The Boarders

The yogurt was wrong.

Maya had followed every step. Heated the milk to eighty-two degrees Celsius, cooled it to forty-six, stirred in the starter culture, wrapped the jar in a towel, and set it on the counter overnight. The bacteria in the starter were supposed to multiply, turn the milk thick and sour. Instead the jar held something that looked like milk with a skin of yellow liquid floating on top.

"That's whey," her mother said, passing through the kitchen with her laptop open, already late for a video call. "Means something went wrong with the temperature. Try again tomorrow."

But Maya had checked the temperature three times. She lifted the jar and tilted it. The failed yogurt sloshed. Under the whey, tiny clumps floated like clouds in a pale sky.

She spooned one out. It sat on the spoon, a soft white lump. Bacteria had done this much. They had started to change the milk, started to break down the lactose, started to build their acid, and then stopped. They had lived in there for hours, eating and dividing and doing what bacteria do, and somewhere in the night they had quit.

She put the spoon down.

Something her science teacher, Mr. Reade, had said last week was bothering her now. He had been talking fast, the way he did when he was excited about something but running out of class time. Bacteria are everywhere, he said. In the soil, in the ocean, in your gut. And then, almost as an aside, tapping his pen on the projector screen: and inside almost every cell in your body, right now, there are structures that used to be bacteria themselves.

He had moved on to the next slide before anyone could ask what he meant.

Maya opened her mother's laptop, which her mother had left on the counter after all. She typed: mitochondria bacteria.

The first result stopped her.

One and a half billion years ago. A bacterium entered another cell, or was swallowed by another cell, and did not die. Did not get digested. Stayed. And the cell that swallowed it did not eject it. They kept living, one inside the other. The bacterium kept doing what it had always done, pulling energy out of oxygen, and the larger cell used that energy to do things it could never have done alone.

One and a half billion years.

Maya read the number again. She had trouble with large numbers not because she could not do the math but because she wanted to feel them, and feeling a billion and a half years was like trying to hold the ocean in her hands.

She read further. The mitochondria in her cells right now, this morning, in this kitchen, still had their own DNA. Not like the DNA in the nucleus. The mitochondrial DNA was circular, a closed loop, the same shape bacteria still used. After a billion and a half years of living inside larger cells, dividing when the cell divided, being passed from mother to daughter to daughter to daughter across every generation of every animal that had ever lived, they still carried their own tiny circular genome.

They had not become the cell. They had not merged completely. They were still, in some way she could feel but not yet say, themselves.

She looked at her hand on the keyboard. Thirty-seven trillion cells in a human body, she had read once. And inside almost every one, anywhere from a hundred to several thousand mitochondria. Each with its own circle of DNA.

She was not one organism. She was a city. She was a cooperation that had been running for longer than animals had existed, longer than plants, longer than anything she could see from this kitchen window.

The yogurt sat on the counter. Free bacteria, doing their own thing in a jar of milk. Failing, today, but free. And inside her, bacteria that had not been free for longer than there had been eyes to see with.

Did they want to leave?

That was a strange thought. She let it sit.

No. That was the wrong question. They could not leave. They had lost most of their genes over the eons, transferred them to the nucleus, become dependent. And the cell could not survive without them. The relationship had gone past the point of return so long ago that the word relationship barely fit. It was more like becoming.

Except they still had the circle. The tiny loop of their own DNA, with only thirty-seven genes left, coding for thirteen proteins. She had to look that up twice because it seemed too small and too specific to be real. Thirty-seven genes. Everything else had been given up or given over.

But those thirty-seven remained.

Mr. Reade would have moved to the next slide by now. Maya did not want the next slide.

She thought about what it meant that the mitochondria in her body right now were dividing on their own schedule. Not when her cells told them to. They divided when they needed to, like bacteria still do, pinching in half, copying that circle of DNA, each daughter getting a copy. Her cells could signal for more of them, or fewer. But the mitochondria did the dividing themselves.

Boarders. The word came to her. Not prisoners. Not pets. Not organs. Boarders. Like someone who moved in a billion and a half years ago and never left, and now you could not tell whose house it was.

She picked up the jar of failed yogurt. The bacteria in there were Lactobacillus, free-living, with their own full genomes, their own complete lives. Given the right temperature they would eat and divide and reshape the world of that jar entirely, no host needed. Her mitochondria had been like that once. Before.

Maya carried the jar to the sink and poured it out. She would try again tomorrow, get the temperature right, give those bacteria exactly what they needed to thrive.

She rinsed the jar. She held it up to the window. She pressed her palm flat against her sternum, feeling the warmth of her own body. Every bit of that warmth was mitochondria, burning oxygen, doing the one thing they had done since before there were bones, or hearts, or kitchens, or hands to feel them with.

She stood there, both of them breathing.