How Evolution Turned A Possum Into A Wolf

You guys remember Vanessa, from Braincraft, right?

Tell these people about the animal you just told me about.

V: Right, so I've been fascinated by the Tasmanian Wolf, AKA the Tasmanian Tiger, Thylacine, because it's like a dog with a pouch.

Dogs don't have pouches.

V: Dog with a pouch!

Ok, we have to get to the bottom of this.

Meet the thylacine, also known as a Tasmanian Wolf, or Tasmanian Tiger, a medium-sized meat-eating mammal that as recently as the early 1900s lived on the island of Tasmania.

Unfortunately, local farmers liked their sheep more than the thylacine, and eventually they were hunted to extinction.

The last known thylacine died at a Tasmanian Zoo in 1936.

But that isn't what makes the thylacine so interesting.

J: So if I was on the island of Tasmania and I walked up to one of these things, I would think it was a wolf or a dog or something.

RV: It's actually a marsupial.

So are kangaroos, koala bears, wombats, possums, things like that.

J: So this is more closely related to a kangaroo or a possum than it is to the wolf that it looks like in its anatomy, shape and... RV: Yes Fossils and DNA tell us that mammals like wolves and mammals with pouches last shared a common ancestor about 160 million years ago.

Another pair with a common ancestor that far back?

A hummingbird and a Velociraptor!

The thylacine is about as distantly related from a wolf or a fox as it can be and still be a mammal.

But... just look at them!

The posture.

The long legs, built for running.

Their skulls built for sniffing, and with massive muscle attachments for biting.

Sure, they have pouches, but they look so similar to a wolf!

Way more similar than a hummingbird and velociraptor.

So how is it that two animals, this distantly related, could come to share so many similarities?

RV: There are two biological explanations for similarity.

Two animals can be similar because they shared a recent common ancestor.

Wolves and coyotes are similar because they share a recent common ancestor.

Humans and chimpanzees, it's not accidental that we look very similar because we have a relatively recent common ancestor.

RV: But there's another explanation for similarity, and that's called convergent evolution, which means two animals their descendants have evolved to be similar to one another though the action of natural selection.

Me, a penguin and an ant walk to a bar, because we all have legs.

If we go back to the common ancestor between me and the penguin, it already had legs, and they were built a lot like mine.

We say a similarity like this is homologous, because the trait was inherited from a common ancestor.

But the common ancestor between me and an ant was so long ago, it didn't have legs.

Our ancestors and ant ancestors evolved legs independently.

We call these similarities analogous.

Analogous similarities are the result of convergent evolution, and one of the most striking examples is in how the wolf and thylacine walk.

Think of your own foot, we're plantigrade animals, so plantigrade animals put their whole foot flat on the ground.

The ancient ancestors of the tasmanian wolf were possums like our neighborhood possums and they're plantigrade animals like we are, they put their whole foot flat on the ground.When a dog stands, or a wolf stands, their heel is up in the air.

So here's the hip joint here's the knee joint, there's the heel joint.

So what they're standing on are their toes.

The tasmanian wolf keeps his heel off the ground, and it's walking on its toes like a dog.

GS: Despite coming from different ancestors, the wolf and thylacine independently evolved this analogous trait: walking on their toes.

J: I'm never gonna look at my dogs the same way again, that they're tiptoeing around the house all the time!

GS: Another example: Take a bird's wing and a bat's wing.

Homologous or analogous?

This one's a little tougher.

J: So let's see if I can guess what this is: This is a bat, right?

RV: Yes, ha, this is a bat.

This is one of the larger species of bats, this is a fruit bat.

J: Ha, it also says "fruit bat' right there!

I was cheating.

Haha.

RV: Haha, yes.

The common ancestor of birds and bats already had a forelimb, so "having an arm-like thing" is a homologous trait.

But using that arm-like thing as a wing evolved independently in birds and bats, so "flying with arm-like things" is an analogous trait.

Birds flap their whole arm, but bats fly in a totally different way... RV: The scientific name for the group that bats belong to is "Chiroptera" which means hand-wing.

So its wing is formed by its hand.

J: I recognize this, this is like the X-ray of your arm.

One bone, two bones.

Does that mean that's a thumb?

RV: This is a thumb!

So, similarity inherited from a common ancestor?

Homologous.

Similarity not from a common ancestor?

Analogous.

Let's run down some examples: The human inner ear and a Dimetrodon's jaw joint?

Homologous.

Our distant shared ancestor had the same structure.

Platypus and duck bills?

Analogous, their shared ancestor didn't have it.

A bird's feather and a reptile's scales?

Different today, but both are inherited from a common ancestor, so they're homologous.

And the wolf-like body of the thylacine?

Now we know the answer: It's analogous.

It formed independently, by convergent evolution.

The key to convergent evolution is a similar environment.

Conditions like the climate, competition, and what food's around determine which traits are favored more than others.

In the open ocean, a streamlined body and powerful swimming tail is proven to work, and we see it in all kinds of animals.

Underground, a legless body seems to work pretty well.

For medium-sized carnivorous mammals that have to run after their prey, a doglike body works really well, so we see it again and again and again.

And these are all just examples of similar body shapes and anatomy, but behaviors can be convergent, too--just ask a bat and a dolphin.

If you speak bat or dolphin.

None of these are "best".

There's no such thing as "perfect" traits.

Having a streamlined body is not the only way to be successful in the ocean--just ask an octopus.

And having a dog-like body is not the only way to be a successful hunter--just ask a bear.

But even though evolution doesn't march toward some ideal form, I've gotta say: thylacines are particularly awesome.

They show us an important lesson: Not all similarities are the same.

Stay curious.