Together in this series we’ve hopped around the metazoan family tree like a… well, a treehopper, and dug at its roots like a skunk.

We’ve seen brainless animals, bone eaters, and what’s basically a parasite turducken which I can never truly unsee. But most of the animals on Earth, particularly invertebrates, are still undescribed and unknown to Western science.

So far in the 21st century, each year professional and amateur zoologists describe roughly 11,000 new animals. But at that rate it’s still going to take us about 600 years just to describe all the animals we think are out there, and many more than that to understand the intimate details of their lives and evolutionary history. It’s going to take a lot of amazing people sharing knowledge and working on really cool topics, like the scientists studying dragonfly evolution, or the biomechanics of things like slingshotting spiders.

We know that the history of zoology shows a lot of white men, but there are many other stories to tell -- both in the past and now as people like you and me get involved in animal science and taking care of the Earth and all its creatures. There are still big, unresolved questions out there about animal diversity and evolution. And animal-inspired tech is showing up in everything, including medicine and construction which means zoology isn’t just teaching us about animals, it’s shaping our lives as humans.

For the last time, I’m Rae Wynn-Grant, and this is Crash Course Zoology! Together we’ve seen some of the truly amazing and bizarre adaptations animals have. Besides selecting for traits that will best help animals thrive in their environments, evolution really has no planned direction.

Which is pretty well demonstrated by a huge variety of invertebrates, or animals without a backbone we’ve seen across 34 out of the 35 phyla in Metazoa. They really deserve a lot more credit than they get. I mean, I still like bears the best, but I’m coming around to paradise spiders, they’ve got great moves.

And many of the big questions zoologists are working on right now have to do with invertebrates. Currently, about 25,000 species of nematodes have been described. But the estimate for how many nematodes there really are keeps climbing as we use increasingly sophisticated models and techniques like DNA barcoding.

Maybe there are 40,000. Maybe even 1 million, which would place them 2nd only to arthropods. Maybe even more!

So rather than lifting up a rock to find a friendly beetle, you might find the most average animal by digging up some soil in your garden for some mostly microscopic worms! 8 to 10 million might also be too low an estimate for the total number of animal species. Despite ruminating on it for a whole episode and some excellent math, we’re still not 100% sure what counts as an animal. We know that an animal is multicellular, eats, sexually reproduces, and moves -- or is at least descended from an animal that could do all that.

But the more we learn about different lifeforms, the more we question what really separates them from animals. Like contagious cancers, like the facial tumour disease that’s affected Tasmanian devil populations since the 1990s. It originated from animal cells and has mostly Tasmanian Devil DNA, and has a lifestyle not too different from a lot of parasites.

So is its own species of animal? And there are lots more cases like this we need more zoologists to figure out. Answering those questions will require knowing more about the millions of years of evolutionary history, or what the ancestors of today’s animals were like, and how they changed over time.

Millions of equally evolved animals took different paths to become the species we know today. But we’re still not sure what some of the first animals to evolve were like. The original animal group to evolve could be either Ctenophora or Porifera.

Back in episode 5 we leaned towards Ctenophora. But in the time it took us to get through 14 episodes, a new paper came out, swinging the momentum back towards Porifera! Regardless of who’s the earliest diverging group in metazoa, it’s going to change how we understand the evolution of a lot of fundamental animal features, like nerves and neural systems!

But the evolutionary mysteries don’t end at the root of the tree of animal life. Like even though we’ve got a few metrics, we’re still figuring out what role social behavior plays in the evolution of “intelligence.” Whales, humans, elephants and most of the other famously smart mammals live very complex social lives. Which suggests that their brain power might’ve evolved to deal with all the extra drama that comes with having roommates.

But then again, lots of invertebrates have complex social lives too, like the Toxeus jumping spider that nurses her spiderlings with milk. And others, like Portia, are wicked smart, but loners. So maybe intelligence has different roots in invertebrates and vertebrates.

Or maybe some animals think in a way that’s so fundamentally different from us that we humans can’t even conceptualize what’s going on in whatever nerve system they’ve got, never mind measure it! Or instead of brains, we could delve into the many ways that animals can make more animals, including why each strategy evolved and their long term implications. Or we could turn to everyone’s favorite animals: parasites!

