When whales walked: Fossils document the rise of the largest animals on earth

Whales provide some of the best evidence that evolution has occurred, with DNA, development, biogeography, and the fossil record all telling the same story: that whales once walked on land.

Now before you start imaging a humpback whale or a bottlenose dolphin with legs, let me back up and clarify. Whales, as we now know them today, are extremely well-adapted to living in an aquatic environment. So much so, that if you took them out of the water for too long, they would certainly die. But scientists think that they had ancestors that could leave the water, much like seals, and even more ancient ancestors that lived completely on the land.

Whales are, after all, mammals, not fishes. They produce milk for their young and have hair (albeit very little). Since the vast majority of mammals are land dwelling, it stands to reason that if evolution is true, then these denizens of the sea likely descended from mammals that walked on land. In fact, when scientists compare their DNA to that of other animals, we find that they’re genetically most similar to hoofed mammals, such as cows, deer, hippos and llamas (Cetartiodactyla).

So are there any fossils that document this supposed transition from a hoofed mammal to the massive blue whale? As illustrated in the figure below [1], there most certainly are.

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Let’s unpack this illustration. At the bottom is an animal called Indohyus, a small, plant-eating, hoofed mammal found in rocks as early as 56–47.8 million years ago [2]. Interestingly, Indohyus possessed dense ear bones which likely facilitated underwater hearing. Sound does not travel underwater the same way it does through air, so an animal that spends significant amounts of time in the water presumably would adapt to spending time in such an environment.

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But why on earth would a land-dwelling animal be spending so much time in the water? One possibility can be found in the modern day water chevrotain. These hoofed mammals will dive into water to avoid predation. Such a behavior may have been the beginning of facilitating the transition to water.

Next is Pakicetus, another hoofed mammal found in rocks of the same age as Indohyus. While very similar to Indohyus, one key difference is its teeth. Whereas Indohyus, like nearly all hoofed mammals today, had teeth that were well-suited to eat plants, Pakicetus has sharp pointed teeth that are good for eating fishes. Perhaps an animal like Indohyus only forayed into water to avoid predators, whereas Pakicetus likely sought water to be a predator.

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Pakicetus also had a more robust tail than Indohyus, suggesting that it may have used its tail to aid in swimming. Modern whales primarily use their tails for swimming, suggesting the beginnings of a new method of locomotion.

Perhaps Indohyus being related to cows, deer and camels doesn’t seem like too much of a stretch to you, but I anticipate that you might find Pakicetus so dissimilar to these herbivores that you might roll your eyes in disbelief. Well, one of the key features that suggests that Pakicetus is related to this group is a so-called double-pulley astragalus. Basically, this is an ankle bone with an indentation on the bottom, and it is one of the defining characteristics of the group that includes camels, pigs and deer (“artiodactyls”). Indohyus, Pakicetus, and some of the other species discussed below (Artiocetus, Rodhocetus) have this special ankle bone, further supporting the case that these are aquatic or semi-aquatic hoofed mammals.

Astragalus of Pakicetus compared to other hoofed mammals
Astragalus of Pakicetus compared to hoofed mammals

Now whereas Indohyus and Pakicetus are approximately the same age, Ambulocetus, the next transitional form, is found in younger rocks dating to 47.8–40.3 million years ago. Whereas the former two animals clearly were land-dwelling, it should be immediately apparent that Ambulocetus was better adapted for life in the water. Looking at the length of its arms relative to the rest of the body, this creature was probably not the best on land.

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Beyond its limbs, Ambulocetus shows evidence of having a fat pad in its lower jaw. These pockets of fat, believe it or not, are used by modern whales to aid in underwater hearing. This suggests that Ambulocetus had a better capacity for underwater audition than either Indohyus or Pakicetus.

Ambulocetus is found in the same aged rocks as Remingtonocetus, and both display another feature indicative of a transition to an aquatic realm. Both fossils lack the telltale signs of a vomeronasal organ.

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The vomeronasal organ is a sensory structure used to detect chemical signals, similar to your nose, but its found at the roof of the mouse. If you’ve ever seen a cat close its eyes, open its mouth and flare its lips, its using its vomeronasal organ.

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For a lot of animals, chemical signaling is not quite as useful underwater, so the loss of the vomeronasal organ suggests a further commitment to living in the water. Interestingly, modern whales have remnants of vomeronasal organ genes in their genomes, providing further evidence of this transition.

In that same group of rocks that Ambulocetus and Remingtonocetus are found in, we find several additional species that display further commitments to the realm of the sea. Rodhocetus, Artiocetus and Protocetus have some distinctive changes to their noses, having shifted their nostrils backwards from the tip of their snouts to higher on their faces.

