What is an Animal? Crash Course Zoology #1

Summary

Crash Course Zoology explores the animal kingdom, tracing the evolution of over 1.5 million species and their relationship with humans. Host Rae Wynn Grant introduces the scientific field of zoology, covering the basics of animal taxonomy, the importance of evolutionary history, and the use of phylogenetic trees to understand species' relationships. The episode delves into defining what makes an animal, examining traits, and the history of life on Earth, emphasizing the importance of understanding an organism's lineage for classification.

Takeaways

• 🎨 The earliest human records include 40,000-year-old cave paintings depicting what was important to our ancestors, such as hand prints, people figures, and various animals.
• 🌱 A significant cultural and technological revolution occurred when farmers in the Fertile Crescent domesticated animals like sheep and pigs, marking the beginning of a deep relationship between humans and animals.
• 🔍 Zoology is the scientific field dedicated to studying animals, encompassing a wide range of professionals including scientists, veterinarians, and conservationists.
• 🧬 Taxonomy is the science of classifying organisms, which is essential for organizing the diversity of life on Earth and can involve reclassification and renaming over time.
• 👤 Aristotle's early zoological work influenced later scientists by distinguishing between plants, animals, and humans based on their capabilities and characteristics.
• 📚 Carl Linnaeus developed binomial nomenclature, providing a unique two-part Latin name for every species, which is still used today in modern taxonomy.
• 🔬 The process of classifying animals can be complex due to shared features like eyes, and zoologists use DNA analysis and the molecular clock approach to determine evolutionary relationships.
• 🌐 Phylogenetics is the study of evolutionary relationships among species, using tools like phylogenetic trees to visualize the branching pattern of descent from a common ancestor.
• 🌿 Four key traits define animals: they eat, move, sexually reproduce, and are multicellular, although some animals may only exhibit these traits for part of their life cycle.
• 🌳 Evolutionary history is crucial for understanding an organism's traits and relationships, with the First Animal likely being a multicellular blob with a mouth that could eat and move.
• 🌟 Zoology aims to answer fundamental questions about animals, including their classification and evolutionary history, and challenges misconceptions about what constitutes an animal.

Q & A

What do the forty thousand year old cave paintings indicate about our ancestors' priorities?

The cave paintings indicate that our ancestors valued depictions of hand prints, human figures, and various animals such as aurochs, bison, giant sloths, and camels, reflecting their daily life and the importance of these elements in their society.

What significant cultural and technological revolution occurred when farmers in the Fertile Crescent domesticated animals?

The significant revolution was the domestication of sheep, pigs, and other livestock, which marked a major shift in human societies from hunter-gatherer lifestyles to settled agricultural practices.

What is the primary focus of the series 'Crash Course Zoology'?

The primary focus of 'Crash Course Zoology' is to explore the animal kingdom by tracing the evolution of over 1.5 million different creatures and understanding the lives of both the animals and the zoologists that study them.

What is the scientific field dedicated to studying animals called?

The scientific field dedicated to studying animals is called zoology.

What are some of the various professions that a zoologist might hold?

A zoologist might be a scientist, veterinarian, biomedical engineer, conservationist, or work in many other related fields.

What is the branch of science that zoologists, ecologists, and other scientists rely on to organize life on Earth?

They rely on taxonomy, which is the branch of science dedicated to naming, describing, and classifying organisms.

Who was the Greek philosopher and early zoologist that influenced the way animals and plants were categorized?

Aristotle was the Greek philosopher and early zoologist who influenced the categorization of animals and plants.

What is the system of giving all animals a unique two-part Latin name called?

The system is called binomial nomenclature.

What is the term used to describe the process of estimating how long ago two species diverged by comparing their DNA sequences?

The process is called the molecular clock approach.

What are the four key traits that scientists generally agree make animals special?

The four key traits are that animals are eaters, movers, sexual reproducers, and multicellular.

What is a phylogeny or phylogenetic tree used for in the study of animal relationships?

A phylogeny or phylogenetic tree is used to visually represent the evolutionary relationships among various species or groups of species, showing their divergence from common ancestors.

What term is used to describe a group with all the descendants of the same common ancestor?

The term used to describe such a group is a clade.

What are the two main approaches used by zoologists to build phylogenies?

