WHALE EVOLUTION 1. The Walking Whale of Indo-Pakistan
Travel back some 50 million years on planet
earth. You find yourself in the Indo-Pakistan region of the relatively newly
formed Himalayan mountain belt. Life has adjusted from one extreme to the other;
the non-avian dinosaurs are distant memories preserved within stone and the
earth is getting hotter. But teetering on the edge of a freshwater environment,
the ancestor of all modern whales was taking its first tentative steps back
into the water.
You look around. Dense shrubbery gives way
to a riverbank and resting on the edge of the slow moving water is an animal
the size of a Labrador. It has four legs, an elongated snout with sharp teeth.
It looks like a mix between an otter and a seal. This is Pakicetus. The Walking
whale of Indo-Pakistan: the ancestor of all modern whales.
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Can’t you see the
family resemblance between this animal and a Blue whale? Wait, seriously?!
(Image by Roman Uchytel)
You might exclaim… “A whale with four legs? What a load of crap!”… But even the forefather of Evolution and Natural Selection, Charles Darwin, was ridiculed for the idea when he first proposed it in 1859. In his first edition of the “On the Origin of the Species”, Darwin tried to explain how something that was once terrestrial (land-bearing) could turn into something like a whale. He used the hypothetical example of the American Black bear (which catches insects with their mouth open, whilst swimming). It wasn’t his best work:
"I
can see no difficulty in a race of bears being rendered, by natural selection,
more aquatic in their structure and habits, with larger and larger mouths, till
a creature was produced as monstrous as a whale,"
he speculated.
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You don't win friends with salad ♪ (Image via http://www.thetaxidermystore.com)
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He would completely drop this speculation in the Second edition. It wasn’t until the Sixth that he would tackle the issue once more.
But why on earth would an animal like Pakicetus move back into the water? The
answer is a lot simpler than you might expect… it has something to do with food… LOTS OF FOOD. And it stems
back to the extinction of the dinosaurs.
The undoubted catalyst of the horrific K-Pg extinction event was a meteorite. Imagine a space rock the size of Mt Everest
travelling twenty times the speed of a bullet. It’s unthinkable. It would wipe
out the non-avian dinosaurs some 66.03 million years ago, but it was more than
just the enigmatic “terrible lizards” that would die out. It killed off 75% OF ALL LIFE… Marine
life suffered considerably more than life on land. The giant marine reptiles,
particularly the mosasaurs (which would have occupied the modern orca niche) had
their abundant food source (the ammonites) effectively scoured from the face of
the earth and died in quick succession.
If you weighed more than 25 kilograms and were “warm-blooded” you DID NOT survive this event. It would take the earth more than ten million years to reach similar biodiversity levels.
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| The 75 million year old Tylosaurus, in all its frightful glory. Imagine a monitor lizard with flippers, the size of a modern Humpback whale. Image via Brain Engh. |
For starters, it had a thickened skull bone known as the
auditory bulla, which (in modern whales) is specialized for underwater hearing (the hallmark of
all cetacean anatomy).
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The auditory (tympanic) bulla of a modern Pygmy Right Whale, my
hand for scale.
It had the ability to accumulate fat on the
body, for use in both warmth (blubber) and energy.
It had a mouth filled with sharp teeth,
capable of grasping slippery prey (to feast on the glut of food that had
accumulated in the early Eocene oceans).
But there were plenty of issues:
1. IT BREATHED AIR and would therefore have to hold its breath when under water.
2. It had a tail, but it could not propel itself in the water with it (in this early stage, it used its back-limbs).
3. It gave birth to live young.
4. SO MANY OTHER ISSUES…
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| Shitty photo/image courtesy of yours truly. |
The obligate marine revolution and return
to the water had begun with “Pakicetus”. It was an animal that was not well
adapted to a marine setting, but in a very short amount of evolutionary time,
approximately 7-8 million years, this primitive walking whale would evolve into
one of the biggest predators the world had ever seen; Basilosaurus, an 18 metre long toothed whale that feasted on
smaller whales and even sharks. (Picture by Roman Uchytel).
The next whale in this evolutionary series is Ambulocetus; it looked superficially like Pakicetus but was an ambush predator, living like a modern crocodile. It too is known as a “walking whale” and lived in freshwater ecosystems in the Indo-Pakistan region 48 million years ago...
References
J.G. Thewissen, E.M. Williams
The early radiations of Cetacea
(Mammalia): evolutionary pattern and developmental correlations Annu. Rev.
Ecol. Syst., 33 (2002), pp. 73-90
M.D. Uhen The origin (s) of whales Annu. Rev. Eart. Planet.
Sci., 38 (2010), pp. 189-219
F.G. Marx, M.D. Uhen Climate, critters, and cetaceans: Cenozoic
drivers of the evolution of modern whales Science, 327 (2010),
pp. 993-996
F.G. Marx, O. Lambert, M.D. Uhen
Cetacean Paleobiology John Wiley
& Sons, Chichester (2016)
Dinosaur extinction event
analogy: Peter Brannen's book "The ends of the World" 2017.
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