Hey.

We've been together for a long time now.

A year, as a matter of fact!

But if it feels like we've known each other for slightly longer than one year, it's probably because, over the course of the first season of PBS Eons, we've explored the history of Earth from the very origins of life right up to the Cenozoic Era which is what we're in right now.

And along the way, you've asked some really excellent questions.

We've tried to address some of them in the comments.

But some of the most common questions we've gotten turned out to have some interesting and complicated answers.

Others ... were just about why we pronounce things the way we do.

Either way, we'd like to celebrate our first anniversary together by answering some of your most frequently asked questions.

One of our most f a'd q's came from our episode on the Great Snake Debate, and the conflicting theories about snake evolution.

In the episode, we noted that, somewhere around 200 million years ago, snakes diverged from lizards.

But were they aquatic or terrestrial?

And both fossil anatomy and modern genetic evidence seem to add support to both theories.

So many of you asked: Why not both?

Well, all snakes need a common ancestor.

If snakes evolved from two different lineages of lizard, and both became "snake-like," that would be an example of convergent evolution.

And this has happened!

It's why there are such things as legless lizards today, as well as true snakes.

Both animals converged on the same body plan that helped them survive and reproduce.

But it's unlikely that some lineages would come from marine reptiles, and others would come from terrestrial reptiles, and then they all wound up being snakes.

That's not how evolution works.. usually.

There is a rare phenomenon known as despeciation or reverse speciation, in which two lineages converge and form a hybrid.

But this has only been observed in modern species like the common raven and stickleback fish.

Teasing this out of the sparse fossil record of snakes would be a challenge...if not impossible.

However I should mention that our understanding of the evolutionary history of snakes, as well as so many things, is still a work in progress.

Now, let's move on to our episode about Spinosaurus, which we described as the only known semi-aquatic dinosaur.

To which many of you responded: What about Halszkaraptor?

Well, it was only in November of 2017 that, Halszkaraptor, a new Dromaeosaurid dinosaur, was described from the Late Cretaceous of Mongolia.

After confirming the specimen was genuine - - it has a shady past -- the researchers found it to be semiaquatic.

The specimen was collected by poachers, smuggled through China and into Europe.

Luckily it was rescued by a paleontologist at the Royal Belgian Institute of Natural Sciences and will be returned to Mongolia once researchers are finished with it.

We probably should have mentioned Halszkaraptor in the episode, but we didn't due to the newness of the specimen.

There hasn't been much time for the scientific community to review the conclusions put forth in the report, and as you'll learn in a little bit, there's a LOT about Spinosaurus that people are still arguing about.

But if you'd like to check out the manuscript and do your own peer review, it can be found in the description.

And of course, I'd be remiss if I didn't mention a question that has come up time and again in the past year, especially in our episode on Triassic animals: Nitch or neesh?

Well, we consulted the online Oxford english dictionary to see how to pronounce this word, and it lists both nitch and neesh as acceptable pronunciations.

So there I'd like to answer a question that's related to Kallie's.

Not the niche thing, the other one.

Was Spinosaurus bipedal or quadrupedal?

Turns out there's more than one controversy about this animal.

Not only is there some debate about whether Spinosaurus was the only semi-aquatic dinosaur, there's also not a real consensus about how many legs this thing walked on, and when.

It may have walked on two legs, or four, or switched back and forth, depending on whether it was on land or in water.

As with so many things in this field, it depends on who you ask.

For example, if you're the person who first found and named Spinosaurus -- that would be Ernst Freiherr Stromer von Reichenbach - - then you'd say it walked on two legs.

Because, one thing that everyone pretty much agreed on was that Spinosaurus was a theropod, and that's one of a theropod's defining features.

So, in his reconstruction of Spinosaurus, he drew it on its hind legs.

But, if you're the person who found the most recent and complete fossils of Spinosaurus, then you'd say otherwise.

Nizar Ibrahim revolutionized our thinking about Spinosaurus when he found fossils of it from Morocco in 2008.

And in 2014, he and his colleagues published a paper suggesting that spinosaurus was an obligate quadruped, meaning that it had to walk on all fours, at least on land.

They based this on the size of its hindlimbs, the location of its center of mass, and the orientation of its hips.

But this model has gotten a lukewarm reception from other researchers.

Some have taken issue with the measurements used in the study.

Others pointed out that even Ibrahim's exquisite specimen was still incomplete, including the legs.

OH but, if you're Jurassic Park, then Spinosaurus is definitely a biped.

