RNCSE 23 (3-4)

Reports of the National Center for Science Education
Volume: 
23
Issue: 
3–4
Year: 
2003
Date: 
May–August
Articles available online are listed below.

Astrobiology and the Search for Alien Life

Reports of the National Center for Science Education
Title: 
Astrobiology and the Search for Alien Life
Author(s): 
David Morrison
Senior Scientist, NASA Astrobiology Institute
Volume: 
23
Issue: 
3–4
Year: 
2003
Date: 
May–August
This version might differ slightly from the print publication.
In another paper in this issue (p 15), I described astrobiology and discussed the ways this field relates to the study of the origin of life on earth. In this paper, I touch on other aspects of astrobiology: studying life in extreme conditions on earth and the initial efforts to search for evidence of life beyond our planet.

In the previous paper, I noted the difficulty of understanding the origin of life, since there is almost no direct evidence from either geological or genetic studies on the first half-billion years of terrestrial history. Here we pick up the story after the emergence of simple cells that already have DNA and RNA and thus are subject to selection as they reproduce and evolve.

Evolution on earth: The highlights

Astrobiology brings a multidisciplinary perspective to biology, as well as to comparing our planet with other potential abodes of life. Someone looking at the history of life from this perspective quickly focuses on microbes. Most of life’s history took place before the first multicellular organisms appeared. Most life today (whether measured by biomass or diversity or chemical interaction with the atmosphere) is still microbial. It is probably microbes, not “little green men”, that we will find when we encounter life on other planets.

Remarkably, the vast majority of microbial life on earth is unknown — recent experiments in molecular taxonomy suggest that fewer than 1% of the microbes in any random sample belong to previously identified or cultured species.

Many popular texts still refer to the microbes as “primitive” and comment on the sluggish pace of evolution prior to the Cambrian explosion. Evolution is sometimes treated as if it did not really begin until the most recent billion years of earth history, and there is an understandable emphasis in museums and popular-level books on evolution of the larger creatures, such as the dinosaurs, or the “smarter” ones, such as dolphins and apes.

When we look at the molecular phylogenetic tree, however, we see hints of a rich evolutionary history spanning the 3 billion years between the emergence of earliest living things and the Cambrian explosion. On the modern tree of life, the metazoans — from mollusks to mammoths — are banished to just a few twigs. From a functional point of view, also, it can be argued that the development of metazoans was just the most recent of several critical milestones in evolutionary history.

The earliest major innovation was the invention of photosynthesis. This may have happened as long as 3.5 billion years before the present (BP). Before that time, living cells had to extract energy (as many still do today) from chemical disequilibria in their surroundings, such as those produced when superheated water dissolved chemicals from the crust in regions of hydrothermal activity. Such strategies are much less efficient than photosynthesis. Once living things developed the complex sequence of chemical reactions that allowed chemical energy to be extracted directly from sunlight, a grand new world of possibilities opened. One of the most important questions in the search for life elsewhere involves our expectations concerning the emergence of photosynthesis on other worlds.

The second great leap, which may have happened at about the same time as photosynthesis, was the emergence of the Eukarya — cells larger and more complex than the prokaryotes (Bacteria and Archaea). Eukaryotic cells contain functionally distinct subunits: a nucleus enclosing genetic information, plus various mitochondria and (for plants) chloroplasts. These microbes are among life’s most successful groups, especially the ubiquitous protists (including amoebae, diatoms, paramecia, and foraminifera). Almost certainly the Eukarya originated in the merging or envelopment of various bacteria; for example, the chloroplasts in many ways resemble cyanobacteria. There is a crucial difference, however, because in the Eukarya the genetic information for each of the subunits as well as for the cell as a whole is combined in the nucleus. Their origin marks a fundamental change in the functionality of the genome.

The third milestone was the invention of sexual reproduction, which happened roughly 2 billion years BP. Certain eukaryotes developed double strands of genes (providing redundant information storage). The next step was to find a way of combining genes from two parents, rather than simply cloning the cell. Sexual reproduction allowed greater genomic diversity, since offspring are not genetically identical. This diversity of populations enhanced the opportunities for selection to favor some genetic combinations over others and accelerate the pace of evolution.

The fourth and final great innovation was multicellular or pluricellular organisms. Sometime before 0.7 billion years BP, life evolved the capability to store and use the genetic information for multiple types of cells within its germ cell. Now a variety of cells types could be manufactured from a single source of genetic information through sophisticated control of gene expression. Once this capability existed, it was relatively simple for these different cell types to form tissues and to link up within a single organism. Thus metazoans — multicellular organisms with cells organized into tissues — became possible.

The preceding is a crude, macroscopic perspective on evolution. When we consider the possibility of life on other worlds, even life that is chemically similar to our own, we must ask ourselves which of these key steps — photosynthesis, multicomponent cells, sexual reproduction, and multicellular organisms — might have taken place there. Our ability to detect and recognize alien life depends on these or similar evolutionary events.

Habitable environments on earth and beyond

To understand the role of life in the universe, we must explore the range of environments that might support living things. Astrobiologists approach this problem in two ways. In this section, I discuss habitability from the perspective of the basic properties of carbon-based life. In the next section, I look at the diversity of environmental conditions on earth where life is found.

Most astrobiologists limit their consideration to carbon chemistry because carbon is abundant and more capable than any other element of forming a wide variety of complex chemical bonds. Besides, carbon-based life is the only sort we could confidently recognize.

Much of organic chemistry is enabled by the presence of liquid water. Water is the best solvent, which is why we use it for washing. The range of temperatures in which water is a liquid (from 0° to 100°C) is precisely the range in which much of carbon-based chemistry is active. At temperatures above 100°C the larger carbon molecules start to come apart, which is why boiling water kills most microbes.

