Test of life and non-life

In a new book, physicist Sarah Walker argues that assembly theory could explain what life is and even help scientists create new forms of it.

For generations, physicists have puzzled over how life works. Their theories about matter and energy helped them understand how the universe gave birth to galaxies and planets. But physicists still struggle to understand how lifeless chemical reactions give rise to the complexity stored in our cells.

In the new book “Life as No One Knows It: The Physics of the Origin of Life“, published August 6, Sarah Walker, a physicist at Arizona State University, offers a theory that she and her colleagues believe could give life meaning. “Assembly theory“, as they call it, looks at everything in the universe in terms of how it was assembled from smaller parts. Life, scientists say, arises when the universe finds a way to create exceptionally complex things.

The book comes at an opportune time, as the theory of assembly has attracted both praise and criticism in recent months. Dr Walker argues that the theory could help us find life on other planets. And it could allow scientists like her to create life from scratch.

“I think alien life will be discovered in the lab first,” Dr. Walker said in an interview.

Dr. Walker entered graduate school planning to become a cosmologist, but soon her attention was drawn to life. She was struck by how difficult it was to explain life using standard physical theories. Gravity and other forces were not enough to create the self-sustaining complexity of living things.

As a result, scientists were unable to explain how a set of chemicals reacting with each other could lead to life. Scientists had no way to measure how life-like a group of chemicals was, the way a thermometer measures how hot something is.

“Without the concept of absolute zero, you don't know what you're doing,” she says.

Dr. Walker's thoughts on life took a major turn in 2015 when she went on a conference to Washington, D.C., on the origins of life. There she listened as Lee Cronin, a chemist at the University of Glasgow, described the theory he was developing.

Dr. Cronin emphasized that proteins and other molecules that make up our bodies do not form overnight. They must be assembled, step by step, from simpler building blocks. Outside of living things, molecules can also be assembled through chemical processes. For example, in outer space, carbon dioxide and other compounds can combine into meteorites to make amino acids. Dr. Cronin set out to develop a way to compare molecules—living or nonliving—by the number of steps it took to form them.

Dr. Walker was so intrigued by the approach that she joined Dr. Cronin to develop the theory further. “Sarah has an incredible ability to articulate complex problems quickly and succinctly,” says Dr. Cronin.

Over the past few years, Dr. Walker, Dr. Cronin and their colleagues have developed methods to measure a molecule’s complexity. Their tests use a number they call the assembly index: a higher index means the molecule requires more steps to assemble.

To determine the assembly index of a molecule, scientists don’t have to painstakingly create it from scratch. Instead, they can destroy it with a laser and count the number of fragments that remain. A molecule with a high assembly index will produce many different fragments.

In 2021, Dr. Walker and her colleagues discovered a striking pattern in the assembly index of the hundreds of molecules they tore apart. When they looked at nonliving molecules, such as those formed in meteorites, they couldn't find a single one with an assembly index higher than 15. But proteins and other molecules formed inside cells scored as high as 64.

The scientists suggested that the number 15 they discovered in their experiments may indicate a threshold for life. Conventional chemistry can only assemble molecules through a limited number of steps, while life can go much further.

If so, the assembly index could be a new way to search for life on other planets or moons, either by sampling molecules with a spacecraft or by studying their atmospheres with a telescope.

Although Dr. Walker and her colleagues have been developing the assembly theory for nearly a decade, many scientists first became aware of it last year, when the team outlined it 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 glowingly of the theory. “Assemblage theory is a potentially profound approach to evolution and its foundations in physics,” he said.

But some biologists criticized the paper's sweeping claims and unclear wording. “How did this nonsense get past peer review?” asked Rosemary Redfield, a microbiologist at the University of British Columbia, on the social media site X.

Others take a more measured view. “I really like the basic idea of ​​assembly theory,” said Robert Hazen, a mineralogist and astrobiologist at Carnegie Science in Washington, D.C., but he doubts that scientists can draw a clear line between life and nonlife. “I’m not sure they should settle on any specific number,” he said.

Dr Hazen and his colleagues attempted to measure the assembly index of some of the most complex minerals known in geology, such as evingite, a golden-coloured crystal made up of calcium, carbon and uranium. In January, they reportedthat these minerals scored 30, which is well above the 15 threshold that Dr. Walker and her colleagues found in their 2021 study.

Dr Walker said Dr Hazen's research was flawed. His team had copied experiments she and her colleagues had designed specifically to study molecules. But minerals differ from molecules in some important ways. Instead of free-floating clusters of atoms, they are lattices with some disorder in their structure.

Dr Walker says she and Dr Cronin and colleagues are working to expand the theory of life assembly. They also have a more ambitious goal: to build what she calls a “life-engine” in the lab. Robots will mix inert chemicals in a huge number of combinations, trying to find those that lead to more complex compounds.

Under the right conditions, chemicals can form droplets capable of self-replication with ever-higher assembly indices. Once they reach a certain threshold, they can become alive—in a form we have never seen before.

If we find life on other planets, Dr Walker believes it will be a milestone in human history. But the idea of ​​how life originates has made her keenly interested in watching new life emerge in the lab here on Earth.

“For me as a scientist, it's more interesting because you can test the theory and see it happen,” she said.