Published: April 10, 2024

CU Boulder is one of five ‘spokes’ of the Leverhulme Centre for Life in the Universe, charged with exploring the nature and extent of life in the universe

For most of human history on Earth, we have looked up and out, gazing into the fathomless cosmos and asking one of our biggest questions: Is there life out there?

It’s a question that scientists, philosophers, theologians and artists have pondered for millennia, and one that guides the work of the Leverhulme Centre for Life in the Universe (LCLU) at the University of Cambridge and its “spokes”—five affiliated institutions of which the University of Colorado Boulder, led by Carol Cleland, a CU Boulder professor of philosophy, is one.

The LCLU and its five spokes—which also are University College London, ETH Zurich, Harvard University and the Center of Theological Inquiry at Princeton University—collaborate on cross-disciplinary research studying the origin, nature and distribution of life in the universe.

Carol Cleland

Carol Cleland, a CU Boulder professor of philosophy, leads the CU Boulder "spoke" of the Leverhulme Centre for Life in the Universe.

LCLU founder Didier Queloz, an astronomer and 2019 Nobel Prize winner for physics, knew of Cleland’s philosophical work on why life can’t be defined, which challenged the definition-based search strategies favored by NASA, and her alternative proposal for searching for potentially biological anomalies (vs. life per se).

He invited her to help make an application to the Leverhulme Trust (UK) to fund a new center for the study of life in the universe housed at Cambridge University.  When the application succeeded, Cleland and CU Boulder and four other researchers and their affiliated universities became “spokes” of a new Center for Life in the Universe at Cambridge University.

“I was invited by Didier because of my work on logical and philosophical problems with defining life and the role of anomalies in facilitating scientific discovery,” Cleland explains. “I wrote a book in 2019 where I argued that rather than coming up with the definition of life, which is impossible, one should be looking for potentially biological anomalies using tentative [vs. defining] criteria. My book explains the thorny logical and philosophical challenges involved in defining life.  [With regard to searching for extraterrestrial life] these problems include the infamous N=1 problem, namely, that known Earth life represents a single example of life, and that current biological theorizing about the nature of life tends to be based on what is now known to be an unrepresentative example of familiar Earth life. Logically speaking, cannot safely generalize to all life from a single unrepresentative example of life.” 

Cleland guesses that “Didier is interested in my work on the role of anomalies in scientific discovery and its application to scientific investigations into the nature, origin, and extent of life in the universe.”

Defining life

In 1995, Queloz and his research colleagues discovered the gas giant planet 51 Pegasi b, the first exoplanet discovered orbiting a Sun-like star. Though scientists had long theorized the existence of exoplanets, the discovery not only earned Queloz the 2019 Nobel Prize for physics, but also helped charge the scientific and philosophical search for life in the universe.

The LCLU was established with a $12.5 million grant from the Leverhulme Trust and charged with exploring the nature and extent of life in the universe. That includes not only working to understand whether the universe is full of life, Cleland says, but how life emerged on Earth and its potential for emergence elsewhere in the universe.

“Characteristics that scientists currently take as fundamental to life reflect our experience with a single example of life, familiar Earth life,” Cleland noted when LCLU was founded. “These characteristics may represent little more than chemical and physical contingencies unique to the conditions under which life arose on Earth. If this is the case, our concepts for theorizing about life will be misleading.”

“Philosophers of science are especially well trained to help scientists 'think outside the box' by identifying and exploring the conceptual foundations of contemporary scientific theorizing about life, with an emphasis on developing strategies for searching for truly novel forms of life on other worlds,” she adds.

Cleland, who began her career, with a degree in mathematics, as a computer scientist interested in artificial intelligence, transitioned into philosophy by considering one of the biggest questions of human existence: What is consciousness?

In pondering life and consciousness, she eventually concluded that we currently lack a scientifically fruitful, conceptual framework for understanding the nature of consciousness and switched to the difficult but, she believed, scientifically more tractable question “what is life?”

