Origins
A Test for Life Versus Non-Life
In a new book, physicist Sara Walker argues that assembly theory can explain what life is, and even help scientists create new forms of it.
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For generations, physicists have puzzled over life. Their theories about matter and energy have helped them understand how the universe produced galaxies and planets. But physicists have struggled to understand how lifeless chemical reactions give rise to the complexity stored in our cells.
In a new book, “Life as No One Knows It: The Physics of Life’s Emergence,” out on Aug. 6, Sara Walker, a physicist at Arizona State University, offers a theory that she and her colleagues believe can make sense of life. Assembly theory, as they call it, looks at everything in the universe in terms of how it was assembled from smaller parts. Life, the scientists argue, emerges when the universe hits on a way to make exceptionally intricate things.
The book arrives at an opportune time, as assembly theory has attracted both praise and criticism in recent months. Dr. Walker argues that the theory holds the potential to help identify life on other worlds. And it may allow scientists like her to create life from scratch.
“I actually think alien life will be discovered in the lab first,” Dr. Walker said in an interview.
Dr. Walker went to graduate school planning to become a cosmologist, but life soon grabbed her attention. She was struck by how hard it was to explain life with standard physics theories. Gravity and other forces are not enough to produce the self-sustaining complexity of living things.
As a result, scientists still struggled to explain how an assortment of chemicals reacting with each other might give rise to life. Scientists had no way to measure how life-like a group of chemicals were, in the way they might use a thermometer to measure how hot something is.
“Without a concept of absolute zero, you don’t know what you’re doing,” she said.
Dr. Walker’s thinking about life took a major turn in 2015, when she went to a conference in Washington, D.C., on the origin of life. There she listened to Lee Cronin, a chemist at the University of Glasgow, describe a theory he was developing.
Dr. Cronin focused on the fact that the proteins and the other molecules that make up our bodies do not jump into existence. They have to be assembled step by step from simpler building blocks. Outside living things, molecules can also be assembled through chemical steps. In outer space, for example, carbon dioxide and other compounds can combine to produce amino acids in meteorites. Dr. Cronin set out to develop a way to compare molecules — living or not — based on how many steps they took to form.
Dr. Walker was so intrigued by this approach that she joined Dr. Cronin to further develop the theory. “Sara has an incredible ability to articulate complex problems quickly and succinctly,” Dr. Cronin said.
Over the past few years, Dr. Walker, Dr. Cronin and their colleagues have developed methods to measure the complexity of a molecule. Their test uses a number they call the assembly index: A higher index means a molecule needs more steps to assemble.
To determine the assembly index of a molecule, it’s not necessary to painstakingly craft the molecule from scratch. Instead, scientists can blast it apart with a laser and count the different kinds of fragments left behind. A molecule with a high assembly index will produce many different fragments.
In 2021, Dr. Walker and her colleagues found a striking pattern in the assembly index of hundreds of molecules they blasted apart. When they looked at nonliving molecules, such as compounds that formed in a meteorite, they never found one with an assembly index over 15. But proteins and other molecules formed inside cells scored as high as 64.
The scientists suggested that the cutoff of 15 they discovered in their experiments might be evidence of a threshold for life. Ordinary chemistry could assemble molecules only through a limited number of steps, whereas life could carry it much further.
If that’s true, the assembly index might be a new way to look for life on other planets or moons — either by sampling molecules with an interplanetary probe, or by inspecting their atmospheres with a telescope.
While Dr. Walker and her colleagues have been developing assembly theory for nearly a decade, many scientists first became aware of it last year, when the team laid it out in a high-profile essay in the journal Nature. In an accompanying paper, George F.R. Ellis, a mathematician at the University of Cape Town in South Africa, wrote about the theory in glowing terms. “Assembly theory is potentially a profound approach to evolution and its foundation in physics,” he said.
But some biologists criticized the paper’s sweeping claims and obscure language. “How did this nonsense get past peer review?” Rosemary Redfield, a microbiologist at the University of British Columbia, asked on X.
Others have developed a more nuanced opinion. “I really like the core idea of assembly theory,” said Robert Hazen, a mineralogist and astrobiologist at Carnegie Science in Washington, D.C. But he questioned whether scientists could draw a clean line between life and non-life. “I’m not sure they should set a number,” he said.
Dr. Hazen and his colleagues tried to measure the assembly index of some of the more complex minerals known to geology, such as ewingite, a golden-hued crystal that includes calcium, carbon and uranium. In January, they reported that the minerals had scored as high as 30, far above the threshold of 15 that Dr. Walker and her colleagues found in their 2021 study.
Dr. Walker said Dr. Hazen’s study was flawed. His team copied experiments that she and her colleagues had designed specifically to look at molecules. But minerals are different from molecules in some important ways. Instead of free-floating clusters of atoms, they are lattices that include some disorder in their structures.
Dr. Walker said that she and Dr. Cronin are working with their colleagues to extend the assembly theory of life. They also have a far more ambitious effort underway: to build what she calls “an origin-of-life engine in the lab.” Robots will mix inert chemicals in a vast number of combinations, looking for ones that produce more complex compounds.
Under the right conditions, the chemicals may form droplets that may be able to bootstrap themselves to a higher and higher assembly index. Above a certain threshold, they might become alive — but as a form of life we’ve never seen before.
If we do discover life on other worlds, Dr. Walker expects it will be a milestone in human history. But the idea of an origin-of-life engine has made her more interested in watching new life emerge in a lab here on Earth.
“To me, it’s more exciting as a scientist, because you can test the theory and see it happen,” she said.
Carl Zimmer covers news about science for The Times and writes the Origins column. More about Carl Zimmer
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