How is a raven like a rhinoceros?

No, this isn’t the setup for a bad dad joke. I promise.

On the outside, the two may seem very different. One is known for its feathery body and keen brains, and the other for its hard skin and pointy horn. But trace their evolutionary histories, and you’ll find they’re not so different after all.

They’re connected to each other – and every other form of life – by evolutionary relationships. And if we hope to understand all the connection points between ravens, and rhinoceroses, and everyone else, we need a way to visualize them. Now, the best way we’ve found isn’t a series of steps.

Because evolution doesn’t function like a ladder in one direction. Instead, we can visualize evolutionary relationships with a tree – the tree of life. Hi!

I’m Dr. Sammy, your friendly neighborhood entomologist. And this is Crash Course Biology.

Now, might I be able to interest you in some theme music? [THEME MUSIC] Behold the tree of life. Each of the different species that currently, or have previously, roamed our world represent a leaf at the tip of each twig. Trace those twigs back, and you reach a branching point, representing a common ancestor between two different species.

Trace the limb further and you find more branching points, and more common ancestors among more species. Like, ravens are birds, a group that descended from now-extinct dinosaurs. Deeper in the past, those dinosaurs descended from an ancestor that they shared with modern reptiles.

And before that, the dinosaur-reptile ancestor descended from an ancestor that they shared with modern mammals – like the rhino. So, the tree of life is kind of like Six Degrees of Kevin Bacon. Look hard enough — or in this case, far enough into the past — and you’re bound to find a link between two species.

By the way, these modern branch divisions don’t always match up to the traditional Linnaean hierarchy that you may have learned in school: kingdoms, phyla, orders, and so on. That system of classification is largely based on how organisms look rather than how they’re actually related. As impressive as the tree of life is, it’ll never be a perfect representation of all life on Earth.

For example, it’s tough to even estimate the number of species alive today because, well, we don’t know what we don’t know. Some species live in the unforgiving deep sea, are too small for human perception, or are otherwise outside of our awareness — so far. On top of that, the tree of life includes extinct species, and there are billions of those to learn about.

So, the tree of life is likely to keep growing and changing as long as life does. The more we learn, the more detailed and refined it becomes. For example, as recently as 2022, a whole new branch of life was added to describe a diverse group of 600 species of fungi.

Many of them look, and act different, but their genes suggest that they’re all descended from the same ancestor. So these species from nine different countries, which were previously organized into seven different classes, are now better understood as one evolutionary group. And this type of discovery that shakes up our understanding of the tree is bound to happen again!

But for now, let’s zoom out. If we want to understand the tree of life as a whole, we need to focus on the main branches, rather than on each and every twig. Major branches on the tree of life correspond to the major groups of living things, with each smaller branch that splits from it representing a smaller subdivision. [quirky music plays] Closest to the roots of the tree, you’ll find the branch of prokaryotes.

These consist of single-celled organisms whose genetic material isn’t contained within a nucleus, but floats freely within the  cell’s goo, or cytoplasm. There are two main groups of prokaryotes – the familiar bacteria and the likely less familiar archaea. Both are microscopic, and to be honest, it can be hard to tell them apart.

So, because bacteria and archaea aren’t much to look at — no offense, prokaryotes – their smaller subdivisions are based on things like their genetic codes, rather than their physical features. And while it’s hard to get a good idea of how many prokaryotes there really are, some scientists think there could be up to a billion species. Or, think of it this way: if you balled up all the living stuff on Earth, bacteria and archaea would be about 14 percent of the total biomass, or the mass of all living things.

And animals would be less than half a percent. [quirky music plays] Us non-prokaryotes live somewhere on the eukaryote branch of the tree of life. The split that created eukaryotes marked a major step in evolutionary history. And today, eukaryotes are defined by having certain kinds of organization within their cells, including genetic material contained within a nucleus.

The eukaryote branch contains a lot of familiar groups: animals, plants, and fungi, as well as the more enigmatic protists. Collectively, protists can be defined as eukaryotes that… just don’t belong to the other three groups. Most are microscopic and single-celled, so you might imagine they’re all more or less the same.

But they actually have more species diversity than any other eukaryote. Some, like rhodophyta, group together to make bigger structures that seem very much like plants. Others, like metamonads, act more like bacteria.

They live inside the guts of wood-eating insects, helping them break down tough fibers. And then there’s rhizaria, which construct hard shells around their single cells to protect them from ocean waves, similar to clams and snails. But they’re not plants, bacteria, or animals.

They’re the beautiful rainbow of protists. And that’s just to name a few examples. Protists are not as prevalent as bacteria and archaea, but still: if you balled up all the protists on Earth, they’d weigh roughly twice all the animals.

Protists come in just under 1% of the Earth’s total biomass. And they’re an important component of virtually every ecosystem. Including our own internal ecosystems.