What kind of evolutionary paths do they take? And is a parasite always a parasite, or can they re-evolve the traits they lost and drop their parasitic ones to become a free living creature again? Those are just some of the questions that have come up since we recently started paying attention to these strange but strangely common animals!

In fact, diseases and parasites are two areas of biomedical research, or the science of preventing and treating illnesses in people and animals, that zoologists are delving into to shape our human lives. A lot of biomedical research, like developing new drugs, finding new genetic risk factors for diseases, learning about how aging and the super complicated immune system works, all depend on animals. Animals like mice, fruit flies, and C. elegans...the nematode show up a lot in research labs.

So we know a lot about them because well, they live in research labs. In fact, C elegans was the first multicellular organism to have its entire genetic code, or genome, figured out. We’ve also mapped out it’s entire nerve system: we know exactly how each of it’s 302 neurons are connected to each other and how each of it’s 959 or 1033 body cells, depending on the individual’s sex, grow and develop.

Which are definitely not things we know about humans...or most living things! But there’s still a lot we don’t know about animals including C. elegans. Like what their lives are like outside the lab.

Most C. elegans in labs are descendants of thousands, even millions, of generations of lab stock populations. So even the animals that we think we know the most about are still a mystery to us! And one way to see how much zoology has changed over time is to examine how we’ve treated animals for research.

Today there are a lot of checks and balances in place to protect animal welfare, including inspections and oversight committees with vets and other animal care experts. But there are very different rules for vertebrates versus invertebrates -- animals without backbones aren’t given anywhere near the same protection or oversight. Animal research isn’t going anywhere anytime soon because there are things we can only learn by bringing animals into a lab.

But there are still lots of important ethical issues we’re still working through. There are also things we can only learn by observing animals in their natural environment, and there’s a lot of work in zoology doing that too! Real life zoologists work in a lot of places: universities, government departments, private companies, museums, zoos, and elsewhere.

Anywhere animals are! Zoologists at universities and other academic institutions are usually in grad school, or have a PhD like myself. You could also become a vet -- sometimes vets assist with research projects that involve some medical procedures, like testing a new drug or implanting GPS trackers in wild animals.

You could also be a zookeeper or technician, which usually requires an undergraduate degree and some zoology coursework. But there are plenty of ways you can get your paws dirty doing some zoology right now and see if it’s something you’d like to pursue in the future. Let’s go to the Thought Bubble.

Say you’re a birder who wants to spend her time taking care of the planet and advancing science. Every year, zoologists in museums, research labs, zoos, and more send out ads looking for interns and volunteers, often during the field season. So you check their websites or job boards as a way to get started in the field!

During the rest of the year, you can get involved with community science initiatives, which is sometimes called citizen science. This is scientific research conducted with the help of non-professional or amateur scientists, usually in collaboration with a researcher at a university or other institution. So if you enjoy fishing, the local river watch or wildlife programs might ask you to call if you land a fish with a special tag.

Another community science initiative might have volunteers send in photos of animals they encounter. There are lots of projects where having as many helpers as possible is crucial. So, get out there!

Help count birds for the Audubon Society’s annual Christmas Bird Count, measure light pollution in your hometown by reporting how many constellations you can see, and log what animals you encounter with apps like eBird and iNaturalist. You’ll be helping zoologists learn more about the animals than they could ever do on their own, and you’ll probably have a lot of fun doing it too! Thanks, Thought Bubble.

Another way to get involved is to volunteer with your local conservation authorities or societies, helping protect animals -- whether we know they exist yet or not -- and their homes so they will be around for future generations. It might mean clearing away invasive plants like kudzu in North America or picking up plastic and other garbage from beaches or hiking trails. But if getting messy in the field isn’t your speed, many conservation groups are also happy to have someone knocking on doors to raise awareness and fundraising to support these initiatives.

Especially for species that normally don’t get a lot of attention, or that a lot of people don’t appreciate. There’s a lot of different things to do in zoology, and a lot of really cool animals to work with, each with their own unique traits and evolutionary history! This might be the end of Crash

Course: Zoology, but it’s only the beginning of the work we need to do to take care of and learn from all the wonderful animals sharing our planet! Thank you for joining us. Now get out there, find some animals, and maybe I’ll see you out in the field some time! Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE.

It’s shot on the Team Sandoval Pierce stage and made with the help of all these nice people. If you’d like to help keep Crash Course free for everyone, forever, you can join our community on Patreon.