Five questions for Carol Cleland

Philosophy professor delves into origins of life, ‘shadow biosphere’


If researchers were to find “life” that is unlike anything we currently recognize as a living organism, would the discovery be understood? It’s a question that has been debated for years and one that has intrigued Carol Cleland, a professor of philosophy at the University of Colorado Boulder.

She has written on the nature and origins of life in several manuscripts and a book, “The Quest for a Universal Theory of Life: Searching for life as we don’t know it.” She also is affiliated with the NASA Astrobiology Institute and is a member of CU’s Center for Astrobiology. Most recently, she was named director of the new Center for the Study of Origins.

Her areas of interest include the philosophy of science, the philosophy of logic and metaphysics. She loves to travel, and whenever she’s invited to speak at workshops or conferences, she’ll extend her trip a day or two to explore the area.

“I particularly like historical monuments, especially medieval history. I was in Scandinavia a couple of years ago and spent a lot of time visiting old Viking ruins and churches and homes,” said Cleland, who also loves natural history, especially paleontology, and is “always trying to talk my way onto an expedition.”

1. Your CV notes that you graduated with a bachelor’s degree in mathematics. How did your career path wind from math to philosophy?


I view myself as an accidental math major. Originally, I was a physics major, but I didn’t like labs. I was a klutz in the lab and my experiments never turned out right, and I was more interested in theoretical science. I tried a number of other fields, including geology. I liked geology because I liked getting out in the field and basically scrounging around looking for stuff and collecting stuff. But it turned out there were no women in geology, and being in the field was isolating and very awkward.

I was trying to figure out what I wanted to do, and I ended up taking a lot of science courses. I took chemistry and biology classes. In my junior year, I discovered philosophy and I felt that was what I wanted to do. It turned out I couldn’t get a degree in philosophy without taking undergraduate classes. I had taken all of the upper-division classes, but I didn’t have the first-year courses on the history of philosophy. I’d been basically taking courses for five years on overload: 180 units were required and I had something like 260 units. I only needed one course to get a degree in mathematics because I had taken all those courses for physics and several others on foundations of mathematics and mathematical logic.

After graduation, I worked as a software engineer in a small company. I did that for about a year, and when my then-husband got his Ph.D. and received a position in Massachusetts, I ended up at Brown University pursuing philosophy, with an emphasis on metaphysics, because I was interested in space, time and causations.

I earned my Ph.D. in philosophy; my dissertation was on causation. I thought I would spend a year writing my dissertation and having a baby, but a teaching position opened up at Wheaton College, and I went there. Between having a new baby and dealing with teaching college for the first time, the completion of my dissertation was delayed by two years. When I found out the position was non-tenured, I applied for, received and accepted a postdoctoral fellowship in both philosophy and computer science at Stanford at the Center for Study of Language and Information. In 1986, I applied for a job at CU and got the job.

2. How did you become affiliated with the NASA Astrobiology Institute (NAI)?

I was doing metaphysics, philosophical logic and computability theory in the mid-’90s at CU. There was a breaking news story about the alleged discovery of fossilized Martian life in an ancient Martian meteorite that had been discovered in the Antarctic. What was found inside this meteorite was puzzling both chemically and structurally. It became very controversial. Bruce Jakosky (Laboratory for Atmospheric and Space Physics) put together a panel discussion and invited me to be part of it. I was teaching the philosophy of science at the time.

I read the articles about the discovery and realized there was a fundamental confusion in the debate. People were saying the researchers hadn’t proved that it was what they had suggested. Science can’t prove a hypothesis, which, no matter how well-established, is always tentative and subject to revision. You can falsify a generalization, but you can’t falsify a claim about the origins of particular structures in a single rock, and these structures were very old, dating back to about 3.8 billion years ago. The search into Earth’s past and the origins and development of our planetary system, as well as other planetary systems and the universe as a whole, is about particular events – events that typically happened a very long time ago

I was intrigued. The philosophical literature hadn’t paid much attention to the practices of historical scientists. It had been assumed that all of science fits a one-size-fits-all model based on experimental science. I gave my talk and a couple of scientists told me they had learned something. A year later, Bruce Jakosky told me he was writing a proposal to the new NASA Astrobiology Institute (NAI), which was created based on the controversy over the meteorite (dubbed ALH 84001). NAI had put out a call for diverse teams of researchers and Bruce asked me to join his team. Over the next 10 years, I was a co-Investigator on two Boulder teams funded by the institute.

I’m currently a co-Investigator on a team, led by Robert Hazen of the Carnegie Institution of Washington’s Geophysical Laboratory, that is looking at the use of biogenic minerals as biosignatures for searching for extraterrestrial life. We recently received funding from the John Templeton Foundation. I am also a core collaborator on a CU-based NAI team, “Rock Powered Life,” led by Alexis Templeton.

3. What does your current research entail?

Since the mid-’90s, I have focused on two separate prongs of research. One is the nature of life and the fact that we don’t have a universal theory of life. We have universal theories of physics: general relativity and quantum mechanics. These theories are tentative, but we don’t have the same type of theories for life.

