At a time far in the future, when animals once again roamed the Earth, there was a family of marmoset monkeys who lived happily in the canopy of a lush rainforest. Papa and Mama Marmoset were proud of their growing infants. They had taught Mark and Malia how to deftly dig up edible insects, skillfully sniff out luscious flowers, and nimbly clamber up towering trees to find mouth-watering fruits. However, Mark was not as industrious as his sister. Often too lazy to climb a tree, he would settle for snacking on insect larvae which, while chock full with protein, did not provide the variety of nutrients he needed to grow strong.

One morning, following a torrential rainstorm which had howled like an angry pack of wolves all night, the Marmoset family awoke very hungry. However, as they started foraging for their breakfast, they found the rain had transformed the jungle floor to a lake as muddy as a day-old coffee. Finding their usual cornucopia of insects and flowers washed away, Malia scampered undeterred to the top of a gangly tree to find fruit, but Mark sat listlessly below.

“Why don’t you go get fresh fruit in the tree?” prodded Mama.

“Too much work,” Mark responded lackadaisically. “Why bother if I can just eat insects crawling around?”

Mama sighed loudly.

Papa gave her a knowing wink and then announced, “Well, Mark, I have a story that may change your mind. Let me tell you and Malia ‘The Legend of the Single Digit Creature’.”
But even as he said the title, Mama groaned. “Everyone knows that story is not true, Papa!” she challenged. “One digit can never be that important.”

“Of course it’s true! My great-great-great-great-grandfather was alive back then and passed this story down through my family!” Papa argued indignantly. “Listen carefully and you will learn how a single digit was responsible for both the meteoric rise and tragic downfall of an entire species.” Malia sat down obediently next to Mark in front of Papa, but Mama harrumphed and turned her back away slightly in subtle protest. Papa ignored her and began his mysterious tale. “A long time ago there was a type of animal that had a very special digit.”

Immediately, Malia interrupted, “What’s a digit?”

“See our four beautiful paws? Each has five digits! These creatures also had five digits on each paw, but on their front paws they had a very special digit called a ‘thumb’ which could bend so oddly that it could touch the tip of the four other digits!”

“EWWW!” cried Malia as she tried to contort her paw into such a structure. “Too strange, Papa! I don’t believe you.”

“It’s true! Because of their special thumbs they could grasp things. But instead of using this ability to climb higher up a tree they invented a…,” paused Papa for theatrical effect, “…thumb machine!”

The kids sat with their mouths agape, as open as a lion about to roar. But Mama only shook her head and muttered, “Thumb machine, haha, Papa, maybe you should call it a ‘dumb’ machine after this dumb story.”

“Now, Mama, let me tell the whole story,” continued Papa, relishing the attention. “These creatures started walking on their back paws so they could use their thumbs to do other things. Instead of adroit quadrupeds they became sluggish bipeds. Then many of them stopped using even their back paws! Instead of walking they spent their days in rolling machines, using their thumbs to steer, as they zoomed everywhere as impatient as wildebeest on migration! They also stopped eating fresh food and just ate convenient food,” elaborated Papa, catching Mark’s eyes before continuing. “This was often less nutritious, processed food. Many of these creatures became so fat it was hard for them to bend over.”

With this sentence, Mark and Malia started giggling. “This story is too much, Papa!” snorted Mark, chiming in for the first time. “What sort of stupid creature would stop eating delicious fruit that grows naturally? Why would it roll around when it could walk? Wouldn’t evolution select against it getting so big that it could no longer escape its enemies?”

“Excellent thinking, Mark,” agreed Papa, his complement enveloping Mark like a hug.

“But these creatures had no predators. Their only enemies were themselves. It was no longer about survival of the fittest, but instead all about convenience. And this led to their doom. They spent all day using their thumb machines: to work, to socialize, to order delivered food. This is why they are remembered as the ‘Single Digit Creatures’.”

Papa gazed calculatedly at his captive audience before continuing. “It was their habits which destroyed the environment: temperatures rose, unpredictable fires charred our jungles, and tsunamis flooded our coasts. Finally, pollution from the rolling machines choked oxygen levels until they were mortally low. Every foot higher off the ground had 10% less oxygen, so a six-foot-tall biped had half as much oxygen as a quadruped one foot off the ground. Upright, these bipeds no longer had enough oxygen to breathe. And, unable to spend all day on all fours, they began to die off, one by one, like overripe fruits dropping from a tree.”

The entire family sat in stunned silence. Finally, Mark looked up with tears in his eyes and asked, “What happened after that, Papa?”

“Well, after the Single Digit Creatures went extinct, it took a very long time to reverse all their damage to Earth. But eventually oxygen levels rose again and the catastrophic weather relented. Surviving quadrupeds like us began to thrive again.”

Mark sat solemnly for a moment gazing up at the trees. The dangling fruit now seemed to sparkle in the sunlight, like bundles of jewels tempting his appetite, and his mouth began to water. His eyes brightened and he grabbed his sister’s paw. “Malia, I’ll race you to the top of that fruit tree!” And as their two dashed off, Mama smiled and reciprocated Papa’s knowing wink. “Great story, Papa.