Reports of the National Center for Science Education

Making Evolution Digestible

The outreach activity "In Case of Cellulose" helps participants understand the evolution of different digestion strategies.

“All that work and only 15 calories!” The exasperation in the 10-year-old’s voice is amplified by the walls of the two-and-a-half-meter-long intestinal tract she’s been crawling through. “That’s like three jelly beans,” one of our interpreters shouts meekly into the tube, unsure to which adult her frustration is directed. Suddenly, a crop of curly hair pops out of the large intestine: “Three? I had 36 jelly beans this morning.”

This child, like many of the more than 400 participants we chatted with while leading our activity called “In Case of Cellulose” recently as part of the Bay Area Science Festival, was learning firsthand about evolutionary tradeoffs in the mammalian digestive system. We were very excited to see that, despite the inherent frustration of “digesting” low-calorie foods, families spent more than 20 minutes on average playing through the activity together. Through this extended engagement time we were able to craft an evolutionary story that avoided many common pitfalls.

Finding "nutrients" as part of the In Case of Cellulose activity. (Photo by Dave Strauss)

While playing “In Case of Cellulose,” participants choose to simulate the digestion of a cow, horse, panda, or a human. We create a digestive bolus of chewed leaves using baking soda, citric acid, cornstarch, and food coloring (you might call it a bath bomb in other circumstances). To digest these boli, participants must first chemically and mechanically process them in the “stomach”—a tray— using digestive enzymes. As a colorful chemical reaction takes place before their eyes, they first notice a few “nutrients” appearing—small, colorful, beads—before stumbling across a hard plastic ball that is unreactive to digestive enzymes. This ball of cellulose contains significantly more nutrients but is incredibly difficult to open. Depending on their chosen species, participants will then get to experience different evolutionary strategies for digesting cellulose and absorbing nutrients in the intestines as they carry the bolus through the “intestines”—a long, collapsible tunnel. Once in the intestines, they will have to match the nutrition they found with the enzyme or part of the microbiome that helps it be absorbed.

Cellulose has been an important structural component of plants since the Paleozoic, but digesting it has presented a challenge for animals. In fact, no animal can digest cellulose without the help of microorganisms inhabiting their guts, though the type and location of these digestive helpers vary. Foregut fermenters, like cows, break down cellulose in their first few stomachs, but need to regurgitate, chew their cud, and pass the bolus through the last few stomachs in order to absorb protein. Hindgut fermenters like horses, by contrast, use the microbiome located primarily in their cecum to break down cellulose. Pandas present a particularly weird case, as they subsist almost entirely on high-cellulose food, but have a short, carnivore gut that makes fermentation impossible and lacks the microbiome to process cellulose (though evidence is mixed on this last point).

While every strategy allows animals to extract at least some nutrition from plant resources, evolutionary trade-offs exist for all three animals. Foregut fermenters extract fewer nutrients overall, while hindgut fermenters often have to rest during digestion. Furthermore, the overall strategy of subsisting on plant resources means that animals often have to spend huge parts of their day foraging. Many participants were shocked to see how little caloric value they get out of each resource and how many hours of their day they would have to spend chewing and digesting in order to stay alive. Pandas, who get almost no nutrients from the leaves they eat, have devised an incredibly risky strategy for digestion, where any deviation in plant resource availability can end in their death. For our participants, it is also an important lesson in how adaptive strategies work.

It is this combination of the intuitive and the novel that leads to early success that then allows for engagement with the deeper evolutionary content.

Creating inquiry-based activities for evolution can be difficult, given that evolution occurs over a time scale that is difficult for humans to grasp. It is easy to introduce misconceptions through oversimplification. Activities that focus on an individual organism—an adorable panda bear, for example, or a hungry bird—are easily relatable, but often participants walk away with the impression that evolution responds to what an individual organism needs, thus undermining understanding of natural selection. Creating activities with too much nuance— that focus on change in allele frequency over generations, for example—on the other hand, can overwhelm visitors and prevent breaking down the barriers to science, the core goal of our program. Mechanistic activi ties that focus on the process of evolution often don’t have enough of an emotional hook to drive deep engagements. While adults typically engage with evolutionary questions that are relevant to their own lives, like health and disease, kids are more drawn to evolutionary activities that involve recognizable species (and allow them to make a mess). “In Case of Cellulose” tries to be mindful of each of these considerations while creating an activity that has resonance with a broad portion of the population.

“In Case of Cellulose” aims to be immersive by allowing participants to take on the role of a digestive bolus, exposing participants not only to cellulose but also to the digestive system as a whole. Such immersive experiences, if not carefully constructed, can prove to be intimidating. We don’t expect participants to know anything about digestive enzymes or the different pathways to absorb different nutrients; we therefore make the process of chemical digestion in the stomach and the matching of nutrient colors in the intestines intuitive. It is this combination of the intuitive and the novel that leads to early success that then allows for engagement with the deeper evolutionary content. It also allows volunteers to scaffold content that is appropriate for each family’s age and interest levels. By creating activities that engage at different levels, we can truly make evolution digestible.

This activity was mailed out to the members of our Science Outreach Collaborative in early December 2019. If you want to do this activity in your community, you can check out a full list of instructions for this and other activities.

This version might differ slightly from the print publication.

Kate Carter
Short Bio

Kate Carter is Director of Community Science Education at NCSE.