What exactly is “life?” Astrobiologists still have more questions than answers

Defining exactly what we mean by “life” — in all its varied forms — has long been a formidable challenge. Physicist Erwin Schrödinger wrote a book titled What is Life?” in 1944. More than 80 years later, despite all our progress in biological science (including the discovery of DNA’s structure), we still don’t have a solid answer.

None of the many suggested definitions has been widely accepted. It seems nearly every researcher in the field has a favorite one. The recent discovery, by Ryo Harada of Dalhousie University and colleagues, of a microorganism with a genome so small it contains, in essence, only enough genes for its own replication, just adds to the complication.

The archaea in question (Sukunaarchaeum mirabile) lives within another organism and appears to be something between a virus and a bacterium. By the traditional dictionary definition, “life” requires metabolism, growth, replication, and adaptation to the environment. Most scientists, therefore, don’t consider viruses alive because they can’t reproduce and grow by themselves and do not metabolize. Yet they possess a genetic mechanism that enables them to reproduce, with the help of a living cell.

Parasites also cannot reproduce without a host, but no one would say an animal such as a tapeworm is not alive. Strictly speaking, viruses fit the traditional criteria of life only part of the time, and under certain circumstances. Even more confusing is that viruses may have evolved from bacteria, which clearly are alive. So, is this a case where a living organism transformed into a non-living state by evolutionary pressures? If so, where would we draw the line between living and non-living? Is it more of a continuum than the clear dividing line many of us seem to expect? 

Life as we might find it

Because astrobiologists think not only about life as we know it but also life as we might find it, some of them gravitated toward the broad definition of life proposed by NASA: “A self-sustaining chemical system capable of Darwinian evolution.” But how helpful is that, really, for scientists working on planetary exploration missions? Imagine an astronaut or a rover on some alien planet, assigned to search for life. Should they really wait around until they can observe Darwinian evolution, which might take generations? And why does it have to be Darwinian evolution? In a world where we talk about designer babies, we can easily imagine new forms of life that would undergo Lamarckian evolution, passing on acquired characteristics rather than evolving only due to natural selection. Would they not be considered alive, too? 

As usual, when considering such questions, we’re hampered by our limited state of knowledge. We still try to define life by properties we can observe, just as 19th-century scientists defined water as a liquid that froze at 0 °C and boiled at 100 °C. Only with the discovery of the molecular theory did they come to define water as H₂O. 

There is also the N=1 problem. How can we expect to arrive at a good definition of life when we have only one example: life on Earth? Even with all its diversity, can we really be sure that Terran life is representative of all life in the Universe? Can we really exclude the possibility that we are the oddballs? 

Maybe it’s partly a linguistic question. Grammatically speaking, “life” is a noun. But in biological terms, it’s more like a verb — more of a process than a thing. Defining life is something like defining wind, which describes air in motion — a state of being rather than a specific object. Wind molecules are the same as those of air, but their dynamic state is what defines them. 

Maybe we should be consulting philosophers. Carol Cleland, a philosopher at the University of Colorado in Boulder, argues that the lack of a single, accepted definition of life is due to the lack of a comprehensive theory of living systems. As we search for life on other worlds, we should focus on anomalies, which in some instances may turn out to be alive. This seems to be an appropriate search strategy, especially when we look for life as we do not know it. Otherwise, if our search parameters are tuned too narrowly to look only for familiar and Earthly lifeforms, extraterrestrial life may be too weird for us to recognize. 

A universal checklist

Despite these dilemmas, there are certain features we should expect from all living entities. There should be some kind of boundary and disequilibrium between the organism and its external environment (if there’s no difference between you and the dirt around you, you’re probably dead!). There should be some intake of external energy to do work inside the organism. And finally, the organism should be able to reproduce itself. 

That last criterion may be the most important one. But it raises another thorny issue: Would we call a machine “alive” if it can assemble another machine (either like or different from itself) from raw materials and pass along the instructions necessary to keep repeating that fabrication process? Or should we reserve that term only for the biological lifeforms who designed these self-replicating machines, even if the designers are no longer alive? 

As we head out into space, beyond our planet of origin, we will face such conundrums, just as the advent of artificial intelligence raises (sometimes uncomfortable) questions about what we mean by sentience and even consciousness. Right now, we have no good answers. But at least we’re starting to think about the possibilities in a more sophisticated way.