales

The story of whales returning to the seas was derived from fossil evidence and genetic studies. The fossil skeletons found in Pakistan and Egypt show a series of animals from different time periods slowly transitioning from walking on land to being more adapted to water.

From land-dwelling Pakicetus to fully aquatic marine mammals like Dorudon, each new step in the evolutionary lineage of whales adds features that help them be better at swimming and lose parts that are needed for walking. Changes that happened over millions of years.

DNA studies also showed that hippos are the closest living relatives of whales. These creatures were able to retain their abilities to walk on land. A trait that ancestors of whales may have had, too. So, when scientists combine all these findings, it tells a story that whales are mammals that lived on land once upon a time, and then evolved back into the sea roughly 50 million years ago, taking advantage of the resource-rich oceans.

Author's Final Thoughts

Whales did not start as the ocean giants that we have come to know them today. They also did not live all their lineage in the water. There was a time when, just like other terrestrial mammals today, they were living on land, hunting, walking, and surviving. But as some of them began to hunt nearby bodies of water, the sea may have offered both safety and an abundance of food supply.

Over millions of years, natural selection may have favored the animals that were better at swimming, diving deeper and deeper into the coastal seas, and eventually learning how to live fully in these environments. Their bodies changed, their way of living transformed, and much later on, they became the whales we know today.

Read next: Why Did Plants First Invade Land Around 470 Million Years Ago? — Here’s What Paleobotanists Found

References & Further Reading

Thewissen, J. G. M., Cooper, L. N., Clementz, M. T., Bajpai, S., & Tiwari, B. N. (2007). Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature. https://doi.org/10.1038/nature06343

Gingerich, P. D., ul-Haq, M., Zalmout, I. S., Khan, I. H., & Malkani, M. S. (2001). Origin of whales from early artiodactyls: Hands and feet of Eocene Protocetidae from Pakistan. Science. https://doi.org/10.1126/science.1063902

Thewissen, J. G. M., Cooper, L. N., George, J. C., & Bajpai, S. (2009). From land to water: The origin of whales, dolphins, and porpoises. Evolution: Education and Outreach. https://doi.org/10.1007/s12052-009-0135-2

Uhen, M. D. (2010). The Origin(s) of Whales. Annual Review of Earth and Planetary Sciences. https://doi.org/10.1146/annurev-earth-040809-152453