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.