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November 6, 2013

From Land to Water: The Evidence That Whales Were Once Terrestrial

 Hey Folks - 
We have a fantastic volunteer, Seth Commichaux, who is a great writer and has been diving into some interesting topics related to environmental education.  We feel that his posts are thought provoking and well researched.  We do, however, want to note that the Green Fork Utah blog does not necessarily reflect the views of USEE.  It is a venue to prompt discussion and critical thinking and the articles are created and posted by staff, interns and volunteers.  Please check out the blog below and tell us what you think in the comments!  We'd love to have an open and respectful discussion.
The USEE Staff

You might already know that whales are mammals and you might already know that scientists currently believe whales evolved from an ancestor mammal who lived on land, but then evolved to life in water again.  But what evidence is this based upon?  When I did some research I found some interesting things that hopefully you'll find interesting too.

Whales come in many shapes and sizes.  Blue whales are the largest animals in the world some being as long as a 100 feet and weighing over 375,000 lbs.!  On land bones would break, the lungs would collapse, and the heart would fail, being unable to support that much strain, but in water where buoyancy counteracts the effects of gravity size seems to be an asset.  Many whales roam the blue planet over covering many thousands of miles every year in search of food and mates (maybe they're just travel junkies?).  They are also known for their complex songs and their long-range, inter-person communication.  They also seem to enjoy playing and making a big splash whenever they get attention!  (maybe I'm exaggerating; judge for yourself)


At first glance whales might seem to be fish because they have fins and a fusiform shape, but wait....................whales have to come up to the surface to breathe; they don't have gills like fish, they have lungs like mammals.  Looking closer one discovers that whales don't lay eggs like fish do either.  Instead, embryos get implanted in the uterus, a placenta forms to receive nutrients from the mother, and when developed the mother gives birth to live young.  The young drink milk from the mother's mammary glands until they are old enough to eat food. In 1859 when ever-observant Charles Darwin looked at these facts he proposed that whales must be mammals who left the land to swim in the sea.  The reason why whales look like fish is because the ocean has had similar selection pressures for both groups of animals and has thus led to similar adaptations.  A fusiform body more efficiently moves through water and fins are better for moving yourself about in water than are hands or paws or hooves.  Biologists would say that the similarity in appearance of whales and fish is an example of convergent evolution meaning that though fish and whales have evolved to look alike, they do not share a common ancestor who passed down those traits.  Rather the water environment had selection pressures that led to a similar body plan.

Other clues about whales mammalian inheritance can be found in their bones.  Whales have hand bones in their fins that aren't like fish fin bones, they're like our hands.  Human hands; land animal hands.

And though greatly diminished they still have the vestiges of a pelvic girdle (hip bone) and hind limbs.
As people began finding older fossils of whale ancestors a surprising similarity in the astragalus bone (an ankle bone) was found that connected whales' ancestry with artiodactyls, or even-hooved animals like camels, deer, pigs, cows, sheep, etc.  (Interestingly, whales also have a multi-chambered stomach which would connect them to herbivorous, hooved mammals as well, even though whales eat fish and krill)  In the figure below Pakicetus is the whale ancestor fossil astragalus bone.  Notice how it is vertically oriented like the pig and deer (artiodactyls) as opposed to the squashed sideways astragalus of the dog (representative of most other mammals).
With a growing collection of ancestor fossils the evolutionary path that led to modern whales can be recreated.  Look at the chart below that show some of the found, fossil ancestors of whales and notice a few things: the changes in the skull, the shrinking of the legs, the straightening and increasing robustness of the vertebral column to strengthen the swimming motion, the movement of the eyes from facing front to the sides of the head, the loss of teeth (in some species), the lowering of the body to the ground.  And though it isn't that visible in this chart the nares or nose holes move from the edge of the snout to the top of the head so that the whales don't have to lift their heads out of the water to breathe.


Of course all of these changes didn't happen over night, it took millions of years of evolution and thousands of generations of changing environments as well as trial and error in body plans, in the arena of life, to get to what we know and call whales today.  More than likely the transition from land to water was gradual and involved many intermediate stages where the ancestors adopted an ever more aquatic existence.  Maybe they lived on land and only went into the water to snag a fish every now and then.  Later they spent most of their time in shallow waters to catch fish and to avoid predators until finally they left land altogether.  This is just me speculating, but for reasons as yet unknown this group of mammals left land to live in the oceans.

Indohyus and Pakicetus, two of the oldest known whale ancestors are thought to have been terrestrial carnivores based upon their sharp teeth and robust skulls.  Whales are still hunters though they mostly eat microscopic plankton and small fish nowadays. 

All of this anatomical evidence is nice and it is quite convincing that whales are the descendants of a land mammal that also gave rise to the even-hooved animals, but now with DNA we can compare all living organisms and compare their genomes.  What animal is the closest, living relative of cetaceans (whales and dolphins)?

That's right.  It's hippos!  Hippos are whales' and dolphins' closest living relative and ironically hippos live in water too.............just not all of the time.

One last interesting thing that connects whales to mammals, and separates whales from fishes even further, that I'll mention though there's probably many more bits of evidence is that when fishes swim they move their bodies back and forth horizontally.  They wave their bodies sideways.  Accordingly, their tail fins are vertically oriented to give them propulsion.  When vertebrates came on land they still moved like fish.  In fact snakes and many lizards still move in a sideways wave motion when they move across the land.  In, other reptiles, dinosaurs and, most commonly, in mammals they move their vertebral column up and down in waves so that their legs can swing like pendulums underneath their bodies.  This is more efficient for movement on land.  If you watch the swimming motion of whales you will see that they, similarly, move their bodies in a wave like motion up and down.  They are still mammals, but rather than galloping on land they are galloping in water.  Their tail fins are horizontal to propel them through the water, not vertical like a fish.

If I have any editorial comment to offer as food for thought, I would note how dynamic life is.  Not just how we change and our environment changes from day-to-day, second-to-second, but how life changes over time so drastically that in one eon you're walking on land and in the next you're living in the oceans.  Everything is in a constant state of change.  Life being no exception.  Humans being no exception.  Who knows what we'll all become as time passes.

-Seth Commichaux

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