While water molecules are abundant in the universe, liquid water is much less so. Most places are either too hot or too cold. In addition, liquid water requires an ambient pressure greater than 0.006 bars (4.5 mm of mercury); at lower pressures, water can have only two states: solid and gas. The requirement for liquid water directs our attention toward planetary surfaces with temperatures between 0° and 100°C.

In addition to liquid water and organic chemicals, life requires an energy source. Early life on earth extracted energy from dissolved chemicals through fermentation and other reactions. Photosynthesis, however, enables life to tap the much greater energy of sunlight itself. Photosynthesis yields carbohydrates and oxygen gas as byproducts, and these can be used as an energy source by other organisms, such as animals.

A habitable environment, then, seems to require the presence of 3 things: abundant raw material in the form of carbon compounds, liquid water (which points us toward planets), and an exploitable energy source.

Life in extreme conditions

On earth, life has evolved to fill many ecological niches, some of them quite different from our everyday experience. Organisms that live and flourish in such environments are called extremophiles — meaning that their environments seem extreme to us. Most extremophiles are microbes, but these are not necessarily simple or primitive — in fact, a great deal of evolutionary adaptation was required for them to function in these environments.

Most life works best at temperatures between about 15°C and 60°C. Microbes that prefer lower temperature are called psychrophiles, and those that prefer heat are called thermophiles. At the low end, life can often survive at temperatures even below 0°C, although metabolism slows down or stops. Microbes that were frozen and dormant for tens of thousands of years in the Antarctic ice have been revived in our laboratories.

At high temperatures, thermophiles have developed mechanisms to make the chemical repairs that are needed as carbon-based compounds begin to come apart. Unlike the dormant state at low temperatures, adaptation to high temperatures requires active chemical intervention. For example, many microbes flourish in the Yellowstone hot springs at temperatures up to 100°C. The record for a thermophile is 113°C at deep-sea vents. (Note that the pressure of the water above the vent is so great that 113°C is still below the boiling point of water there.)

Other environmental extremes involve moisture, salt, and acid. Many microbes tolerate desiccation in much the way they survive low temperatures, by going into a dormant state and waiting for better conditions to return. Some microbes are so tolerant of high salinity that they can live even in the waters of the Dead Sea. The range of acidity in which life has been found goes from pH less than 0 to greater than 9. One example of an acid environment is the Rio Tinto of southern Spain, which originates in a region of extensive mineral deposits and maintains a steady pH of 2.5 all the way to its mouth (by comparison, lemon juice has a pH of 2). A rich microbial community inhabits this river, with some of the microbes helping to maintain the acidity, because that is the environment they like.

One of the most surprising cases of tolerance to extremes is exhibited by an “atomophile”: the bacterium Deinococcus radiodurans, which is found (among other places) in the cooling water of nuclear reactors. With its highly developed chemical repair mechanisms, D radiodurans can survive ultraviolet or particle radiation up to 6000 rads per hour, a thousand times more than a human can tolerate. It is also resistant to many unpleasant industrial chemicals and is commonly found in toxic waste dumps.

Life has evolved to survive on earth in a remarkable range of environments. Nearly every ecological niche seems to be filled, although in many cases the rate of metabolism is very low. But there are some exceptions. No organism has learned to extract the water it needs directly from ice: the ice sheets of Greenland are not green, in spite of ample sunlight. There are also no organisms that carry out their life cycles entirely in the air. Life (as we know it) requires liquid water and something substantial such as land or an ocean to make a home.

Life on Mars

Within our solar system, Mars is probably the other planet most likely to harbor life. This conclusion has nothing to do with the “canals” of Percival Lowell; the life we are discussing is microbial, not metazoan. Nor today do most scientists support the initial hypothesis put forward in 1996 that the Martian meteorite ALH 84001 contains fossil microbes (although the point is still debated). Rather, optimism about Mars derives from the fact that several billion years BP its climate was different. There is abundant geological evidence from spacecraft exploration that Mars once had a thicker atmosphere and liquid water on its surface.

The 1976 Viking landers carried instruments designed specifically to detect microbial life in the soil, but the results were negative. Although Mars has the most earthlike environment of any other planet in the solar system, the surface is too dry and too cold and too affected by solar ultraviolet radiation to meet the requirements for habitability. However, there is no reason to think that life could not have begun on Mars about 4 billion years BP, since earth and Mars apparently had similar surface conditions then. Future missions will include the return of samples, selected from sedimentary rocks at sites (such as ancient lake beds or hot springs) that once held water. The most powerful searches for Martian life (past or present) will thus be carried out in our laboratories here on earth.

Finding fossil life in ancient rocks would motivate an accelerated search for survivors. We will look for life that evolved to deal with the deteriorating climate of Mars, perhaps by finding some refuge that is warmer and wetter than most of the Martian surface. NASA’s theme in searching for life is “follow the water”. The most likely source of liquid water on Mars today is deep below the surface, where extensive aquifers may exist. Perhaps someday astronauts on Mars will drill deep wells down to this layer of liquid water and finally encounter living alien life.

One interesting twist on the search for life is derived from the presence of ALH 84001 and nearly 30 other Mars rocks that have been identified on earth. These are rocks blasted off the surface by meteorite impacts. Mars and earth are close enough together that they have exchanged material in this way throughout their history (although most of the traffic has been from Mars to earth, as a consequence of the lower surface gravity on Mars). It is possible that some of these rocks may have contained viable microorganisms. Mars might have seeded earth, or the two planets could have exchanged biological material. It is therefore conceivable that if we eventually find living things on Mars, they will be genetically similar to terrestrial life, for the good reason that they are our distant cousins. If so, it will be fascinating to study life that has evolved independently for close to 4 billion years, but we will be no closer to answering the fundamental question of how life began. The “holy grail” of astrobiology is not just to find life elsewhere, but to find and study life that had an independent origin from that on earth.

