The Fifty Percent Argument

The banana pulp had gone gray and stringy in the jar, and floating on top of it was a cloud of white threads that Soren had not expected to see with his own eyes.

"That's it," the volunteer said, packing up her cooler. "That's DNA. Rubbing alcohol, dish soap, a little salt. You didn't need a lab." She waved at the door. "Lock it when you go. Just pull the handle."

Then she was gone, and it was just the two of them and a gray banana and a wad of its own genetic material sitting there like spit.

"She said something weird earlier," Maya said. "During the boring part. She said we're half banana."

"Fifty percent of our genes," Soren said. "I wrote it down. I think she meant it as a joke."

"It's not a joke, though. Either it's true or it isn't."

Soren pulled the jar closer. "Then it's wrong. Look at it." He tilted his head at the banana. "That's a banana. I'm not half that. I don't have a peel."

"Right," Maya said. "So where's the fifty percent?"

"That's my point. There isn't one."

"No, I mean actually. Where would it be." She was doing the thing where she stopped listening to the argument and started listening to the problem underneath it. "If half your genes match a banana, they're not the peel genes. Obviously. So which half is it."

Soren opened his notebook. He drew two circles, a person and a banana, and made them overlap in the middle. "The overlap," he said. "Whatever's the same."

"So what's the same between you and a banana."

"Nothing." He said it fast, then heard it. "Wait."

"Yeah," Maya said.

He looked at the white threads. "It's alive. It was alive. It's made of cells."

"So are you."

"So am I." Soren tapped the pen against his teeth. "And a cell has to do stuff. It has to make energy. It has to read the DNA and build things off it. It has to copy itself."

"Every cell," Maya said. "Yours. The banana's. The mold on the fence out there."

"There has to be machinery for that," Soren said slowly. "Little machines inside that read the instructions and build proteins. And if the banana has them and I have them..."

"They'd be the same machines."

"They'd have to be almost the same. Because they do the same job." He stopped. "That's the overlap. It's not the peel. It's the part that keeps the cell running."

Maya was already ahead of him, walking a slow circle around the workbench. "But that's backwards from how it sounds. When she said it, it sounded like we're close to bananas. Half close. But that's not it, is it."

"Say the rest."

"It's the opposite. The banana's not close to us. The banana and us are both close to something older. The thing that figured out how to run a cell in the first place. And neither of us ever changed it, because you can't. If it breaks, the cell dies."

Soren stared at her. "So the fifty percent is the oldest part."

"The oldest part of you," Maya said, "is the part you have in common with a banana."

They were both quiet. Outside, a hose hissed somewhere in the garden.

"Okay, but," Soren said, and he flipped to a clean page, "then I was wrong about how it works. I thought matching meant looking alike. Same genes, same result. But the banana and I have the same instructions for the cell machinery and I still don't have a peel."

"Because the other half is different," Maya said. "The half that isn't shared. That's the part that decides banana or boy."

"So the shared half builds the factory," Soren said, "and the unshared half decides what the factory makes."

Maya stopped walking. "That's the thing that gets me."

"What."

"The instructions for staying alive are so good that nothing's allowed to mess with them. For like, billions of years. A banana kept them. A mushroom kept them. That fly kept them." She pointed at a fruit fly doing loops near the window. "That fly is running some of the exact same code you are."

Soren looked at the fly. He had swatted at flies his whole life without once considering that they were reading the same book he was, in the same language, in the parts that mattered most.

"I don't feel related to a fly," he said honestly.

"You don't have to feel it," Maya said. "It's just true. In the part you can't see."

Soren wrote: the machinery is shared. He looked at the white threads in the jar, the pale tangle he had pulled up out of a piece of fruit with soap and salt. Somewhere in that mess were instructions that also existed, letter for letter almost, inside his own hand holding the pen.

"Here's what I want to know now," Maya said, and her voice had dropped "If the important part is that old, and it's in everything, then how far back does it go? Fly, banana, mushroom. What about the stuff in the soil you can't even see. What about the thing all of it came from. There had to be a first cell that figured it out. One. And it worked so well that it never got thrown away."

"You're saying it's still here," Soren said. "That first solution. Just, running. In us."

"I'm saying you're carrying around the oldest working thing on the planet and you didn't know it until ten minutes ago."

Soren looked down at the back of his own hand. Under the skin, in every cell, the little factories were running the same ancient instructions the banana had used, and the fly, and whatever came before either of them, an unbroken chain that had never once stopped copying itself in more than a billion years.

"We should test something," he said.

"Test what."

"Everything else in here." He stood up and grabbed the empty jars. "The tomato from the bin. That leaf. If it's alive, it should have the machinery. It should make the white stuff too."

Maya was already tearing a leaf into pieces and reaching for the salt.

Outside the shed window, the fruit fly landed on the rim of the gray banana jar, folded its wings, and cleaned its face with two front legs that ran on the same code as their hands.