Some of our earliest records of humanity are forty thousand year old cave paintings that show what was important to our ancestors: there are hand prints and little people figures, but there are also aurochs, bison, giant sloths, and camels.

Thousands of years later, one of the most significant cultural and technological revolutions occurred when farmers in the Fertile Crescent domesticated sheep, pigs, and other livestock. For as long as there have been humans, there have been creatures sharing our lives, planet, and history.

And together in this series we'll walk, crawl, fly, and swim through the animal kingdom, tracing the evolution of the over 1.5 million different creatures  we know about and what the lives of both the animals and the zoologists that study them are like.  But before we can do that, we need to figure out what it means to be an animal.  I’m Rae Wynn Grant, and welcome to Crash Course Zoology! Animals have always been part of our lives, but there’s still so much to learn, even just about one particular animal. Take this bear.

We could study how it’s related to other bears, like polar bears. Or how and why it can smell food miles away. Or we could track where this bear lives and what happens when it crosses paths with humans.  All of these questions are part of zoology, which is basically the scientific field dedicated to asking and answering questions about animals.  Today, zoologists are many different things -- scientists, veterinarians, biomedical engineers, conservationists, and so much more.  So really, the question isn’t who is a zoologist, but what is an animal.

We’re pretty sure beetles or fish are animals. But amoebas? Or sea sponges?

Drawing the line can be surprisingly difficult.  To organize the chaos of life on Earth, zoologists, ecologists, and other “ists” rely on taxonomy, the branch of science dedicated to naming, describing, and classifying organisms.  It’s tricky work because no two types of animals are exactly the same even though some features like eyes are shared by lots of animals. So it’s not unusual for an animal to be recategorized and renamed over time. Zoologists have a long tradition of proposing different ways to categorize life...with varying degrees of success.  Like to Aristotle, the Greek philosopher and influential early zoologist, plants were sort of the baseline: they grew and produced new baby plants, but that’s it.

Animals also grew and reproduced, but were separate from plants because they moved and sensed their environment. And while we now know humans are a type of animal, Aristotle grouped us separately because we’re capable of deep thought and reflection. [Well, sometimes.]  Aristotle and his plant-animal-human system influenced generations of zoologists, including. Carl Linnaeus who developed binomial nomenclature, the system of giving all animals a unique, two-part Latin name.  We remember both Aristotle and Linnaeus as important men, but no matter what we’re studying, scientists are people making choices about what’s worth paying attention to, and they’re not always right or fair.  Some of Linnaeus’s other work is considered scientific racism, a debunked pseudoscience that categorizes humans into “varieties'' based on their skin color and stereotypes.

These views are widely discredited, but there’s still a lot of work to do in dismantling racism in science.  In his work on binomial nomenclature, Linneaus set-up a similarity hierarchy where we move from most similar groups to least similar groups.  So on one end we divide by species, which is a group of all the animals of the same type that can breed together over multiple generations. Then in the next level, different animals that are the most similar they can be without being part of the same species are grouped into a genus. And we build up from there to bigger ranks like family, class, all the way up to kingdom.
 The genus-species combo is how we identify animals in modern binomial nomenclature because no two types of animals have the same one.

So Ursus americanus is the North American black bear, and Danaus plexippus is a Monarch butterfly.  Distantly related animals always have a different genus, but they could have the same species name. Some words are just such useful descriptors, like elegans, meaning elegant, or vulgaris which means common. But scientists are an efficient bunch, and given the chance, we abbreviate almost anything.

So Cyprinodon elegans, Cyriocosmus elegans, Caenorhabditis elegans, and Cyclanorbis elegans are all C. elegans. So now we’ve got C. elegans the pupfish, C. elegans the tarantula, C. elegans the nematode, and C. elegans the turtle.  [You know you’re a zoologist when you start thinking of worms, turtles, fish, and tarantulas as “elegant.”]  Using the same abbreviation is confusing. But, it’s also an opportunity to explore just how related some animals are with something called a taxonomic sandwich.

Let’s go to the Thought Bubble. Think of two of our C. elegans as our bread and the evolutionary time, or the years since the two species last shared an ancestor, as the filling. But calculating evolutionary time and organizing animals isn’t easy!  Originally, scientists like Linnaeus grouped animals based on their looks.

Many animals have similar traits because they’re related, called homologous traits. But they can also have similar traits that evolved completely independently, which are called analogous traits. So if we thought the wings of insects, bats, and pterosaurs were homologous traits, we’d group them together.

And we might think it hasn’t been too long since the animals were related. But if wings are an analogous trait, we can't use them to tell how closely related the animals are. Figuring out if a trait is homologous or analogous can be really hard.

So to get more information, scientists also look at an organism’s DNA to suss out evolutionary relationships. Like we can use the molecular clock approach, which estimates how long ago two species diverged by comparing their DNA sequences.   Basically in the molecular clock approach, we assume that DNA sequences mutate or change over time at predictable rates. By combining information about mutation rates with the fossil record, we can then estimate how long ago two animals shared an ancestor.

Using observations and DNA, zoologists estimate the tarantula-turtle C. elegans sandwich has 600 to 800 million years of filling, while the turtle-fish sandwich has “only” 443 million years. That’s when these animals last shared a common ancestor, which likely had traits the two have in common! Thanks, Thought Bubble.