Life elsewhere in the solar system

Are there other locations in the solar system where there is liquid water? Recent studies of the satellites of the outer planets suggest positive answers, in spite of the cold surfaces of these worlds. In most cases, the putative liquid water is deep beneath the crust. The most tantalizing site, however, is Jupiter’s satellite Europa. Data from the Galileo spacecraft (in orbit about Jupiter 1995–2003) on this Moon-sized world indicate the presence of a global ocean of liquid water beneath an ice crust only a few kilometers thick.

Life requires an energy source, and sunlight does not penetrate below the frozen crust of Europa. Life is therefore unlikely to have evolved photosynthesis there. But internal energy sources may be present in the Europan seas. To remain liquid, Europa’s global ocean must be warmed by heat generated by tides and now escaping from the interior of Europa. Hot (or at least warm) springs might be active, analogous to those we have discovered in the deep oceans of the earth. Europa might therefore support life that derives its energy from the mineral-laden water in such springs.

Although some scientists think that Europa is the most likely place beyond the earth to find life in the solar system, others question whether life could originate in a dark ocean heated only by hot springs. Since we do not know exactly how life formed on earth, it is impossible to evaluate this possibility for Europa. One thing is clear, however: if there is life in the Europan oceans, it is likely to be unrelated to terrestrial life. There is no exchange of rocks between earth and Europa to provide the possibility of cross-contamination, as is the case for Mars. Thus Europa holds out the tantalizing prospect of a second genesis — an independent origin of life. If so, we can hardly guess what that life might be like. Will it be carbon-based? Will it utilize protein chemistry? Will it possesses a genetic material something like DNA or RNA? Or will it be more exotic than we can now imagine?

Habitable planets orbiting other stars

One of the most important recent developments in astronomy has been the detection of planets circling distant solar-type stars — with more than 100 such planets discovered by the end of 2002. These planets cannot be seen directly; they are detected by the “wobble” of their parent stars in response to the gravitational tug of the planets. So far, we can detect only giant planets (like Jupiter and Saturn), and such large planets (without solid surfaces) seem unlikely as the home of life. Further, many of the newly discovered giant planets are on eccentric orbits or cluster close to their parent stars, where temperatures are far too high for liquid water. Still, their existence holds out the prospect of smaller worlds (either earth-like planets or satellites orbiting the giants) that might support liquid water and the other conditions necessary for biology.

In evaluating the prospect for life in distant planetary systems, astrobiologists have developed the idea of a habitable zone — a region around a star where suitable conditions might exist for life. Since the focus is on the presence of liquid water, the usual definition of a habitable zone is the range of distance from the star where water will be liquid on the surface of a terrestrial-type planet (that is, a planet with roughly the mass of the earth).

Obviously the earth is in the habitable zone for the solar system. Note, however, that earth’s surface is above the freezing temperature of water only because the greenhouse effect in our atmosphere raises the average temperature by about 25°C. In the past, when the Sun was fainter, we were even more dependent on a greenhouse effect to maintain clement conditions. Thus, we must consider the nature of any atmosphere as well as the distance from the star in evaluating the range of habitability.

Our neighbor worlds provide some insight into the habitable zone within the solar system. Venus, the next planet closer to the Sun, has evolved through a runaway greenhouse effect into an oven where life is impossible, but it was once probably inside the habitable zone. Mars today is too cold and dry for surface life, but in the past it had a thicker atmosphere and apparently supported surface water (although perhaps its lakes and seas were ice-covered). Today Mars seems to be outside the habitable zone, but if the earth (with its greater ability to retain an atmosphere) were in the orbit of Mars, it might still be relatively warm. The current inhospitable nature of Mars is as much a consequence of its small mass as its distance from the Sun.

This is all very complicated, and scientists still differ in what they consider to be a habitable zone. Roughly speaking, however, the habitable zone in our solar system is limited to the terrestrial planets earth and Mars, and (perhaps early on) Venus.

Biomarkers

From the discussion above, our prime candidate worlds in the search for life beyond the solar system are terrestrial-type planets within the habitable zones of their stars. Astronomers are unable to detect such planets with current technology, but within a decade or so, space missions should allow us to determine how common such habitable planets are and to identify nearby candidate systems for further study. The NASA mission called Kepler, which is to be launched in 2007, is designed to determine the frequency of occurrence of terrestrial planets within the habitable zone of solar-type stars.

The fact that a planet is within the habitable zone does not ensure, of course, that it is actually inhabited. Indeed, one of the most important questions in astrobiology is just that: will life arise naturally when the environmental conditions are correct? It is thus important to consider how we might recognize the signature of life on a distant planet.

Even with the largest space-based telescope we can contemplate, we will never be able to obtain images of distant planets, as we do the worlds in our own solar system, let alone visit them with robot spacecraft. Astrobiologists therefore need a global biomarker — something distinctive to separate a live world from a dead one. To be detectable, these biomarkers should involve changes in atmospheric or surface chemistry that can only be the result of life.

If we observed earth from a great distance, and took sensitive visible-light and infrared spectra, we might just see such biomarkers. The most easily detectable evidence is the presence of abundant free oxygen in the atmosphere, which produces distinctive features in near-infrared spectra. On earth, oxygen is the byproduct of photosynthesis; if life on earth should cease, the oxygen in the atmosphere would disappear within a few thousand years. The oxygen, therefore, is a biomarker. A similar atmospheric gas is methane, produced by microbes. Without the presence of life, methane would be quickly oxidized and disappear from the atmosphere. Probably the most distinctive biomarker for earth is the simultaneous presence of these two gases: oxygen and methane.

Unfortunately, for two billion years the earth was a living planet without the oxygen/methane biomarker in its atmosphere. Astrobiologists are therefore looking in more detail at the possible interactions between ancient microbial life and the atmosphere. One of the themes of astrobiology is to study the co-evolution of life and the planet. Our ability to detect the presence of life someday on an extrasolar planet depends on a better understanding of the complex interactions that could reveal the presence of a biosphere to our spectrometers.

