In 2019 they set out to find it by carrying out a survey of scientists and other scholars. They put together a list of things including people, chickens, Amazon mollies, bacteria, viruses, snowflakes, and the like. Next to each entry the Lund team provided a set of terms commonly used to talk about living things, such as order, DNA, and metabolism.
The participants in the study checked off all the terms that they believed to apply to each thing. Snowflakes have order, for example, but they don’t have a metabolism. A human red blood cell has a metabolism but it contains no DNA.
The Lund researchers used a statistical technique called cluster analysis to look at the results and group the things together based on family resemblances. We humans fell into a group with chickens, mice, and frogs — in other words, animals with brains. Amazon mollies have brains, too, but the cluster analysis put them in a separate group close to our own. Because they don’t reproduce by themselves, they’re set a little apart from us. Further away, the scientists found a cluster made up of brainless things, such as plants and free‐living bacteria. In a third group was a cluster of red blood cells and other cell‐like things that can’t live on their own.
Furthest away from us were things that are commonly not considered alive. One cluster included viruses and prions, which are deformed proteins that can force other proteins to take their shape. Another included snowflakes, clay crystals, and other things that don’t replicate in a lifelike way.
The Lund researchers found that they could sort things pretty well into the living and the nonliving without getting tied up in an argument over the perfect definition of life. They propose that we can call something alive if it has a number of properties that are associated with being alive. It doesn’t have to have all those properties, nor does it even need exactly the same set found in any other living thing. Family resemblances are enough.
One philosopher has taken a far more radical stand. Carol Cleland argues that there’s no point in searching for a definition of life or even just a convenient stand‐in for one. It’s actually bad for science, she maintains, because it keeps us from reaching a deeper understanding about what it means to be alive. Cleland’s contempt for definitions is so profound that some of her fellow philosophers have taken issue with her. Kelly Smith has called Cleland’s ideas “dangerous.”
Cleland had a slow evolution into a firebrand. When she enrolled in the University of California, Santa Barbara, she started off studying physics. “I was a klutz in the lab, and my experiments never turned out right,” she later told an interviewer. From physics she turned to geology, and while she liked the wild places that the research took her to, she didn’t like feeling isolated as a woman in the male‐dominated field. She discovered philosophy in her junior year and was soon grappling with deep questions about logic. After graduating college and spending a year working as a software engineer, she went to Brown University to earn a Ph.D. in philosophy.
In graduate school Cleland mulled space and time, cause and effect.
When Cleland finished grad school, she moved on to subjects that were easier to talk about at dinner parties. She worked at Stanford University for a time, contemplating the logic of computer programs. She then became an assistant professor at the University of Colorado, where she remained for the rest of her career.
In Boulder, Cleland turned her attention to the nature of science itself. She examined how some scientists, like physicists, could run experiments over and over again, while others, like geologists, couldn’t replay millions of years of history. It was while she was reflecting about these differences that she learned about a Martian rock in Antarctica that was posing a philosophical conundrum of its own.
[The Martian rock, a meteorite designated Allan Hills 84001, was examined in 1996 by a NASA team led by David McKay. They reported seeing signs of ancient life in it, including microbial fossils, but most scientists dismissed the evidence as too ambiguous to be credible.]
A lot of the arguments over Allan Hills 84001 had less to do with the rock itself than with the right way to do science. Some researchers thought the NASA team had done an admirable job of studying it, but others thought it was ridiculous to conclude from their findings that the meteorite might contain fossils. The planetary scientist Bruce Jakosky, one of Cleland’s colleagues at the University of Colorado, decided to organize a public discussion where the two sides could air their views. But he realized that judging Allan Hills 84001 required more than running some experiments to measure magnetic minerals. It demanded thinking through how we make scientific judgments. He asked Cleland to join the event, to talk about Allan Hills 84001 as a philosopher.
What started as a quick prep for a talk turned into a dive into the philosophy of extraterrestrial life. Cleland concluded that the fight over Allan Hills 84001 sprang from the divide between experimental and historical sciences. The critics made the mistake of treating the meteorite study as experimental science. It was absurd to expect McKay’s team to replay history. They couldn’t fossilize microbes on Mars for 4 billion years and see if they matched Allan Hills 84001. They couldn’t hurl a thousand asteroids at a thousand copies of Mars and see what came our way.
Cleland concluded that the NASA team had carried out good historical science, comparing explanations for the ones that explained their evidence best. “The martian‐life hypothesis is a very good candidate for being the best explanation of the structural and chemical features of the martian meteorite,” she wrote in 1997 in the Planetary Report.
Cleland’s work on the meteorite impressed Jakosky so much that he invited her in 1998 to join one of the teams at NASA’s newly created Astrobiology Institute. In the years that followed, Cleland developed a philosophical argument for what the science of astrobiology should look like. She informed her ideas by spending time with scientists doing different kinds of research that fit under the umbrella of astrobiology. She traveled around the Australian outback with a paleontologist searching for clues to how giant mammals went extinct 40,000 years ago. She went to Spain to learn how geneticists sequence DNA. And she spent a lot of time at scientific meetings, roaming from talk to talk. “I felt like a kid in a candy store,” she once told me.
But sometimes the scientists Cleland spent time with set off her philosophical alarms. “Everybody was working with a definition of life,” she recalled. NASA’s definition, only a few years old at that point, was especially popular.
As a philosopher, Cleland recognized that the scientists were making a mistake. Their error didn’t have to do with determinate attributes or some other fine philosophical point understood only by a few logicians. It was a fundamental blunder that got in the way of the science itself. Cleland laid out the nature of this mistake in a paper, and in 2001 she traveled to Washington, D.C., to deliver it at a meeting of the American Association for the Advancement of Science. She stood up before an audience made up mostly of scientists, and told them it was pointless to try to find a definition of life.
“There was an explosion,” Cleland recalled. “Everyone was yelling at me. It was really amazing. Everyone had their pet definitions and wanted to air them. And here I told them the whole definition project was worthless.”
Fortunately, some people who heard Cleland talk thought she was onto something. She began collaborating with astrobiologists to explore the implications of her ideas. Over the course of two decades she published a series of papers, culminating in a book, The Quest for a Universal Theory of Life.
The trouble that scientists had with defining life had nothing to do with the particulars of life’s hallmarks such as homeostasis or evolution. It had to do with the nature of definitions themselves — something that scientists rarely stopped to consider. “Definitions,” Cleland wrote, “are not the proper tools for answering the scientific question ‘what is life?’”
Definitions serve to organize our concepts. The definition of, say, a bachelor is straightforward: an unmarried man. If you’re a man and you’re unmarried, you are — by definition — a bachelor. Being a man is not enough to make you a bachelor, nor is being unmarried. As for what it means to be a man, well, that can get complicated. And marriage has its own complexity. But we can define “bachelor” without getting bogged down in those messy matters. The word simply links these concepts in a precise way. And because definitions have such a narrow job to do, we can’t revise them through scientific investigation. There is simply no way that we could ever discover that we were wrong about the definition of a bachelor as being an unmarried man.
Life is different. It is not the sort of thing that can be defined simply by linking together concepts. As a result, it’s futile to search for a laundry list of features that will turn out to be the real definition of life. “We don’t want to know what the word life means to us,” Cleland said. “We want to know what life is.” And if we want to satisfy our desire, Cleland argues, we need to give up our search for a definition.
From the book Life’s Edge: The Search for What It Means to Be Alive by Carl Zimmer, published by Dutton, an imprint of Penguin Publishing Group, a division of Penguin Random House LLC. Copyright © 2021 by Carl Zimmer.