But that's mainly because Jurassic Park III came out before Ibrahim's paper.

And anyway, we try not to use Jurassic Park movies a whole lot in our research Another common question came from our episode on the Cenozoic era, the era that leads right up to today.

And that question was basically variations on: "Is the Anthropocene a thing?"

The answer here is a very clear: Not yet.

To give you some background here, back in 2000, two scientists -- atmospheric chemist Paul Crutzen and aquatic biologist Eugene Stormer - - proposed that the current epoch of geologic time that we're now in, be given a new name, the Anthropocene.

They argued that humans have had such a dramatic impact on the planet that -- just as with any other game-changing event -- that impact should mark the beginning of its own epoch.

And yes I pronounce it "epoch."

You can say it in your head however you want.

Now, even though this proposal is nearly 20 years old, it's still not an official part of the Geologic Time Scale.

But it has stirred a really interesting debate, not just about what our impacts are on the planet, but how we can actually measure them.

For instance, if we're gonna create a new epoch, then which of our many marks on the land would signal its beginning?

Crutzen and Stormer first proposed that the Anthropocene should begin in the late 1700s.

That's when ice cores begin to show a rapid rise in concentrations of CO2, as the burning of fossil fuels first became widespread during the Industrial Revolution.

But others have suggested that more visible boundaries might be useful for future geologists.

Like, some have proposed that concrete and cement be the markers, because they started to come into widespread use in the late 1800s.

Others have suggested plastics, which have been filling our landfills since the early 20th century.

And one paper, which Crutzen co-wrote, proposed that the Anthropocene should begin exactly on July 16th 1945, when the first nuclear bomb was detonated in New Mexico.

Because decades of nuclear testing have left an unmistakable concentration of isotopes like plutonium-239.

Now, some experts have argued that the whole thing is a bad idea.

They point out that the concept itself is kind of human-centric instead of geology-centric, and that human impacts have escalated at different times in different parts of the world.

In any case, the fate of the Anthropocene Epoch now rests with the official keepers of the Geologic Time Scale, a group known as the International Commission on Stratigraphy.

The proposal was submitted in 2016, and since then the commission has been ... thinking about it.

As you can imagine, decisions about geologic time do not happen quickly.

Ok. And here's another question that I personally would like to address.

It's this one: Now, that's not really a question.

It's more of an equation.

But for what it's worth, Merriam Webster lists the pronunciation of that word as either "nitch," or "neesh".

Finally, one of our most frequently asked questions came from our episode about the Mesozoic Era, the so-called Age of Reptiles.

And since the Mesozoic was the heyday of the dinosaurs, a lot of you asked: Are dinosaurs reptiles?

And does that mean that birds are also reptiles?

And the answer is: yes and no.

But also, yes and yes.

It depends on what system you use to classify organisms.

The system that's been around the longest is the Linnaean system, named after Swedish biologist Carl Linnaeus.

It classifies living things based on their physical characteristics, and then ranks them based on how similar they are, from the general to the specific -- from domain to species.

And in this system, the class Reptilia includes air-breathing animals with backbones and scales.

To be a reptile, you had to have those things.

And birds do!

But the thing is, technically, the class Reptilia does NOT include birds.

Instead, they're part of another class called Aves.

This is partly because Linnaeus defined the classes of modern animals in the 1700's, long before we understood the evolutionary relationships of things.

But still, dinosaurs were closely related to -- and they shared traits with -- animals that are un-arguably reptiles, like crocodiles.

So, according to Linnaean Taxonomy, dinosaurs are in class Reptilia; they're reptiles.

But birds aren't, mainly for historical reasons.

Now, many biologists and paleontologists today want to name things in a way that reflects their shared evolutionary history.

This system classifies organisms into clades, and it's called cladistics.

and in this system, since dinosaurs derived directly from reptiles and share a close evolutionary relationship with them, they're still considered reptiles, because they belong to the reptile clade.

And, so are birds!

Because birds descended from dinosaurs, so they belong to the dinosaur clade, which itself belongs to the reptile clade.

This makes birds both dinosaurs AND reptiles.

Which is weird, I know.

But a lot of science comes down to arguing about things -- like, whether Halszkaraptor was aquatic, or whether we should name a new epoch after ourselves, or who belongs in what clade.

The things to remember is that there can be many different, conflicting, proposed answers to a single question.

And by arguing with each other over the facts, we just might sort some of them out.

Oh, and when it comes to how you pronounce this word?

I pronounce it: "jif."