Conclusions

The search for life is just beginning. It may require decades for a thorough exploration of Mars, and Europa is even less accessible. To search for biomarkers on distant earthlike planets requires not only that we discover such planets, but also that we construct enormous telescopes in orbit to measure the composition of their atmospheres in the search for biomarkers. A third possibility, of course, is that a successful SETI program will detect signals broadcast by a technical civilization somewhere in the galaxy. While this seems like a long shot, the rewards of success are certainly sufficient to motivate the search for such signals.

Astrobiology is a science that broadens our perspective on biology to include other worlds. It includes the study of evolutionary adaptations to extreme environments on earth, as well as the potential for life to develop on other planets. Because of its fusion of astronomy and biology (plus generous contributions from geology and chemistry), astrobiology has great public appeal. Dozens of books have been published in the past few years, and more recently astrobiology has found its way into college curricula, especially as a general education introduction to science. Two new textbooks have been published for such “Astrobiology 101” courses (Life in the Universe by Jeffrey Bennett, Seth Shostak, and Bruce Jakosky [San Francisco: Addison Wesley, 2003], and The Search for Life in the Universe, 3rd edition, by Donald Goldsmith and Tobias C Owen [Sausalito (CA): University Science Books, 2001]), and more than 100 college courses are currently offered (see http://nai.arc.nasa.gov/institute/college_courses/). Evolution, of course, lies at the heart of these studies — taken in the wide context of an evolving universe and of the coupled co-evolution of life with its host planet.

Astrobiologists are asking the big questions about the origin, evolution, and distribution of life in the universe. While it may be a long time before we get definitive answers to such questions, the quest for this knowledge is fascinating to educators and the public as well as the research community.

About the Author(s): 
David Morrison
NASA Ames MS 240-1
Mountain View CA 94035-1000
dmorrison@arc.nasa.gov

Faith, the Environment, and Evolution

Reports of the National Center for Science Education
Title: 
Faith, the Environment, and Evolution: An Interview with John F Haught
Author(s): 
Phina Borgeson
Volume: 
23
Issue: 
3–4
Year: 
2003
Date: 
May-August
This version might differ slightly from the print publication.
John F "Jack" Haught is Landegger Distinguished Professor of Theology at Georgetown University in Washington DC and director of the Georgetown Center for the Study of Science and Religion. His area of specialization is systematic theology, with a particular interest in issues pertaining to science, cosmology, ecology, and religion. He is the author of several books, including Science and Religion: From Conflict to Conversation, God After Darwin: A Theology of Evolution, and Responses to 101 Questions on God and Evolution (reviewed by Phina Borgeson on p 52); his newest book, Deeper Than Darwin: Evolution and the Question of God, was published in 2003 by Westview Press.

Haught visited the University of California, Davis, on March 18 and 19, 2002, to deliver the annual St Augustine Chair Lecture of The Belfry campus ministry, which was concomitantly was the keynote address of a conference on Care for God’s Creation: Spirituality and Environmental Stewardship. NCSE Faith Network Project Director Phina Borgeson attended and interviewed Haught afterward.


RNCSE: The organizers of the Care for God’s Creation event wisely, in my opinion, asked you to speak on evolution as an important foundation for any faith-based environmental activism. It has seemed to me that there are similarities between people who deny evolution because of their beliefs and those who disparage environmentalism out of faith. What would you say is the theological common ground between those two groups?

Haught: Well, first, they are both dualistic in their thinking; it is their view of reality that we humans are essentially spiritual beings only accidentally imprisoned in a material universe. Second, and perhaps most important, it is their view of ultimate destiny that militates against their taking the environment seriously. I grew up in the country, on a family farm in rural Virginia. There is a radio program I listen to on Sunday afternoons that plays the kind of music common in that area, "Stained Glass Bluegrass". I still have an affection for the music, but not for the theology, which says that our ultimate home is elsewhere: earth is like a school for souls, and when it is all over, we will be harvested away from earth to heaven. To such a mindset, taking care of this planet seems pointless. And even if those who espouse these beliefs accept evolution, they are not interested in doing anything with it theologically. The environment is not important when the destiny of the individual is deemed significant and the destiny of all of creation is not. Those of us doing theology after Darwin, though, can speak with even more certainty about the inseparability of cosmic and human destiny.

RNCSE: What about parallels between the two in activism and methods?

Haught: The detractors project onto both evolution and environmentalism their own sense of what they have been taught is evil. Basically, evolution is seen as evil and the avenue by which modernity has allowed in all kinds of ills. For many who would deny it, the word "evolution" is so symbolically charged that of itself, it arouses a moral impulse toward activism.

Evolution, of course, is change over time. Evolution’s detractors have a concept of God and a concept of order according to which change is considered demonic or even satanic. And they have a lot of conservative money to fight change.

They do not want perfection in Whitehead’s sense, which includes both novelty and order. they want it in a trivial form devoid of novelty. For Whitehead, perfection is not attainable, but a goal, the highest possible integration of novelty and order (which he also called beauty). Too much novelty is chaos; too much order is banality.

Creationists and those who reject environmentalism also cannot distinguish between a sacramental outlook, in which nature is symbolic or revelatory of God, and pantheism, in which nature is equated with God. There does seem to be less creationism in sacramental churches, but there is a growing "intelligent design" movement in Catholicism.

RNCSE: Of course — Michael Behe is a Roman Catholic, and the University of San Francisco, a Jesuit institution, hosted the 2002 IDEA conference.

In your lecture, you talked about how any Christian theology must be related to the revelation of God in the person of Jesus. You identified three key attributes of Jesus: humility, self-gift, and opening up of the future, or promise. It seemed to me that each of these runs counter to the theology implicit in "intelligent design".

For example, when talking about God’s self-gift, you said, "Revelation is not the passing of information from heaven to earth, but the infinite entering the finite world." This seemed to allude to the theological uses, or misuses, of information in the work of Phillip Johnson and William A Dembski. Would you care to comment?

