What constitutes life, and how did it originate?

Life likely emerged from nonliving matter aided by asteroids striking a turbulent, chaotic Earth.

Natural selection may not be the primary driver of adaptive evolution.

Self-organizing molecules slowly acquired memory, agency, and, eventually, consciousness.

I watch Rufus, a friend’s dog, chew enthusiastically on a rubber bone. I know the dog is alive, and his toy is not. I know that intuitively. But can I prove it scientifically? Can science parse the distinction between life and inert matter?

To be considered alive, does something have to be able to reproduce, move, grow, and process energy? Any definition along these lines is riddled with exceptions. For instance, is a virus alive? While viruses do evolve, they don’t replicate on their own. They use the host’s tissues to make copies of themselves.

In Star Trek: The Next Generation, the definition of life as something that “absorbs compounds from its environment,” “excretes waste,” and “grows” is famously challenged by characters like Data to show its limitations. Fire or crystals, for example, also consume nutrients, which are energy, “excrete” waste, and grow, yet are not considered alive. Throughout Star Trek, the ultimate definition of life, particularly when discussing androids or artificial intelligence, often centers on sentience, consciousness, and self-awareness rather than just biological or metabolic functions. However, these are abstract concepts that are difficult if not impossible to define scientifically.

One of science’s most enduring riddles is what constitutes life and how cells that metabolize, replicate, and adapt emerged from matter that was once inert. Many scientists speculate that the early bombardment of Earth by asteroids set in motion a cascade of chemical and environmental changes, culminating in the appearance of the “last universal common ancestor,” endearingly called LUCA.

After impacting Earth, materials from asteroids may have undergone further transformation in hydrothermal settings before being transferred to surface environments where cycles of drying, re-wetting, and ultraviolet exposure concentrated them, packaged them into primitive membranes, and allowed rudimentary selection to occur. Evolutionary biologists believe that organisms with cellular nuclei appeared around 1.8 billion to 2.7 billion years ago. Then there was the transition to multicellular organisms around 1.6 billion to 2 billion years ago, the abrupt diversification of body forms in the Cambrian explosion 540 million years ago, and the appearance of central nervous systems around 520 million years ago.

In the last few decades, major scientific breakthroughs across disciplines, from chemistry and biology to astrobiology, geology, and even philosophy, have shed new light on the evolution and definition of life. Several recent international meetings have addressed these issues.

A few months ago, in Kyoto, Japan, the ALIFE 2025, Artificial Life Conference, was held. The theme was “Ciphers of Life,” exploring information, computation, emergence, and what counts as life in artificial and natural systems. Mike Levin of Tufts University was one of the many distinguished speakers. In his talk, he emphasized the importance of acknowledging and, in fact, utilizing the goal-directedness of life as a critical step in the maturation of the life sciences. His focus on agency in living systems seems to me closely related to the dominant theories about life on Star Trek. Kudos, Star Trek.

At the Oxford 2026 Evolution Conference last month, Raju Pookottil gave a presentation called “BEEM: Biological Emergence-based Evolutionary Mechanism: How Species Direct Their Own Evolution.” Pookottil, an engineer and a self-taught evolutionary scientist, advanced the heretical idea that natural selection may not be the primary driver of adaptive evolution. Rather, he said, “Organisms may direct their own evolutionary trajectories.”

In this view, organisms assess challenges, devise solutions, and transmit them across generations. Ants act as agents within colonies; complex protein networks perform analogous roles within cells. Genes, he argues, are tools rather than tyrants. Organisms can regulate gene expression and repair many accidental changes.

Mutations, Pookottil suggests, are often not random, and when they are, they are frequently corrected within a few generations. Many studies show that mutations are uncommon, and in medicine, the mutations we most often notice are often harmful (disease and cancer).

A substantial amount of ink and lately, electricity, has been spent by scholars in efforts to pin life down with a tidy definition. Following a suggestion by Carl Sagan, NASA adopted a definition of life as “a self-sustaining chemical system capable of Darwinian evolution.” The word “system” was chosen deliberately, acknowledging that components of living systems—cells, viruses, even organisms—may not individually exemplify life in isolation. “Self-sustaining” was meant to exclude entities that require constant external intervention to exist. “Darwinian evolution” served as shorthand for replication with heritable variation and differential fitness.

From the outset, this process required mechanisms that allowed molecules to change adaptively over time. As Pookottil suggested, there are many other processes besides Darwinian evolution responsible for the long ascent from matter to Homo sapiens.

The transition from non-life to life was a gradual process involving planetary habitability, prebiotic chemistry, molecular self-replication, self-assembly, and the emergence of membranes aided by asteroids striking a turbulent, chaotic Earth. What began as self-organizing molecules slowly acquired memory, agency, and, eventually, consciousness.

From the cacophony of the voiceless early Earth, over vast stretches of time and space, a few notes gradually arose, forming a melody played on various instruments in various combinations until it triumphed in a symphony: life.

This article also appears on The Globe and Mail.

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