[Daily Trivia] Evidence that whale ancestors were four-legged mammals closely related to hippos [Issue 154]

A giant whale swimming leisurely in the ocean.
If you were told that this overwhelming king of the sea once walked around on land on four legs, what kind of scene would you imagine?
Not looking like a mouse or a wolf, but having blood ties to that sturdy animal we are familiar with today.

Let's trace the incredibly dramatic footprints that evolutionary history has left in fossils and genes.

The deep relationship between whales and hippos revealed by genes

Scientists once believed that the ancestors of whales were primitive, extinct carnivorous animals called "mesonychids." This was a guess based on appearance, as their skeletal structures looked somewhat similar.
In 1999, advances in molecular biology completely overturned that established theory.

Data analyzing transposable elements called "SINEs" (Short Interspersed Nuclear Elements) in the genomes of organisms revealed a new fact.
The data showed that the closest living relative to the whale is not a cow or a horse, but a "hippo."

When the gene sequences were broken down in detail, several DNA sites were found that are shared exclusively by hippos and whales.
They are like relatives that branched off from the same ancestral line long ago.

Hippos living covered in mud on land, and whales migrating across the vast ocean.
Although they look completely different, they strongly carry the blood of the same group known as "artiodactyls."

The reality of the "walking whale" found in Pakistan

What did they look like when they walked on land, and how did they enter the sea?
Fossils sleeping in the strata of Pakistan and India teach us this as a three-dimensional scene.

About 53 million years ago, an animal called "Pakicetus" existed on Earth.
It was about the size of a wolf. It had four sturdy legs, and each toe had a small hoof.
Looking only at its appearance, it is every bit a mammal that runs on land.

When examining its skull closely, a decisive feature found only in modern whales was discovered.
It is the unique skeletal structure of the "ear bone (inner ear)."
Pakicetus already possessed the whale-specific ear parts for hearing underwater while living on land.
Its eyes were positioned toward the top of its head, a configuration suitable for lurking with its eyes above the water surface like a crocodile.

After an unimaginably long series of generations from there, a species called "Ambulocetus" appeared about 47 million years ago.
The meaning of its scientific name is literally "walking whale."

It was about 3 meters long, reaching the size of a mature sea lion, and had well-developed webbing between its toes.
Analyzing their bones reveals interesting movements.
There are traces that they swam by pushing water with the webbing on their hind legs while undulating their backs up and down.
The very beginning of the motion modern whales use to swim by flapping their tail flukes up and down is right here.

Why did they bother to return to the sea?

One question arises.
Why did a mammal that had gone to the trouble of adapting to land life and acquiring four legs feel the need to abandon everything and return to the sea?

A negative theory was often told, suggesting that losers who lost territorial disputes on land escaped to the sea for their lives.
In current research, it is considered to have been a more proactive choice.

The Earth at that time was not that long after the dinosaurs had gone extinct.
In the ocean, there were no longer any terrifying natural enemies like the large plesiosaurs or mosasaurs that once reigned.
The shallows were overflowing with abundant, untouched food such as fish and shellfish.

I interpret it as the ancestors of whales finding an untapped place without competition and voluntarily going to seize those abundant resources.

In fully specializing for life underwater, they made an intense equivalent exchange.
They completely lost the olfactory nervous system that is useless in water, and also discarded the taste genes for sensing sweetness and umami.
Instead, they applied the mechanism of picking up vibrations by placing their jawbones on the ground on land to develop bone-conduction hearing to the extreme, catching sounds underwater with astonishing precision.

The reproductive strategy unique to mammals—giving birth after raising the young to a certain size inside the womb rather than laying eggs—meshed perfectly with the environment of the sea, which had few external enemies.
Over tens of millions of years, they climbed to the absolute pinnacle of the marine ecosystem.

The beautiful coincidence of embracing change

When looking at the evolutionary history of whales, one is amazed at how flexible biological bodies can be.

A small life that once ran on land looking like a wolf sought a place to live by the water, and over generations, it gradually spread its toes, lost its fur, and moved its nostrils to the top of its head.
At the end of that tens of millions of years of gradation, we have that blue, giant, beautiful fish-like form we see today.

The reason we feel a sense of familiarity when we look at whales is that deep within their thick fat, the "memory of four legs" that once trod the earth just like us, still quietly breathes within them.