The two main approaches are maximum parsimony, which assumes the fewest number of gains or losses of a trait, and maximum likelihood, which calculates the probability of mutations needed to change one DNA sequence into another.

Why is it important to consider an organism's evolutionary history when determining if it is an animal?

An organism's evolutionary history is important because it provides a genetic record of how it came to have its traits, describes its relationships with living relatives and extinct ancestors, and shows how they have evolved over time.

What is the role of the molecular clock in estimating evolutionary relationships?

The molecular clock helps estimate the time since two species shared a common ancestor by assuming that DNA sequences mutate at predictable rates and combining this information with the fossil record.

How does the script define 'zoology' in the context of this series?

In this script, 'zoology' is defined as the scientific field dedicated to asking and answering questions about animals, including their evolution, classification, behavior, and interaction with humans.

Outlines

00:00

🖌️ Ancient Art and the Origins of Zoology

This paragraph delves into the historical significance of cave paintings as early records of human civilization, highlighting the importance of animals in our ancestors' lives. It introduces the concept of the animal kingdom and sets the stage for the Crash Course Zoology series, hosted by Rae Wynn Grant. The paragraph emphasizes the multidisciplinary nature of zoology, encompassing various professions like scientists, veterinarians, and conservationists. It also touches upon the complexities of defining what constitutes an animal, referencing the scientific field of taxonomy for classifying organisms. The historical contributions of Aristotle and Carl Linnaeus to zoology are acknowledged, along with a critique of Linnaeus's controversial views on human categorization. The paragraph concludes with an explanation of binomial nomenclature and the hierarchical structure of biological classification.

05:03

🔬 Taxonomy and the Evolutionary Journey

This section explores the intricacies of taxonomy, the scientific method for classifying organisms, and its challenges due to the diversity and evolution of species. It discusses the concept of 'taxonomic sandwiches' to illustrate the evolutionary relationships between species, such as the C. elegans examples. The paragraph explains the difference between homologous and analogous traits and the importance of DNA in understanding evolutionary relationships through molecular clock approaches. It also introduces the idea of a phylogenetic tree as a tool for visualizing these relationships and the concept of clades as groups of organisms sharing a common ancestor. The paragraph concludes with an overview of how scientists build phylogenies using principles like maximum parsimony and maximum likelihood.

10:04

🌿 Phylogenetics and Defining Animal Traits

The focus of this paragraph is on phylogenetics, the study of evolutionary relationships among organisms. It discusses the use of phylogenetic trees to understand the branching patterns of species from common ancestors and the significance of branch lengths in indicating relatedness. The paragraph outlines the process of building clades and the principles of maximum parsimony and maximum likelihood in phylogeny construction. It also addresses the complexities and subjectivity in creating phylogenies and emphasizes the importance of an organism's evolutionary history in determining its classification. The paragraph concludes with an example using choanoflagellates to illustrate the application of phylogenetic analysis in defining whether an organism is an animal.

15:05

🎥 Production Credits and Further Exploration

In the final paragraph, the video script shifts from the educational content to the production aspects of the Crash Course Zoology series. It credits Complexly for producing the series in partnership with PBS and NATURE, and acknowledges the filming location at Porchlight Studios. The paragraph invites viewers to support Crash Course on Patreon to keep the educational content free and accessible. Additionally, it promotes PBS's new show 'Animal IQ' hosted by Trace Dominguez, which explores the intelligence of various animals, and encourages viewers to watch for a deeper understanding of animal cognition.

Keywords

💡Cave paintings

Cave paintings refer to ancient artworks found in caves, often dating back to prehistoric times. They are significant as they provide a glimpse into the lives and beliefs of early humans. In the video, forty-thousand-year-old cave paintings are mentioned to illustrate the long-standing relationship between humans and animals, as these paintings often depict animals that were important to our ancestors.

💡Domestication

Domestication is the process by which wild plants and animals are transformed into cultivated or tamed forms primarily for human use. In the script, the domestication of livestock such as sheep and pigs by farmers in the Fertile Crescent is highlighted as a pivotal moment in human-animal relations and agricultural development.

💡Zoology

Zoology is the branch of biology that studies the anatomy, behavior, and physiology of animals. It is central to the video's theme as it encompasses the scientific exploration of animals. The script explains that zoology involves asking and answering questions about animals, including their relationships, senses, and interactions with humans.