Haught: Well, that view is based on Karl Rahner’s theology, especially his thinking that revelation is at root the mystery of God pouring itself without reservation into the creation. I have an intuition that if you look upon nature simply as design, it tends to freeze out the novelty that brings life. Dawkins said that design is "brittle". But there are always new forms of order pouring in from the infinite. Because the fullness of divine infinity cannot be received all at once by the finite cosmos, something new is always coming into the universe. The rigidity of design is a barrier to the self-gift of the divine. Good evidence for this is that we see no perfect adaptations.

RNCSE: Of opening up the future, you said, "The universe is seeded with promise rather than design." Your comments?

Haught: Theologically, promise is a field of endless possibilities. Jürgen Moltmann (a contemporary German theologian, influential, among other things, in the renewal of interest in eschatology among liberal Protestants and in propounding a theology of hope) has developed some interesting ideas here. He reminded us that in the biblical view of things, the word "God" means "Future". Possibilities are more powerful than actualities. Possibilities can become actual, but the actual can no longer become possible. The conception of God as the Designer is just too hard and dead to capture the rich way God relates to nature, drawing us into the future in a Teilhardian way.

RNCSE: What about humility? Is there a way you would contrast your understanding of that with the theology of the "intelligent design" crowd?

Haught: It seems to me that fear drives people toward design: fear of change, of novelty, of the infinite. "Intelligent design" puts a sacred canopy over their lives. But design does not truly conquer fear. Design is about a God of power and might rather than a God who shares in our sufferings. Like most natural theology, "intelligent design" fails to make room for the cross. Yet it is a trust in the self-emptying Jesus in passion and crucifixion that drives out fear.

RNCSE: I know that you are aware of the efforts to insert "intelligent design" into the science education standards in Ohio. What would you say to clergy and other religious leaders there who want to oppose "intelligent design" from a theological perspective?

Haught: Well, before I say anything about theology, let me say that from the point of view of science, it is just plain inappropriate. Appeals to "intelligent design", for example Michael Behe’s "irreducible complexity", are theological diversions, not scientifically fruitful suppositions.

Theologically, "intelligent design" trivializes both science and the scriptures by bringing in God at the level of science. It has religion moonlit in an explanatory slot that belongs to science.

Proponents of "intelligent design" and of evolutionary materialism agree that there is one explanatory slot — so we need to fit God into the slot or he will not be present at all. If there is only one slot, there is going to be conflict. In contrast, in my new book Deeper than Darwin, I argue for explanatory pluralism, on which such a conflict need not arise.

The proponents of "intelligent design" seem unable to separate evolution from evolutionary materialism. They throw the baby out with bath water, discarding good science, and at the same time turning God into a tinkerer rather than a creator.

RNCSE: How did you get into this anyway?

Haught: Well, when I was in Catholic seminary, I got into reading Teilhard de Chardin, whose work struck a chord with my cosmic romantic sense. I have also delighted in Whitehead and his romantic reaction to the dominant philosophy of his place and time. I find that Science and the Modern World is still the most influential book I have read, and the best critique of scientism. I think Whitehead was the first postmodernist.

When I left the seminary, I worked in systematic theology, with no idea of going into science and religion as a field. But when I arrived at Georgetown more than thirty years ago, I realized that there was no course to help students to integrate what they were learning in science with what they were learning in the humanities. I like to think that my present work on evolution and theology is helping people — not just my students — undertake a religious voyage of discovery that respects both classical spirituality and the evolutionary discoveries of modern science.

The Astrobiological Perspective on Life's Origin

Reports of the National Center for Science Education
Author(s): 
David Morrison
Senior Scientist, NASA Astrobiology Institute
Volume: 
23
Issue: 
3–4
Year: 
2008
Date: 
May-August
This version might differ slightly from the print publication.
Astrobiology is a new term for the study of the origin, evolution, distribution, and destiny of life in the universe. It uses multiple scientific disciplines and space technologies to address some of the most profound questions of humankind: How did life begin? Are there other planets like earth? What is our future as terrestrial life expands beyond the home planet? For the first time in human history, advances in the biological sciences, informatics, and space technology make it possible for us to provide some answers.

In this paper, I discuss contributions that the new field of astrobiology can make to questions of life’s origins. I am an astronomer and space scientist, not a biologist or biochemist. My perspective is therefore that of an interested outsider. But as an astrobiologist, I look at the state of knowledge in the field and try to make some judgment about directions in which current research seems to be taking us.

The paper has 3 parts. First is a discussion of the nature of astrobiology, using the NASA Astrobiology Roadmap as a way of organizing the subject. Second is a review of the conditions on earth when life began. Third is a perspective on current origins research.

The nature of astrobiology

The United States National Aeronautics and Space Administration (NASA) has encouraged the new discipline of astrobiology by organizing workshops and technical meetings, establishing a NASA Astrobiology Institute, providing research funds to individual investigators, ensuring that astrobiology goals are incorporated in NASA flight missions, and initiating a program of public outreach and education. NASA’s role comes from its history of studying the origin of life and searching for evidence of life on Mars and elsewhere in our solar system. These studies have traditionally been called “exobiology”. Under the broader umbrella of astrobiology, however, research has expanded to include the search for life within other planetary systems, as well as investigation of the response of terrestrial life to global changes on the earth and to exposure to conditions in space and on other worlds. Astrobiology addresses not only our origins, but also our aspirations to become a space-faring civilization.

One description of astrobiology is provided by the NASA Astrobiology Roadmap (available at http://astrobiology.arc.nasa.gov/roadmap/). This Roadmap, completed in 1999, defines the content of astrobiology as perceived by scientists at its birth. It is a starting point only, and astrobiology is maturing as new information is obtained and diverse scientists bring their own perspectives to this discipline.

Astrobiology addresses 3 basic questions, which have been asked in some form for generations.