We’ll make more taxonomic sandwiches throughout this series to explore animals' often surprising evolutionary relationships. And to help us decide what’s really an animal. It’s taken centuries of exploring evolutionary time, but scientists generally agree that four key traits make animals special.

Animals are eaters, movers, sexual reproducers, and multicellular...ers. Like this lioness is made of millions of cells. Her cells work together as she hunts and digests her prey.

And her cubs are born by combining her genetic information with her mate’s. But nature loves to break rules. Some animals only do these things for part of their life -- like mayflies that feast as larvae, but lack mouths and guts as adults!

And even some non-animals have animal traits. Like carnivorous plants that trap and eat bugs! So to resolve these tricky edge cases, we need more information about where these traits come from.

We have to look back in time.  A living thing’s evolutionary history is like a genetic record of how it came to have all the traits it has today. It describes the living thing’s relationships with any living relatives and extinct ancestors, and how they’ve all evolved, or changed over time. By studying evolutionary histories through fossils and DNA, 19th and 20th century zoologists figured out that there was one ancestor species that had multiple cells, and ate, moved, and sexually reproduced.  Zoologists have deduced this First Animal was probably a blob with a mouth, but we don't have a fossil of it or anything to know just how blobby it was.

But from it came everything we'd call an animal, even if they've lost some traits over time. So those non-eating mayflies are still animals because their evolutionary histories trace back to that original animal ancestor.  But carnivorous plants aren’t animals because they aren’t descended from the original animal ancestor.  Studying homologous and analogous traits, evolutionary history, and other relationships among living things is called phylogenetics. Keeping track of who’s related to who can get messy, so we study animal relationships using a diagram called a phylogeny or a phylogenetic tree.

In a phylogeny, individual species or groups of species sit at the tips of the tree. And the branches represent all the different lineages that diverged from common ancestors. Branch lengths also show how related species are.

The longer the branch, the more distantly-related two groups are. So using observations and the molecular clock approach, we can decide which traits will help us group animals into clades, or a group with all the descendants of the same common ancestor, and fit those clades together into a phylogeny.  Now clades aren’t a rank, like species or phylum. They’re a type of group and can work kind of like nesting dolls.

Clades can be very large -- like the clade that includes all animals, called metazoa. Or tiny, like the haplorhine clade of monkeys, tarsiers, and apes. And we can have clades within clades within clades.

It just depends on which common ancestor we focus on. To actually build our clades, the simplest approach is to go for maximum parsimony, where the phylogeny with the fewest number of gains or losses of a trait wins.  Like, it’s more parsimonious to assume that a single dinosaur evolved feathers, and passed feathers onto its descendants, including birds. It’s less parsimonious to assume that the ancestors of ostriches, chickens, and songbirds all evolved feathers independently.

So, as zoologists using maximum parsimony, we’d choose the phylogeny that shows feathers evolving once. Another popular approach is to focus on maximum likelihood, which predicts evolutionary relationships by calculating the probability of the thousands of mutations needed to change one sequence of DNA into another.  Using the maximum likelihood approach, we’d end up with a phylogeny where the sequence of events has the highest probability.  Phylogenies are complicated because they’re tracking many different traits, animals, and time all at once. And they can rotate around their nodes, so this phylogeny, this phylogeny, and even this phylogeny are exactly the same.  Visuals can be misleading, and so can words like “advanced” or “primitive” because no living species is more evolved than any other.

Instead, zoologists use terms like early-diverging clades, to describe splits that happened a long time ago, and late-diverging clades, which split off more recently.  There’s no best way to make a phylogeny, and one phylogeny is really just a hypothesis for all the evolutionary relationships between species or clades based on specific traits or groups of traits.  So zoologists will often make several phylogenies using different approaches. If we keep getting the same answer, we know our phylogeny is a good guess for how different animals are related to each other. And we can use it to help answer our big question: what is an animal?

Like these little creatures called choanoflagellates. First on our checklist, we know animals move. Well, choanoflagellates have little flagella that whip back and forth to move them from place to place. [Check.] Next, animals eat.

Choanoflagellates eat bacteria they catch themselves. [Check.] Animals sexually reproduce. So do choanoflagellates, another check! And finally, animals are multicellular.

As we can see, choanoflagellates are single celled-organisms, but we know animals don’t always have all four animal traits. So let’s go to the phylogeny. Unlike animals, choanoflagellates can’t trace their lineage back to the last common ancestor of all animals, according to many in-depth studies into the genetics of these organisms.

So choanoflagellates aren’t animals! But by making phylogenies and examining DNA, we do know they’re the closest non-animal relative we’ve got. [Well that’s just fascinating.] Ultimately, zoology is asking and answering questions about animals, and hopefully busting some myths along the way.  All living animals have been evolving for the same period of time -- since the common animal ancestor first existed.  This is why knowing an organism's evolutionary history is so important. But now that we have a good handle on what an animal is, next episode we’ll tackle how many of them there are.

Want more Zoology? Then you’ll want to check out PBS’ newest show Animal IQ. Hosted by Trace Dominguez, Animal IQ features deep dives on animal minds to find out just how smart the animal kingdom really is.

We know that humans are clever, but can you find your friends in a crowd as well as a baby penguin? Drive a car as well as this rat? To see full episodes of Animal IQ click the link in our description below and be sure to tell them, "Rae wants more bear content!" Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE.

It is shot on the Team Sandoval Pierce stage at Porchlight Studios in Santa Barbara, 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.