We wouldn’t stand a chance of digesting our food without the diverse community of protists — plus bacteria and other microbes — that call it home. [quirky music plays] And speaking of things that outnumber us while benefiting us in multiple ways, the next major branch on the tree of life is plants. As a group, plants are defined as multicellular organisms that can make their own food by photosynthesis. Their cells have rigid walls made from a substance called cellulose, and they don’t make a habit of moving around by themselves.

But working with that basic definition, plants have nevertheless managed to evolve some impressive diversity. There are vascular plants like trees that transport water and sugars through their stems, leaves, and roots, using dedicated plumbing networks. And there are non-vascular plants like liverworts that don’t have specialized transport cells.

There are plants like ferns that reproduce with spores, and seed plants who do it, as the name suggests, with seeds. And so much more. Plants are by far the most dominant living thing on Earth, accounting for over 80 percent of the Earth’s biomass. [quirky music plays] And while plants live their days in the warmth of the Sun — typically, at least — the next major branch of the tree, fungi, are much more secretive, often lurking underground or out of sight.

Members of the fungi group are united by being heterotrophs, meaning that they eat other organisms to get their food rather than making their own, like plants, which are usually autotrophs. Fungi are mostly multicellular, although some can be single-celled. And their cell walls are strengthened with chitin, a biopolymer found in nature that bolsters things like exoskeletons.

For most of us, our relationship with fungi extends to the mushrooms on our pizza, or maybe the toadstools popping up deliciously, or poisonously, in the forest. But this is only a tiny part of their story. Fungi also include yeasts, like our friend Yeasty the 212th over here.

And molds, like the green and pink fur that grows on old bread. In the wild, fungi play an important role in the food chain, helping to break down all kinds of organic matter and release nutrients back into the soil for plants to use. They also play a similar cooperative role inside many plants and animals, keeping everything cycling smoothly.

Fungi represent about 2% of our planet’s biomass:  way less than plants, but  still way more than animals. [quirky music] [buzzing] And with that, we’ve finally reached the last, major branch on the tree of life: animals! Animals are heterotrophic like fungi, although they typically digest their food inside their bodies instead of outside. They’re all multicellular, but unlike fungi, those cells have membranes that allow for things to move in and out, rather than cell walls.

Most animals arrange their cells into groups of similar cells called tissues, but there are some, like sea sponges, that don’t. You can literally squeeze the cells of a sea sponge through a sieve and it will regroup and carry on as if nothing happened. Most of us… aren’t like that.

Major animal groups can be divided based on what kind of symmetry their bodies have. Some, like jellyfish and sea anemones, have radial symmetry, from the center outward. Whereas many others, like you and me, have bilateral symmetry.

That is, we’re pretty much mirrored down the middle. And then, the animal branch splits further based on how the animal develops before it’s born. In some groups, the mouth is among the first to develop.

So, among these ‘mouth-firsters’ you have all insects, plus molluscs (like snails, mussels and octopuses,) and worms. So many worms. Like, more worms than you can shake a, well, a worm at.

If the mouth isn’t the  first to develop in animals,  then the alternative is….the anus. Animals whose anus appears first include sea urchins, starfish, plus all vertebrate animals — from fish to people. That’s right, we’re ‘butt-firsters’!

Anyway, since we’re animals ourselves, it’s natural that we typically give this group the most attention. And there is a vast diversity of animal life around us. But as we’ve seen, we’re well-matched and even  dwarfed by several other  branches of the tree of life.

And that’s not even counting the many branches of living things that have been cut off due to extinction. Individual species and entire groups, like the dinosaurs and the ammonites are long gone, and we have to rely on their fossils to help place them on the tree. For some, like the mysterious Ediacaran organisms that lived 600 million years ago, we struggle to place them among the known groups, because they have features that just don’t fit the existing branches -- earning them a place in the phylum “Problematica,” which is a real thing by the way.

And still, others didn’t  leave a fossil record at all. What’s more, the most recent research into evolutionary relationships is revealing that the tree of life might be less like a tree and more like a web. This is sometimes called reticulated evolution.

In this model, branches that once split off from one another come back together, as organisms with very different evolutionary histories breed with each other in what’s called hybridization. So, even though it’s constantly evolving, and in places looks more like a thorny thicket than a proud solitary oak, the tree of life is still a powerful way of representing the mind-boggling diversity of life. It can show us how a raven is like a rhinoceros, and that gives us the tools we need to chart the unexpected course of evolution over billions of years.

We can zoom out to see the biggest branches defining the dominant groups, and zoom right back in to see how each twig and leaf is linked in an intricate and ever-growing system. Next time, we’ll focus on the interlocking branches that contain us, and discover just how bushy and tangled that human offshoot really is. I’ll see you then.

Peace! This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit BioInteractive.org/crashcourse for classroom resources and  professional development  related to the topics covered in this course.

Thanks for watching this episode of Crash Course Biology which was filmed at our studio in Indianapolis, Indiana, and made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.