A String Theory for Animal Relationships
How a simple, hands-on game reveals the deep connections between all living creatures.
Look at a spider, a salmon, and a squirrel. At first glance, they seem wildly different. But what if you could hold the story of their shared ancestry in your hands? What if you could literally see the branches of the evolutionary tree that connects them? This isn't a thought experiment reserved for PhDs; it's a dynamic classroom activity that uses nothing more than cards and string to bring the science of systematics—the study of evolutionary relationships—to life. By playing this game, we become explorers, retracing the footsteps of Darwin to map the magnificent and complex family tree of all animal life.
Before we dive into the activity, we need a key concept: the clade. A clade is a group of organisms that includes a single common ancestor and all of its descendants, both living and extinct. Think of it as a complete branch on the tree of life. If you were to cut that branch off, the entire lineage would come with it.
Scientists build these branches, or phylogenetic trees, by looking for shared derived characteristics. These are new evolutionary features that appear in a common ancestor and are passed exclusively to its descendants. For example, the presence of hair is a shared derived characteristic for the clade Mammalia; it unites platypuses, kangaroos, and humans, and distinguishes them from reptiles and birds.
Recent discoveries, especially in genetics, have constantly refined this tree. For instance, we now know that what we once called "reptiles" isn't a true clade unless birds are included, because birds are the descendants of certain dinosaurs! This activity helps us visualize why such reclassifications are necessary.
Animals with backbones
Four-limbed vertebrates
Animals with amniotic eggs
Milk-producing animals
This activity transforms you into a systematic biologist. Your lab is the table, and your tools are cards and string. Let's walk through the methodology.
You start with a deck of cards, each featuring a picture and name of a different animal. A typical set might include: a Jellyfish, an Earthworm, a Snail, a Grasshopper, a Starfish, a Shark, a Frog, a Lizard, a Robin, and a Cat.
Lay all the cards out on a table. Your first task is to group them based on your intuition. You might put the Robin and Cat together because they are both "furry or feathery," and the Snail and Earthworm together because they are "slimy." This initial grouping reflects our often misleading reliance on superficial similarity.
Now, you are given a list of key evolutionary traits, which act as your "shared derived characteristics." These are the real clues to ancestry.
This is the core of the experiment. You take a piece of string and loop it around the cards that share a specific, important trait. For example, you would loop a string around the Shark, Frog, Lizard, Robin, and Cat because they all have a vertebral column (backbone). This string now defines a clade: Vertebrata.
You add more strings for other traits. A string might loop the Frog, Lizard, Robin, and Cat for having four limbs (the Tetrapod clade). Another string would loop just the Lizard, Robin, and Cat for having an amniotic egg (the Amniote clade). Each string represents a branch point in your growing tree.
As you add more strings, a hierarchical, branching pattern naturally emerges from the table. The animals are no longer in vague groups; they are connected by a web of shared evolutionary history.
The final arrangement of cards and strings is a tangible, physical model of a phylogenetic tree. It demonstrates that the diversity of animal life is not a random collection, but a structured genealogy written in the language of anatomy and DNA.
The strings are placed based on careful analysis of traits. Here are the data tables that guide the construction of the phylogenetic tree.
(A "1" indicates the presence of the trait, a "0" indicates its absence.)
| Animal | Vertebral Column | Bilateral Symmetry | Four Limbs | Amniotic Egg | Feathers/Milk |
|---|---|---|---|---|---|
| Jellyfish | 0 | 0 | 0 | 0 | 0 |
| Earthworm | 0 | 1 | 0 | 0 | 0 |
| Snail | 0 | 1 | 0 | 0 | 0 |
| Grasshopper | 0 | 1 | 0 | 0 | 0 |
| Starfish | 0 | 0* | 0 | 0 | 0 |
| Shark | 1 | 1 | 0 | 0 | 0 |
| Frog | 1 | 1 | 1 | 0 | 0 |
| Lizard | 1 | 1 | 1 | 1 | 0 |
| Robin | 1 | 1 | 1 | 1 | 1 (Feathers) |
| Cat | 1 | 1 | 1 | 1 | 1 (Milk) |
*Starfish exhibit pentaradial symmetry as adults, but their larvae are bilaterally symmetrical.
| Clade Name | Defining Trait (Synapomorphy) | Member Animals in our Set |
|---|---|---|
| Bilateria | Bilateral Symmetry | Earthworm, Snail, Grasshopper, Shark, Frog, Lizard, Robin, Cat |
| Vertebrata | Vertebral Column | Shark, Frog, Lizard, Robin, Cat |
| Tetrapoda | Four Limbs | Frog, Lizard, Robin, Cat |
| Amniota | Amniotic Egg | Lizard, Robin, Cat |
| Mammalia | Production of Milk | Cat |
This table represents the branching pattern your string model would create.
| Evolutionary Branching Order | Resulting Clades |
|---|---|
| 1. All animals diverge from non-bilaterians (Jellyfish). | |
| 2. Bilateria splits into Protostomes (Earthworm, Snail, Grasshopper) and Deuterostomes (Starfish, Shark, Frog, Lizard, Robin, Cat). | |
| 3. Within Deuterostomes, Vertebrates (Shark, Frog, Lizard, Robin, Cat) diverge. | |
| 4. Within Vertebrates, Tetrapods (Frog, Lizard, Robin, Cat) diverge. | |
| 5. Within Tetrapods, Amniotes (Lizard, Robin, Cat) diverge. | |
| 6. Within Amniotes, Mammals (Cat) and Birds (Robin) diverge from Reptiles (Lizard). |
What do you need to run this experiment? The beauty lies in its simplicity. Here are the essential "research reagents" for your systematic investigation.
These represent the taxonomic units (operational taxonomic units, or OTUs) being studied. They are the data points for your tree.
This is your character matrix. It lists the evolutionary traits (like "vertebrae" or "amniotic egg") that provide the evidence for grouping organisms.
The string acts as the clade delineator. Each loop of string physically represents a monophyletic group on the phylogenetic tree.
This provides the scientific definitions and images of the traits, ensuring all participants are interpreting the evidence consistently, much like a standardized protocol in a lab.
Small clips can be used to attach trait labels to the strings, making the tree easier to read and interpret, similar to annotating a complex diagram.
The tangle of string on the table is more than just a game. It is a powerful metaphor for the interconnectedness of all life. By physically building the tree, we internalize a profound scientific truth: a cat and a robin are cousins, a frog and a lizard are closer cousins, and all of them share a distant, aquatic ancestor with a shark.
This interactive journey makes abstract concepts concrete and reveals that systematics is not about static classification, but about dynamic, evidence-based storytelling. It shows us that the history of life on Earth is a single, glorious, and branching narrative—one that we can now hold in our hands.