💡Taxonomy

Taxonomy is the scientific method of classifying and naming organisms based on shared characteristics. It is vital for understanding biodiversity and evolutionary relationships. The video discusses taxonomy as the system used by zoologists to organize the vast diversity of life on Earth, including the reclassification and renaming of animals over time.

💡Binomial nomenclature

Binomial nomenclature is a two-part naming system for species developed by Carl Linnaeus. Each species is given a unique name consisting of its genus and species. The script mentions this system as a way to identify animals uniquely, such as Ursus americanus for the North American black bear.

💡Homologs and Analogs

Homologous traits are similar features in different species due to shared ancestry, while analogous traits are similar due to convergent evolution. The video explains the importance of distinguishing between these for understanding true evolutionary relationships, using the example of wings in insects, bats, and pterosaurs.

💡Molecular clock

The molecular clock is a method used in evolutionary biology that assumes a constant rate of genetic mutation over time. It helps estimate when species diverged by comparing DNA sequences. The script refers to this approach to illustrate how zoologists determine the evolutionary time between species, such as the tarantula-turtle C. elegans sandwich.

💡Phylogenetics

Phylogenetics is the study of evolutionary relationships among groups of organisms. It involves the construction of phylogenetic trees to represent these relationships. The video describes how phylogenetics helps in understanding the shared ancestry and traits of animals, such as the First Animal hypothesis.

💡Clades

A clade is a group consisting of an ancestor and all its descendants. It is a fundamental concept in phylogenetics. The script explains how clades help in categorizing animals based on their common ancestry, from large groups like metazoa to smaller ones like haplorhine primates.

💡Phylogenetic tree

A phylogenetic tree is a diagram that represents the evolutionary relationships among various biological species or other entities based on similarities and differences in their physical or genetic characteristics. The video uses the concept of phylogenetic trees to show how zoologists visualize and study the evolutionary history and relationships of animals.

💡Choanoflagellates

Choanoflagellates are a group of single-celled organisms that are closely related to animals but are not considered animals themselves. The script uses choanoflagellates as an example to demonstrate the process of determining what qualifies as an animal based on phylogenetic analysis and the presence of key animal traits.

Highlights

Earliest human records include 40,000-year-old cave paintings depicting important aspects of ancient life, such as hand prints, people figures, and various animals.

A major cultural and technological revolution occurred when farmers in the Fertile Crescent domesticated livestock like sheep and pigs.

Zoology is a scientific field dedicated to understanding animals, encompassing a wide range of professionals including scientists, veterinarians, and conservationists.

Taxonomy is the science of classifying organisms, which is crucial for organizing the diversity of life on Earth.

Aristotle's early zoological work differentiated between plants, animals, and humans based on growth, reproduction, movement, and sensory perception.

Carl Linnaeus developed binomial nomenclature, providing a unique two-part Latin name to each species.

Some of Linnaeus's work has been criticized for scientific racism, highlighting the need to dismantle racism in science.

Binomial nomenclature uses a hierarchy of similarity, from species to kingdom, to classify organisms.

The term 'elegant' is commonly used and abbreviated in scientific names, leading to confusion among different species.

A 'taxonomic sandwich' is a method to explore evolutionary relationships between species based on the time since their last common ancestor.

Homologous and analogous traits can be distinguished through DNA analysis to better understand evolutionary relationships.

The molecular clock approach estimates the time of species divergence by comparing DNA mutation rates.

Four key traits define animals: they eat, move, sexually reproduce, and are multicellular.

Evolutionary history is essential for understanding an organism's traits and relationships with relatives and ancestors.

Phylogenetics studies the relationships among living things through traits, evolutionary history, and other factors.

A phylogenetic tree is a diagram that represents the evolutionary relationships among species or groups.

Clades are groups of all descendants of a common ancestor and can vary in size from large, like metazoa, to small, like haplorhine.

Maximum parsimony and maximum likelihood are methods used to build phylogenies based on the fewest number of trait changes and the highest probability of DNA mutations, respectively.

Phylogenies are hypotheses that can be tested and refined over time as more data becomes available.

Choanoflagellates, while not animals, are the closest non-animal relatives to animals based on genetic studies.

Transcripts

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

occured 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.