How We Know Whale Ancestors Walked on Land

One of evolution’s most iconic stories is of a fish that developed the ability to walk on land. However, equally notable is when an aquatic creature walked on land and then returned to the sea. This was the evolutionary path of whales—and one part of the planet is replete with evidence that it happened.

Wadi Al-Hitan, Arabic for Valley of Whales, is part of Egypt’s Faiyum Desert and is a UNESCO World Heritage Site. About 93 miles southwest of Cairo, Wadi Al-Hitan may have an arid climate now, but tens of millions of years ago it was a sea teeming with wildlife, including early whales.

The enormous aquatic mammals we know today looked quite different—most notably, they had legs. Over decades of excavation, Wadi Al-Hitan has yielded extraordinary fossils that show us a whale that dwelled both underwater and on land.

“This is like a hot spot for when whales actually started their journey and totally left the land behind,” says Hesham Sallam, a vertebrate paleontology professor at Mansoura University in Egypt.

Sallam’s first journey to Wadi Al-Hitan was in 2005, when he held a 40 million-year-old whale vertebra. He says the desert’s sands reveal hundreds of exposed skeletons and hide many more under the surface. These whales swam in what is known as the Tethys Sea during the Eocene epoch, which occurred between 56 and 33.9 million years ago. However, Wadi Al-Hitan was only discovered in 1902, and has been excavated since the 1980s. 

The crucial discoveries out of Wadi Al-Hitan shape our understanding of modern whales. One ancestral species, Dorudon atrox, surfaced in the form of a nearly complete skull. About the size of a modern beluga whale—between 11 and 15 feet long—Dorudon was a common, medium-sized whale, according to Sallam.

Another critical find in this valley is Basilosaurus isis (pictured at top), which eventually lost its legs and became fully aquatic. And if the name Basilosaurus sounds more fitting for a dinosaur, that’s because it literally means “king lizard.”

“In the beginning, people thought that might be a type of lizard,” Sallam says.

Basilosaurus and Dorudon depict whales that lived both in the ocean and on land. They “spent most of their time in the water and very little time outside,” says Abdullah Gohar, a member of Sallam’s lab and a vertebrate paleontology doctoral candidate at Oklahoma State University. While their lifestyle may be called amphibious, whales are mammals and always have been. Their fossils remind us just how much they’ve changed.

“The whales that lived 40 million years ago are radically different from the whales we know today,” Gohar says. Most notably, Wadi Al-Hitan’s fossils reveal that the whales of 40 million years ago had hind legs. Gohar describes how Basilosaurus and Dorudon had front flippers with back limbs. But these creatures didn’t use these hind legs for walking. They went “on the land either mainly for reproduction or just relaxing on the shore,” he says, comparing them to modern seals and sea lions.

Today, whales retain vestiges of these hind legs. Gohar describes how if you dig deep into a whale’s flesh, you’ll find a “very tiny bit of bone” that is actually a vestigial hind limb that was once a pelvic bone. Other animals have vestigial femurs and tibia, or thigh and shin bones. “These bones are clear evidence of this evolution of the hind leg in the earliest forms of whales,” Gohar says. And, if you look at the evolutionary development of whale embryos, “you can see the appearance of the legs” before they disappear.

But that’s not the only notable change. Gohar adds that whale nostrils migrated over millions of years of evolution, moving from the front of their face to the top of their head, resulting in the recognizable blowhole. It’s also possible, Sallam says, that whales were covered in hair or fur—but their fossils cannot confirm that.

Sallam and Gohar have also described their discovery of the whale Tutcetus rayanensis, which is a member of the Basilosauridae family. It’s the smallest basilosaurid as well as one of the oldest known fossils of this family from Africa, dated to about 41 million years old.

“Tutcetus from Egypt is the one of the most primitive, fully aquatic whales that totally no longer has a connection with the land,” Sallam says. 

Tutcetus helps illustrate the transition whales made from an amphibious lifestyle to a completely aquatic one—a change that Sallam says researchers have attributed to different hypotheses. One suggests that global warming pushed whales into the sea, while another says that amphibious-living whales preferred to eat greenery at the water’s edge, and as they spent more time in the water they better adapted to swimming.

Though whales have evolved into the charismatic megafauna we know today, Wadi Al-Hitan is brimming with proof that whales had a long, laborious journey to the form they now embody.