Our current understanding of life is very Earth-centric. We know there are ways in which life on Earth could be different at the molecular level. We know, for instance, that proteins could be modestly different in composition — known Earth life constructs proteins from only 20 out of more than 100 amino acids found in nature — but we don’t know how different life could be from familiar life. In a sense, we have a single example of life, but we don’t know if it is representative or unrepresentative, and if it is unrepresentative, we don’t know in what ways it is unrepresentative. So we’re in a real pickle trying to generalize from life as we know it to all of life.

My book is about that problem. What I argue is that we shouldn’t define life based on a single sample. Because definitions supply necessary and sufficient conditions for the categories that they define, we would completely set the boundaries of what counts as a living thing. But we are in no position to do that. If we use a definition of life, we would automatically exclude life that differs from ours, especially if we are searching for extraterrestrial life.

We want to remain open to being able to discover a system we’re uncertain about. I call this the search for anomalies. I suggest using tentative criteria in order to search for extraterrestrial life rather than using defining criteria. I point out in the book that scientific theories don’t give you definitions for core concepts; they function very differently than commonly supposed.

4. You coined the phrase “shadow biosphere.” Please explain what you mean by this.

I coined the term “shadow biosphere” when I spent five months at the Centro de Astrobiología in Spain while on sabbatical in 2002-2003. I went there because they were affiliated with the European Space Agency and had a team of scientists working on the problem of how to search for extraterrestrial life on planets and moons in our solar system.

I was interested in learning about biochemical and molecular methods for doing this. I asked them how they would detect microorganisms in a sample of Earth soil because most astrobiologists believe that microbial life is probably fairly common in the universe, but big organisms – animals and plants – are very rare. The scientists at the center listed things like microscopy with staining techniques, cultivation and metagenomic methods.

I was stunned because it occurred to me that they would never be able to recognize alien forms of microbial life if they encountered it. How would you explore for life, not as we know it, but as we don’t know it? I argued that you would need a collection of diverse tentative criteria, which would raise the suspicion that a system exhibiting provocatively lifelike characteristics but nonetheless differing in ways that one wouldn’t expect in terrestrial life, might be a truly novel form of life. It also occurred to me that we might have microorganisms on Earth that are undetected for the very same reasons. I called these hypothetical microbes a “shadow biosphere" because, like all organisms, they would leave traces (shadows) in their environments, extracting energy and material for metabolic purposes and releasing waste products back into their environments.

Another theme in my forthcoming book is that the microbial world doesn’t fit well into concepts and principles that have been traditionally used to explore complex multicellular organisms like plants and animals. I recommend that we consider rethinking biology from a more microbial perspective instead of trying to fit them into categories and principles applying to large, complex organisms. That’s the central thesis of the book.

5. What is the Center for the Study of Origins and how will it operate? How did it come about?

The other prong of my current research is focused on natural history. What methodologies are used by historical natural scientists who are studying paleoclimate or Darwinian evolution or the beginning of life on Earth? Can these methods be rationally justified, despite the fact that, as mentioned earlier, they differ in important ways from those of experimental scientists? Are the methods of experimental science really the paragon for all of science? I have written extensively on this topic. That’s where the Center for the Studies of Origins (CSO) comes in.

CSO grew out of my sudden realization, while a member of CU Boulder’s Grand Challenge Steering Committee, that almost every discipline, from the natural sciences to the social sciences to the humanities, studies origins, but nobody has talked much about how different disciplines explore the past and the similarities and differences among different approaches. The center is devoted to exploring the study of the past through the lenses of different academic disciplines.

When you study the past, you don’t study generalizations as much as you study particular cases. Take the collapse of the Chaco Canyon Anasazi culture. What caused this particular culture to collapse? The collapse of every culture is a unique event. There may be similarities, but there will also be important differences, and these similarities and differences will vary from culture to culture. It is unlikely that the culture of ancient Egypt collapsed for the same reasons as the Chaco Canyon Anasazi culture.

Our theme for next year is climate and human history. We’ll bring together social scientists, natural scientists and humanists and hold several workshops. One of the issues that will be discussed is the Little Ice Age. What caused it? How did it affect climate around the world, and how did it affect civilizations and cultures around the world, including the Anasazi? Another question is whether we see influences in the intellectual development of Europe, changes in the development of philosophy and influences from the Little Ice Age on political and cultural institutions. Bringing social scientists, humanists and natural scientists together may help us trace historical connections that might not have been noticed before.

Oxford University Press has invited us to submit a proposal for a handbook on origins. Oxford also expressed an interest in funding volumes for our yearly themes. We anticipate that papers jointly authored by scientists and scholars will come out of these workshops and meetings. A freshman seminar is also planned, and will start this fall. It will be team-taught by CSO faculty as a survey of research on origins across the disciplines. We’re hoping that other courses come out of this, but because the center is so focused on interdisciplinary work, a course would be hard to fit into any particular department, which is required by the new core curriculum.

Our plans for the center are exciting because we truly are trying to cross three broad areas of intellectual inquiry and that’s something we don’t do enough, especially as we emphasize STEM more and the humanities less.