Lucas Mix: What is Life?

I would like to kick off our discussion of the societal implications of astrobiology by asking a fundamental question. What is it about Earth life that motivates the search for life “out there”? With a play on the words from Psalm 8:4, I want to ask “what is life that you are mindful of it – alien life that you look for it?” Astrobiologists seek to understand the origin, extent, and future of life in the universe. For a long time, I asked the question of how we define life. If you’re looking for life in space, what are you looking for? None of the answers satisfied me. Worse yet, the more specific the answers became, the less useful they seemed to be (Cleland 2012, Mix 2015). I have come to believe that we all value life, but we think of it in a number of very different ways. Astrobiology promises to give us deeper understanding of life, which makes it both hopeful and challenging – hopeful because we will see more clearly, challenging because we will come face to face with foundational beliefs. We will discover that not all of us understand the world in the same way.

What is life that you are mindful of it – alien life that you look for it? The discussion can help us make astrobiology more successful; if we know what we are looking for, we will know when we have found it, and how to look. Unfortunately, we do not know exactly what life is and astrobiologists might be tempted to ask, “Why does life have to be so hard?” Indeed, many astrobiologists have given up on the question “what is life?” believing we can do great science without knowing. We can look for interesting things related to life and continue to make discoveries. This stepping back from the problem has allowed us to talk more meaningfully about some of the interesting aspects of life amenable to scientific investigation. Thus, we often turn to astrobiology as the study of the origin, extent, and future of several features of life. The “NASA definition” holds life to be “a self-sustaining chemical system capable of Darwinian evolution.” (Joyce 1994; it is not held by NASA, but was produced at a NASA conference and labelled the “NASA definition” only 15 years later, Benner 2010). This highlights two interesting aspects of life: evolution – as adaptation produced by inheritance, variation, and natural selection – and metabolism – as a system of chemical reactions with feedback loops that maintain an organism out of equilibrium with its environment. Starting with those two concepts, large swaths of astrobiology (and synthetic biology and origin-of-life research) have turned more concretely to investigating the origin, extent, and future of RNA (or other information encoding molecules), proteins (or other metabolic workhorses), and cell membranes (or other metabolic boundaries). The “RNA World” hypothesis suggests that the earliest life (or proto-life) had only RNA, without proteins or membranes and often comes packaged with the idea that the most interesting property of life is evolution and the nominal mechanism of inheritance is RNA. Other researchers have focused on information, complexity, or feedback loops as more mathematical properties of life.

I fully support this approach to astrobiology as a science. I think we have identified the most interesting aspects of life…that are scientifically tractable. At the same time, I want to recall other aspects of life that motivate our quest. We are not always driven by scientifically tractable questions and we may do ourselves a disservice by conflating life – as something that evolves – with life – as one of our fundamental categories for understanding the universe. We must ask whether these other aspects of life are open to other types of inquiry. “Life” plays a crucial role in theology, ethics, medicine, and law, just to name four. Astrobiology impacts these inquiries as well.

What is life that you are mindful of it – alien life that you look for it? I want to suggest three common understandings of life that may color our concerns – that may impact how astrobiology is done, talked about, and received.

First, life goes hand in hand with our ideas about health and wholeness, from biological well being to “the good life” to “spiritual life.” We speak of “life” so often, it can be hard to tell where the plain meaning ends and metaphor begins. “Life of the mind” and “mental health” seem to be outgrowths of biological life, and yet people say they have never felt so alive as when they face sickness and death. In the Bible, Jesus says “those who want to save their life will lose it, and those who lose their life for my sake will find it” (Luke 17:33, NRSV). We frequently speak as though one kind of life – higher life or deeper life – trumps simple biological existence. At first this may seem to be a contrast between science and the humanities, and yet even in medicine and evolutionary biology we have no trouble with the idea that the life of the organism is more important than the life of its cells. The cells may be sacrificed for the good of the whole. And, likewise, we speak of the organism having died, even when many of the cells are still alive. This hierarchy and normativity of the whole cannot be completely disentangled, even in science.

Second, we think of ourselves when we speak of life. In my book, Life in Space: Astrobiology for Everyone, I point out the notorious imprecision of both “intelligence” and “life,” suggesting that we often use them to mean “very much like us” and “somewhat like us.” Humans are not only necessary members of the group “living things”; we are the type species. Particularly in astrobiology, we must always keep in mind the difference between life and Earth-life, between a life-as-it-is and life-as-we-know-it. We cannot generate new knowledge about life without reimagining ourselves.

Finally, in Christian and Jewish scriptures, life means like God, or at least possessed of God in a unique way. In the book of Genesis all animals, including humans, become living beings by having God’s breath within them. In both Greek and Hebrew thinking, life comes through breath and heat moving within a body, often connected to the circulation of blood. Scriptures identify that breath with the Spirit of God. Nor are Judaism and Christianity the only faiths to privilege life with a particular connection to transcendence. Hinduism and Buddhism, for example, speak of all life (or at least all sentient life) having a reality that transcends an individual incarnation.

What is life that we are mindful of it – alien life that we look for it? Astrobiology reveals just how deeply we are invested in the concept of life. It gives us an opportunity to reflect on our own life, what it means to live fully, and how we think about living in community – or living alone. I, for one, hope that life “out there” will help us discover the meaning of life here. The search has already told us more about ourselves, and the world we inhabit, than we ever could have asked or imagined.



Benner, SA. 2010. Defining life. Astrobiology 10: 1021–1030.

Cleland, C. 2012. Life without definitions. Synthese 185:125–144.

Joyce, GF. 1994. Foreword. In Origins of Life: The Central Concepts. Eds. DW Deamer and GR Fleischaker, Boston: Jones and Bartlett.

Mix, LJ. 2009. Life in Space: Astrobiology for Everyone. Cambridge, MA: Harvard.

Mix, LJ. 2015. Defending definitions of life. Astrobiology. 15(1): 15-19.


About the Author: 

Lucas Mix studies concepts of life as approached in biology, history of science, and theology. He holds a Ph.D. in evolutionary biology from Harvard and serves as a priest in the Episcopal Church. Mix has been involved with the NASA Astrobiology Institute since 1999, working on interdisciplinary communication, and producing introductions for specialists (The Astrobiology Primer) and the public (Life in Space: Astrobiology for Everyone). He works as a college chaplain, lecturer and research scholar and specializes in facilitating communication between fields.

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