  • How does life begin and evolve? (Where did we come from?)
  • Does life exists elsewhere in the universe? (Are we alone?)
  • What is life’s future on earth and beyond? (Where are we going in space?)
These very general questions are then explored by means of 10 scientific goals:

1. Understand how life arose on the earth.

Terrestrial life is the only form of life that we know, and it appears to have arisen from a common ancestor. How and where did this remarkable event occur? We can now perform historical, observational, and experimental investigations to understand the origin of life on our planet. We should determine the source of the raw materials of life, either produced on this planet or arriving from space. We should seek to understand in what environments the components may have assembled and what forces led to the development of systems capable of deriving energy from their surroundings and manufacturing copies of themselves.

2. Determine the general principles governing the organization of matter into living systems.

To understand the full potential of life in the universe, we must establish the general physical and chemical principles that lead to the emergence of systems capable of energy extraction and growth (catalysis and metabolism), generating offspring (reproduction), and changing as conditions warrant (evolution). Must all life be based on something similar to terrestrial biochemistry and molecular biology? How can laboratory experiments and computational simulations help us to understand life as a more general phenomenon?

3. Explore how life evolves on the molecular, organism, and ecosystem levels.

Life is a dynamic process of changes in energy and composition that occurs at all levels of assemblage, from the molecular level to ecosystem interactions. Much of traditional research on evolution has focused on organisms and their lineages as preserved in the fossil record. However, processes such as the exchange of genetic information between organisms and changes within DNA and RNA are key drivers of evolutionary innovation. Modern genetic analysis, using novel laboratory and computational methods, allows new insights into the diversity of life and evolution at all levels.

4. Determine how the terrestrial biosphere has co-evolved with the earth.

Just as life evolves in response to changing environments, changing ecosystems alter the environment of earth. Scientists can trace the co-evolution of life and the planet by integrating evidence acquired from studies of current and historical molecular biology (genomics) with studies of present and historical environments and organismal biology. We seek to understand the diversity and distribution of our ancient ancestors, to identify specific chemical interactions between the living components of the earth (its biosphere) and other planetary subsystems, and to trace the history of earth’s changing environment in response to external driving forces.

5. Establish limits for life in environments that provide analogs for conditions on other worlds.

Life is found on the earth anywhere liquid water is present, including such extreme environments as the interior of nuclear reactors, ice-covered Antarctic lakes, suboceanic hydrothermal vents, and deep subsurface rocks. To understand the possible environments for life on other worlds, we must investigate the full range of habitable environments on our own planet, both today and in the past.

6. Determine what makes a planet habitable and how common such worlds are in the universe.

Where should we look for extraterrestrial life? Based on our only example (life on earth), liquid water is a requirement. We must therefore determine what sort of planets are likely to have liquid water and how common they might be. Studying the processes of planet formation and surveying a representative sample of planetary systems will determine what planets are present and how they are distributed, essential knowledge for judging the frequency of habitable planets.

7. Determine how to recognize the signature of life on other worlds.

We are poised on the brink of searching for life, past or present, on a variety of worlds. This search requires that we be able to recognize extraterrestrial biospheres and to detect the signatures of extraterrestrial life. We must learn to recognize structural fossils or chemical traces of extinct life that may be found in extraterrestrial rocks or other samples. And we must develop a catalog of possible signatures of life that can be identified astronomically in planets circling other stars.

8. Determine whether there is (or once was) life elsewhere in our solar system, particularly on Mars and Europa.

Exciting data have presented us with the possibility that at least two other worlds in our solar system have (or have had) liquid water present: Mars and Europa. Extensive exploration of the Martian surface will be required to evaluate the total potential for life on that planet, both past and present. Furthermore, exploration of the subsurface probably offers the only credible opportunity to find extant life on either Mars or Europa.

9. Determine how ecosystems respond to environmental change on time scales relevant to human life on earth.

Research at the level of the whole biosphere is needed to examine the habitability of our planet over time in the face of both natural and human-induced environmental changes. To help to ensure the continuing health of this planet and to understand the potential long-term habitability of other planets, we need predictive models of environment–ecosystem interaction.

10. Understand the response of terrestrial life to conditions in space or on other planets.

What happens when terrestrial life is moved off its home planet and into space or to the moon or Mars, where the environment is very different from that of earth? Can organisms and ecosystems adapt to a completely novel environment and live successfully over multiple generations? Are alternative strategies practical, such as bioengineering organisms for specific environments? The results from attempting to answer such questions will determine whether earth’s life can expand its evolutionary trajectory beyond its place of origin.

Although it is defined in terms of a research agenda, astrobiology also lends itself to education and outreach. The three theme questions strike a chord of interest among both students and the public. Courses built around these questions offer a powerful platform to discuss issues such as deep time, astronomical and biological evolution, and our place in the universe. On a slightly more sophisticated level, this multidisciplinary field illustrates different styles of approaching science such as contrasting the historical versus experimental research and exploratory versus hypothesis-driven research. A new NSF-supported upper-school curriculum, “Voyages Through Time”, provides a highly appealing introduction to evolution on multiple levels: evolution of the universe, planets, life, and intelligence. At the college level, many astronomers (in particular) have begun to offer general-education courses on “astrobiology” or “life in the universe”. Two new college-level textbooks have been published, and the popularity of such courses is rapidly growing.

The origin of life on earth: Context

The first goal of astrobiology discussed above is to understand the origin of life on earth. Such a study requires that we look at the astronomical and planetary evidence concerning the early environment of earth, as well as the likely chemical pathways that led to life. This study overlaps with the two goals that deal with the general conditions for the origin of life in the universe and with understanding the evolution of life on earth, especially in the microbial world.

Let us be clear at the beginning that we do not understand the origin of life on earth in any detail. Indeed, we are not even sure that life began here. There are some arguments that Mars might have been a more suitable environment for the origin of life 4 billion years ago. Since Mars and Earth have exchanged materials throughout their history, it is possible that life has migrated from one planet to another. This modern form of panspermia has its advocates, but the simplest hypothesis is that life formed on earth. If it began on Mars instead, the processes are probably similar to those that we hypothesize for our own planet.

