Whale ancestor research: how we know what we know

I watch a lot of whale videos. I have two videos from ben g Thomas on extinct and extant cetaceans downloaded on my phone at all times. Whenever anyone makes a video about archaeocetes (whale ancestors), they undoubtedly will reference Hans Thewissen’s book ‘the walking whales’. He’s the guy who discovered pakicetus, the oldest known whale ancestor. You know, the critter from this.

The book not only presents you with his discoveries about whales in an exciting way and informs you of current threats faced by cetaceans, it also has a vivid account of his travels through Pakistan and India. He discovered Pakicetus in 2001, so yeah this was right after 9/11. I like how he addressed the xenophobia brewing around that time and talked about the experience he had with the locals. Aside from his work as a researcher, I also really like him as a writer, and I like how he approaches new cultures with respect and interest.

Anyway, it’s whale time! So in this post I’m going to tell you about some interesting things about the discoveries of early whale ancestors that I learned about from ‘the walking whales’.

So first of all, before Thewissen, whales were thought to be related to mesonychians, an extinct group of carnivorous mammals. Turns out, theyre  actually related to artiodactyls, which are even toed ungulates.

To clear that up, ungulates are hoofed mammals. Artiodactyles are even toed, which includes horses, hippos and deer. Mesonychians aren’t exactly artiodactyls, they’re separate but still related to them.

So the reason that we found out that early whale ancestors were ungulates is because of their double-trochleated astragalus, a unique structure found in artiodactyls ankle bones which allows even weight distribution despite them lacking bones in their feet. This was not present in mesonychians.

There was a really cool chapter in the book about this, had me on the edge of my seat. It totally threw everything people thought they knew about whale history in the trash, it was crazy.

Mesonychians, you are NOT the father!/ref

You might have heard of manatees and dugongs. These are called sirenians, they’re very cute and special. They have literally not evolved to be able to feel aggression. While im a diehard whale fan, I can appreciate other marine mammals as well. Anyway, Hans found a bunch of sirenian fossils around the same sites as his whale fossils. How could he tell them apart? Well, sirenian fossils are ‘pachyosteosclerotic’. This means their bones are very fat and heavy.

Sirenians are sometimes called ‘sea cows’ because they generally just graze in shallow water. Their bones have to be very heavy to stop them from constantly bobbing up to the surface. If a whale was this heavy, it’d never get anything done. And grazing is just not how they roll.

Next, how do we know that hippos are whales’ closest relatives? Well, we know thanks to Norihiro Okada and his colleagues.

And yes, if you’re wondering, the humour of a guy whos name is ‘nori’ like seaweed who researches aquatic animals was not lost on them.

You might know that the goal of a virus is to hijack a cell and make it produce the virus’s DNA instead of the cell’s DNA (or RNA). However, not every part of the virus’s DNA is harmful. When a virus enters a genome, the non-harmful parts never actually get phased out, so these act as like evolutionary graffiti that lets us know ‘a virus wuz here’.

These are called ‘endogenous retroviruses’, and Nori and his team spotted the same sequences in both whales and hippos, and no other mammals. So that’s how we know!

Next, how do we know if a whale ancestor was a freshwater or marine critter just from its fossilized bones? The answer is oxygen!

Being able to tell if an archaeocete was freshwater or marine is very important. If it was marine, that’d mean it came later in the evolutionary timeline as they adapted more and more to being fully aquatic, because of course they cant just venture out into the open ocean with less adaptations for aquatic life.

You can’t always expect to find a complete fossil, so you gotta work with what you have and just figure shit out.

The person who helped Hans figure shit out was Holly Smith. I need a physics nerd friend too. Who’s gonna be the Holly to my Hans????

Now, im going to be real with you, I am NOT a chemistry fan. I got 47% on my final and I think its boring to study and don’t ask me about that shit w the hexagons. However, when its put into context w my fav topic, whales, then you have my attention.

First, you need to get what an isotope is. Simply, an isotope is atoms of the same element with a different number of neutrons in them. Neutrons are subatomic particles with no charge, hence the name, neutrons. They make different atoms of the same element lighter or heavier.

Stay with me this will get fun in a moment. If ur a nerd like me I mean. Although if you’ve read this far, you probably kinda are.

Right now just keep in mind two guys: oxygen-18 and oxygen-16. Freshwater and saltwater have different ratios of oxygen isotopes. Since oxygen-16 is lighter, it evaporates more easily. So this is what makes up clouds, which later travel to land and form lakes, rivers and your other freshwater bodies. What’s left behind in the ocean is the heavier oxygen-18.

Here's the cool part: when an animal drinks water, the oxygen isotopes get incorporated into their teeth and bones. A machine called an Isotope Ratio Mass Spectrometer (IRMS) is used to figure out if there’s more of oxygen-16 or oxygen-18.

How the IRMS works, now even our boy Hans himself was confused about this. You don’t often find biology people who also really like chemistry/physics. I am not an exception, regardless I will try to explain.

So first a piece of bone sample is taken and turned into a gas heating it in the absence of oxygen (pyrolyzing). Hans was pained to hand over one of his precious fossil finds, but it had to be done.

Next, its pumped up full of electrons in the IRMS, which excites the particles. They bounce around a bunch inside a magnetic field. The path that a heavy isotope takes and a light isotope takes are different. The machine notes this and calculates which isotope is found more.

I can’t get into the minutiae of how the calculations are done and all that shit, but that’s the main idea.

Side note, you can also figure out the place a person is from by isotope analysis because there are different specific ratios of oxygen and strontium and stuff.

Teeth are so special. They’re my favourite part of the body. They work better for isotopic analysis because they preserve better due to the hard enamel. Plus you can tell an animal’s diet, where they’re from, how old they probably were when they died. Teeth are the best.

So if you’re like me, you might have some questions right now. Why oxygen-18 and oxygen-16? My dad and I now (like literally minutes before I typed this) tried to figure this out. At first, he suggested that the isotopes must form through radioactive decay. They lose neutrons over time, hence the different weights. But this can’t be, because these happen to be ‘stable isotopes’, as in they don’t undergo radioactive decay. They just ‘exist in nature’. But the question remains: how did they form? The answer is cooler than you can imagine.

STARS EXPLODING!

When a star forms, lighter elements fuse to form oxygen. This can form both oxygen-18 and oxygen-16. When it eventually goes supernova, the atoms get scattered and that’s how they ended up here, on Earth, in whales and in us!

I love science. Science is awesome. When I first understood the chemosynthetic theory, I though to myself, is this how religious people feel?

Alright, end of post. Got a bit ramble-y there towards the end, but I hope it was fun to read. It was definitely fun to write.