This is an important transition towards a more whale-like form, if you consider where a whale’s nose is. If you look at pictures of dolphins or humpback whales, you won’t find any nostrils on their snouts. So where are they? They’re on top of their heads! Blowholes are just nostrils, but by being on their noggins, they likely facilitate breathing when the animals surface. These fossils show the beginning of that transition from forward facing nostrils to nostrils on the top of their head.

Southern right whale blowhole
Southern right whale blowhole

These fossils also have anatomical evidence of the development of tail flukes, a distinctively whale-like horizontal fin that aids in aquatic propulsion. Despite having fully intact limbs, these animals were probably already swimming much like whales do.

Yet another fossil found in these rocks is Georgiacetus, which has the distinctive trait of having a pelvis detached from its spine. In humans and most other land vertebrates, the pelvis is attached to the spine to allow for maximum stability during running and walking. Georgiacetus, however, was likely spending so much time in the water that it’s hind limbs no longer needed to support its body on land.

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Jumping ahead in time, Dorudon and Basilosaurus arrive on the scene some 41.3–38 million years ago, post-dating the previous eight species I’ve described. It isn’t difficult to see that these are the most whale-y looking fossils so far, with perhaps the most obvious new feature being the reduction of the hind limbs into tiny vestiges. After animals like Georgiacetus detached their hips from their spine, their limbs may have simply gotten in the way. Reducing limbs is an excellent way to streamline one’s body, allowing for increased swimming efficiency.

However, if you look closely, you’ll see that these reduced hindlimbs aren’t tiny indistinguishable bones. In fact, they still have a thigh bone (femur), a kneecap (patella), shinbones (tibia, fibula), and toe bones (phalanges). In modern whales, you can see the remnants of a pelvis and occasionally a femur, but nothing as complex as the hindlimbs of Dorudon and Basilosaurus. Interestingly, fetal whales show evidence of this transition, initiating the development of hindlimbs before they disappear entirely.

After the origination of Dorudon and Basilosaurus, the fossil record begins to show evidence of more modern whales. Based on DNA and anatomy, scientists believe that there are two general groups of whales, toothed whales (Odontoceti) and baleen whales (Mysticeti). Toothed whales include dolphins, sperm whales, belugas, porpoises and others, all of which have teeth, whereas baleen whales are characterized by their typically enormous size and having sheets of baleen in place of teeth. Baleen is made of keratin, a type of protein, and it is used like a filter to capture millions of tiny crustaceans for food.

baleen-whale-5

Though baleen whales lack teeth, all of the earliest whale-like fossils described so far had teeth. Indeed, scientists think even some of their more recent predecessors had a mouth full of pearly whites, such as Aetiocetus, which post-dates Basilosaurus and Dorudon at 33.9–28.1 million years old. What’s amazing about Aetiocetus, however, is that it had distinctive features suggesting it had baleen too. Nutrient foramina, blood vessels that nourish the baleen, can be found on the upper palate of Aetiocetus, indicating that this animal had teeth and baleen side by side!

i-800672a4d9865241f89bf3c41d25a603-toothedbaleen

Just a little bit later in the geological record, we begin to find fossils that resemble modern baleen whales. Eomysticetus, dating to 28.1–23.03 million years ago, is still certainly primitive in some respects, such as the lack of bowed mandibles (lower jaw bones) and a stouter skull, but it distinctively lacks teeth, having shifted entirely to baleen. Notably, the genomes of modern whales have genetic remnants of tooth genes, providing further evidence of this transition from toothed ancestors to baleen-bearing descendants.

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Together these fossils provide step-by-step transitions from land-dwelling to aquatic mammals, all of which share features with modern whales. Given that whales are charismatic organisms that generate much interest, keep an eye out for additional fossil discoveries providing evidence of the evolution of these impressive animals.

Questions for Creationists

Where did all of the proto-whales go? Since many of them were aquatic, shouldn’t they have been able to survive Noah’s flood? If the flood jumbled up animals randomly, why do we find these fossils in a sequence that is suggestive of evolutionary transitions? Why do we never find bones of modern whales, such as bottlenose dolphins and blue whales, alongside fossils such as Ambulocetus, Dorudon or Aetiocetus? Is it just a coincidence that the transitions inferred by the fossil record are also supported by genetics and development?

References

1. McGowen, M. R., Gatesy, J., & Wildman, D. E. (2014). Molecular evolution tracks macroevolutionary transitions in Cetacea. Trends in ecology & evolution. 

2. all fossil ranges derived from the paleobiology database

 

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2 thoughts on “When whales walked: Fossils document the rise of the largest animals on earth

  1. Pingback: Whales walked in India and Pakistan before swimming around the world – Evolution For Skeptics

  2. Pingback: DNA suggests whales descended from land mammals – Evolution For Skeptics

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