The solar system formed 4.5 billion years ago from a collapsing cloud of gas and dust that already contained a rich complement of organic material. Astronomical investigations of similar “molecular clouds” that exist today have revealed more than 120 molecules, including such complex substances as ethyl alcohol. The so-called biogenic elements (oxygen, carbon, nitrogen, sulfur, and phosphorus) are among the most common interstellar constituents, once we get beyond hydrogen and helium, which make up 99% of the visible universe. Given the abundance of hydrogen and oxygen, water is one of the main molecules. The simple building blocks for life were thus readily available even before the formation of the planets.

The planets themselves condensed from a disk of gas and dust spinning around the protosun. Some of the pre-existing organic chemicals probably survived this formation process, but most may have been destroyed and then reconstituted within the cooling disk, and perhaps destroyed a second time as the planets coalesced. We know from the study of the oldest meteorites that organics were abundant in the disk; the common carbonaceous meteorites are composed of a few percent carbon by weight, partly elemental and partly in the form of organic compounds. One of these, the Murchison meteorite, yielded 74 separate amino acids. Most of these included equal amounts of right- and left-handed molecules, indicating their non-biological origin. The earth and other rocky planets accreted a veneer of volatiles (including water) and organics from the rain of comets and meteorites that continued for the first half-billion years after the surface cooled. These external sources may have been a more important source of organics than Miller-Urey–type synthesis in the atmosphere and ocean, especially as the initial atmosphere of earth is now thought to have consisted largely of carbon dioxide and been neither strongly reducing nor strongly oxidizing.

What were conditions like on the early earth? Since no rocks have survived from that era, we do not know for sure, but some generalizations seem robust. Although initially hot, the surface layers cooled quickly, and oceans formed. The hot interior undoubtedly contributed to a high rate of volcanism, but surface conditions were then, as now, dominated by solar heating, not volcanism. From their study of stellar evolution, astronomers are confident that the early sun was about 35% less luminous than today (a condition called the “faint young sun paradox” by those who note the contradictory evidence for a relatively constant surface temperature over the history of the earth). Therefore either the earth had a large atmospheric greenhouse effect to maintain surface temperatures above freezing or else the primitive oceans froze. We can imagine an initial carbon dioxide greenhouse effect that partly compensated for the faintness of the sun but left frozen oceans like the Arctic Ocean today. The marine environment thus paradoxically included both a relatively cold surface and an abundance of volcanically-driven hydrothermal systems in the depths. However, there probably were not any of Darwin’s “warm little ponds” on the surface, and the surface might have been bathed in ultraviolet light, depending on the mass and composition of the early atmosphere.

Other external agents in addition to the faint sun influenced the environment of the early earth. The lunar cratering history, among several lines of evidence, shows that the rate of asteroid and comet impacts on the earth was much higher before 3.9 billion years ago. Although it is unclear whether there was a short-lived burst of impacts (a “late heavy bombardment”) or a steadily declining impact rate dating all the way back to the accretionary period, the impacts were sufficient to influence the surface environment. Then as now, the greatest effects are from the rarest, largest impacts, happening at intervals of millions of years. It is likely that the earth was struck several times with sufficient energy to boil away most or all of the oceans. Although the surface would cool and the oceans recondense within a few thousand years of such an impact, the effects on any nascent life would have been catastrophic. This bombardment by a few projectiles hundreds of kilometers in diameter has been termed the “impact frustration” of the formation of life. It suggests that life might have formed several times and then been wiped out in such a sterilizing catastrophe. It also suggests the presence of one or more thermal bottlenecks in the early evolution of life, a topic I will return to below.

The origin of life on earth: Evidence

There is very little surviving geological evidence from the first 500 million years. What we know of impact history, for example, is derived from studies of the moon, not directly of the earth itself. The earliest fossils date from sometime after the end of the heavy bombardment.

Study of the early geological record of life dates back half a century, when Stanley Tyler, Elso Barghoorn, and William Schopf identified fossil microbes in the 2.1 billion–year-old Gunflint chert. By 1993, Schopf had found what appeared to be the oldest fossils in the Apex chert of Western Australia at 3.46 billion years. Schopf also suggested on the basis of morphological evidence that these fossil microbes were probably photosynthetic cyanobacteria. However, this work has recently come under attack, and at this writing the situation remains unresolved. In particular, the crucial conclusion that photosynthesis was operative on earth 3.5 billion years ago is in dispute. In any case, there seems to be no question that microbial fossils can be dated to at least 3.0 billion years. Macroscopic fossils in the form of stromatolites — layered constructs built up by generations of microbial mats — have also been found with similar ages.

A complementary approach is to look for an isotopic signature that indicates the presence of life in sufficient quantities to influence the global chemistry of the planet, even if individual fossils have not survived. Stephen Mojzsis and others argue on this basis for the presence of diverse bacteria on earth before 3.85 billion years. If these interpretations are correct, the interval between the end of the late heavy bombardment and the development of a robust global biota is remarkably short.

The major alternative way to study early life is to examine genomic evidence. Similarities and differences in DNA and RNA sequences illustrate relationships related to their lineage. In the case of the metazoans whose fossil remains dominate natural history collections, genomic analysis is a powerful supplement to more traditional studies of evolution. In the microbial world, such studies provide us with almost our only access to the lineages of life. Given that life on earth was exclusively microbial for the first 85% of its history, and that microbes still dominate in terms of biomass and range of habitats, these tools are invaluable for the astrobiologist. Much of astrobiology research is focused on the smallest but most numerous of life’s creatures.