In her 2002 paper “Defining ‘Life,’” co-authored with astronomer Christopher Chyba, Carl Sagan’s last student, she developed an analogy for thinking about whether life can be defined:

“Before the invention of molecular theory, people may (or may not) have believed that ‘water’ could be precisely defined, but the best they could do in ‘defining’ it would be to discuss its sensible properties. In the absence of a compelling molecular theory, attempts at definition were doomed to interminable bickering over which of its sensible properties were essential to water’s nature.

“We suggest that current attempts to define ‘life’ face exactly the same quandary. It is possible that in the future, we will elaborate a theory of biology that allows us to attain a deep understanding of the nature of life and formulate a precise theoretical identity for life comparable to the statement ‘water is H20.’ In the absence of that theory, however, we are in a position analogous to that faced by someone hoping to understand water before the advent of molecular theory by ‘defining’ it in terms of the observable features used to recognize it.”

Generalizing to all life in the universe from a single example

In her book, Cleland emphasizes that understanding—rather than defining—life must necessarily focus on discovering forms of life descended from alternative origins of life and that the best way to do this is to hunt for potentially biological anomalies.

View of southern North America from space

“Why go looking for life like our form of life? Our form of life emerged on a particular planet, Earth, under a set of distinctive physical and chemical conditions that may not generalize to other life bearing planet," Carol Cleland, CU Boulder professor of philosophy, says.

“Why go looking for life like our form of life? Our form of life emerged on a particular planet, Earth, under a set of distinctive physical and chemical conditions that may not generalize to other life bearing planet," Cleland says.

She argues that recent laboratory work “that claims that we are on the verge of creating life in a test tube has limited application for telling us much about either how life originated on Earth or the intrinsic nature of life.”

As an analogy, she gives the example of quartz, which can form in hydrothermal pools by precipitation or in cooling magma by crystallization or be made in yet another way via industrial processes.

“Just as there are a variety of different ways of producing quartz there may be a variety of different ways for producing life, under natural and artificial conditions,” she says, adding that it is important to distinguish questions about the origin of life from questions about the nature of life.

“Long before the discovery of the molecular composition of quartz (SiO2), which depended upon the development of the periodic table in the 19th century, people knew that quartz is produced in hydrothermal vents. Analogously, discovering a way of making life artificially in a lab may not tell us very much about the general nature of life, especially if our theorizing is based on a defective conceptual framework for understanding life.”

Based on these considerations, Cleland recommends searching for potentially biological anomalies. “We just don’t know how different life could be from familiar Earth life or the variety of different chemical and physical conditions under which life might emerge. The best way to search for life as-we-don’t-know-it is thus to look for phenomena that ‘shouldn’t be there’, that is, phenomena resembling familiar Earth life while also differing from it in ways that we wouldn’t expect a nonliving system to exhibit. Such phenomena are anomalous in a special sense, namely, a potentially biological sense, and hence are worthy of further investigation for the possibility of an unfamiliar form of life, as opposed to being dismissed as nonliving because they fail to conform to a favored, earthcentric, definition of life.”

These ideas, Cleland says, dovetail with the four themes that LCLU scientists and philosophers pursue: identifying the chemical pathways to the origins of life; characterizing the environments on Earth and other planets that could act as the cradle of prebiotic chemistry and life; discovering and characterizing habitable exoplanets and signatures of geological and biological evolution; and refining our understanding of life through philosophical and mathematical concepts.

Cleland says she hopes to expand CU Boulder’s role as a LCLU spoke by establishing partnerships across campus, which could lead to enhanced collaboration with researchers around the world.

“We are one planet that we know is actually occupied by life,” she says. “We don’t know if we’re unique in our solar system, and since almost all stars have planets around them, there are likely to be other forms of life. And unless life is a scientific miracle—and scientific miracles almost always turn out to be anomalies thar are later explained by novel approaches—then there is other life in the universe.”

Top image: James Webb Space Telescope NIRCam Image of the “Cosmic Cliffs” in Carina Nebula (Photo: NASA)

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