Carl Woese pioneered the comparison of 16s mitochondrial RNA, a highly conserved sequence that can be found in almost every living thing. By the late 1980s, he had established the division of life into 3 domains, Bacteria, Archaea, and Eukarya. The molecular phylogenetic “tree of life” based on mitochondrial RNA provides us with an entirely new way to look at the diversity of earth’s biota. This diversity, and by implication its evolutionary history, is dominated by microbes within all three domains; the metazoans that have evolved since the Cambrian explosion are banished to a few outlying twigs. Although we do not know the rate of change for mitochondrial RNA in any absolute sense, the conclusion is clear that natural selection has been at work throughout the development and diversification of the microbial world. Today’s microbes should not be called primitive; they are in fact highly versatile creatures that occupy a much greater range of ecological niches than do the more familiar Cambrian metazoans.

Molecular phylogeny is based on the relationships among extant biota. It cannot be used to analyze the mineralized fossils that make up most of the historical record of life on earth — we cannot, for example, use gene mapping to compare an Eohippus with a modern horse, as we can a human and a chimpanzee. Still less are we able to determine the genomic content of ancient microbes, which must have been quite different from anything that survives today. But it is possible to determine which extant microbes are probably similar to the inferred precursors of modern life. This is sometimes ambiguous, especially when we consider that there has been a history of gene transfers among different lineages that can shuffle the deck in ways that make reconstruction nearly impossible. With these caveats, however, a number of suggestions have been made that the most “primitive” organisms today are anaerobic thermophiles — that is, microbes that are happy in oxygen-free environments at high temperatures. Many are also methanogens, microbes that generate methane. These studies suggest, even if they do not prove, that our earliest common ancestors had similar properties.

Even if the common ancestor or ancestors of today’s life were high-temperature, methane-producing microbes, this does not mean that these are representative of the first life. Almost certainly there were many precursors that existed and evolved before the invention of DNA. In addition, however, the last common ancestor is likely to have been the survivor of a “bottleneck” resulting from a catastrophe that wiped out its predecessors. One such possibility is the largest impacts of the heavy bombardment. If the surface and upper layers of ocean were sterilized by an impact, the most likely survivors would be thermophiles from the ocean depths, and it is these survivors who could have repopulated the planet.

The origin of life on earth: Theory

Putting the pieces together to form the first life is a daunting problem. Many scientists who look at the great progress that has been made in understanding the chemical steps along the road toward life are justifiably pleased and optimistic. Others look at the huge gaps that remain and are more cautious. The following is the briefest overview of many complex issues. In preparing this summary, I have been influenced by Belgian Nobel laureate Christian de Duve (Vital Dust: Life as a Cosmic Imperative [New York: Basic Books, 1995]; Life Evolving: Molecules, Mind and Meaning [New York: Oxford University Press, 2002]) and by Australian physicist Paul Davies (The Fifth Miracle: The Search for the Origin and Meaning of Life [New York: Simon & Schuster, 1999]).

The earliest life needed to acquire several basic capabilities. These include assembly of the necessary raw materials within a structure, metabolism (extracting useable chemical energy from the environment), and reproduction, which ultimately involved information-storing molecules such as RNA and DNA that were themselves capable of replication. Each of these is a challenge, and they can hardly have appeared simultaneously.

The first step was surely the chemical factory that extracts energy and uses it to assemble complex molecules. Many such chemical reactions were possible, especially in a rich organic “soup” of amino acids and other organic chemicals. The key was to be able to select and control the rate of these reactions using the biological catalysts called enzymes. The energy sources could have included the conversion of sugars to alcohol or lactic acid by fermentation, or the formation of methane from carbon dioxide and hydrogen by oxidation, depending on available raw materials.

As chemical synthesis became more important, it was necessary to segregate different materials physically. Such segregation can be accomplished by membranes composed in part of lipids, which react with water to form nearly impenetrable barriers. A number of recent experiments and computer simulations have studied simple membranes and the ways they can incorporate proteins to permit partial permeability. A successful cell (or protocell) must eventually develop the ability to admit food and expel waste. Such simple membranes can readily form closed quasi-spherical chambers. In one interesting experiment, organic materials extracted from a meteorite spontaneously formed such closed systems when exposed to water.

Today, DNA is life’s primary information storage and retrieval system, but we also use a simpler system based on RNA, which has the property of participating in protein synthesis as well as storing information. Most workers now think that an RNA world must have preceded the development of DNA. Gerald Joyce of the University of California, San Diego, among others, has carried out extensive experimental studies of the RNA world, demonstrating the ability of RNA to evolve in the test tube. DNA could later have been developed as a more stable information storage system by something akin to the reverse transcription process that can still be observed today.

What processes brought these components together? De Duve makes the case that it cannot have been chance — the probabilities are far too small for self-assemblage of even the simplest such systems in the lifetime of the universe. To paraphrase de Duve, the process must have involved many chemical steps that had a high probability of taking place under prevailing conditions. This progression must have led from prebiotic organic chemistry to biochemistry, and selection effects must have been important in favoring certain chemical pathways. If this is correct, the process could have been rapid, and life should have been able to start in the few million years of stable conditions that separated major impact catastrophes.

The product of this sequence of events — the first protocells — may have been quite different from life that survives on earth today. Even the oldest common ancestor of today’s life probably represents a much more sophisticated system than the first recognizable life. But once natural selection came into play, the means existed for life to evolve. The key challenge, it seems to me, is to understand the selection processes that acted before the formation of the first protocell. It is these processes that must have guided the complex sequence of chemical changes that gave birth to life on the early earth, and, if life exists there, in the rest of the universe, too.

About the Author(s): 
David Morrison
NASA Ames MS 200-7
Mountain View CA 94035-1000
dmorrison@arc.nasa.gov

The Scary Story

Reports of the National Center for Science Education
Title: 
The Scary Story: Cartoon commentary
Author(s): 
Jay Hosler
Volume: 
23
Issue: 
3–4
Year: 
2003
Date: 
May-August
This version might differ slightly from the print publication.