Monday, March 22, 2010
Astrobiological Disparity: A Commentary on the International Year of Biodiversity
From left to right: Deinoccocus radiodurans, a hardy extremophile capable of life in nuclear reactors, middle, the strange body plan of the now-extinct cambrian animal Opabinia Regalis (As envisioned by Nobu Tamura), an afican wild cat (as photographed by Wikipedia user Sonelle). General Sherman, a sequoiadendron, the tallest tree in the world at 275 feet.
2010 is the International Year of Biodiversity, following up 2009, the International Year of Astronomy. This makes it a particularly good time to discuss the field that links these two subjects, Astrobiology. Much of astrobiological work today occurs along two linked themes. The first is assessing habitability and the potential for life elsewhere in the Universe. This is what we are trying to do by following the water on Mars. However, this endeavour cannot proceed without input from the second theme, understanding the origins of life and its early development on the earth.
Unfortunately, both of these themes face a fundamental problem. Even though there is great diversity between extant forms of life on Earth, there is remarkably little disparity, from a cosmic perspective. This difference is a subtle, but important one. While diversity is a measure of the number of different forms in a collection of organisms (usually taken as the number of different species, or non-reproductively mixing groups), disparity is an expression of the degree of differentiation between these forms often in terms of body plans and survival strategies. So a collection of 500 species of shrimp is more diverse, but less disparate then a collection of 100 species made up of plants, fish, crustaceans and plankton. Notably, neither measure takes into account any measures of the success of a particular species in terms of number of organisms, range, species longevity, etc.
Since we only have one example of a planet with life, it is worth asking: how disparate is life on Earth? While there may be as many as 100 million different species present on the planet today (most remaining as yet undiscovered), these can be divided into just three domains of life based upon the form of their constitutive cells. These domains are Bacteria, Archaea, and Eucarya. Yet even these large meta-groups have inter-relationships. Eucarya, the domain of which we and nearly all other macroscopic life are a part, is thought to be the result of a beneficial symbiosis between an Archaean and a Bacterium at some time between 1.7 and 2.7 billion years ago. More fundamentally, all three domains are based on the replicative abilities of a single polymer, DNA and share a common ancestor. Thus in terms of strategies for propagation, the disparity of life on Earth is zero!
The three domains of life with Archea in Green, Eucaryotes in Red and Bacteria in Blue. Note that all three domains share a common ancestor which would be located at the center of the tree. The close relationship between the Archaea and Eucarya is shown as a larger subgroup before linking back to the last universal common ancestor.
Part of the reason for this could be the surprising observation that while diversification increases in time, disparity actually tends to decline. For instance, Stephen J. Gould observes that the number of different body plans (loosely equivalent to the classification level of phyla) in animals present just after the Cambrian Explosion is significantly greater than today. Analogously, it has been hypothesized that several different biopolymers, including RNA, PNA and TNA might have been able to perform functions similar to that which is played by DNA today. All may have been present on the early earth, but DNA, having advantages, outcompeted all of these other forms. The history since has been written by the victorious molecule.
However, this also suggests that even on the earth there may have been greater disparity in the past and that had conditions been different, then the balance could have been tipped in favour of other forms or strategies. As a result, we are left contemplating not just where in the Universe we might find life that has been successful on Earth, but where other kinds of life, as yet unknown, might be possible. There are some theoretical bounds we can put on such a problem; however, I expect that this is an area in which we will be surprised by discovery in the future. As many prognosticators are aware, it is always a dangerous proposition to define the limits of the possible.
Instead, we can proceed by determining what factors will tend to improve the odds of life beyond the earth, based on our limited earthly experience. For instance, liquid water certainly helps the chemistry that we require to function. The presence of certain elements in particular Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorous and Sulphur (collectively referred to as CHNOPS) are also helpful, allowing for systems that can replicate and store energy. Similarly the presence of an energy source to power cellular reactions is critical; both chemosynthesis and photosynthesis, in which energy is gained from chemical disequilibrium or radiation, are practiced on the Earth. This has led to the hypothesis that the powerful oxidants found in the martian soil by the Phoenix Lander could represent a power source for a martian biochemistry.
Above: a soil sample is collected for analysis. On Mars, solar UV causes oxidants to form in the soil, building up to as much as 1 percent by weight of the upper layer. Perchlorate, discovered by the MECA instrument aboard Phoenix represents a potential power source for chemosynthesis, if there is an organism available to metabolize it.
But the most important factor seems to be time. Over time, organisms evolve to move into new habitats that were previously empty. To illustrate this, consider that despite the incredible biodiversity on the Earth, certain niches remain unfilled. Why do the deserts or the summits of mountains not flower with plants and animals? Turn the question around and ask why the continental surface was barren half billion years ago? And why, before that, no animals, plants or larger creatures beyond bacterial colonies filled the seas?
It is worth keeping in mind that all of these biomes, including the terrestrial abodes not filled today, are far more clement locations for the kind of life we know then exists on Mars, Enceladus or Europa. That these are the leading candidates for life elsewhere in our Solar System underscores not only the difficulty of our Astrobiological quest, but also the fragility of life on our planet. It requires that we protect something so rare in all its diverse forms. This is the realization that is at the foundation of the year of Biodiversity.
From left to right: Earth, Mars, Europa and Enceladus (showing water plumes)
As a final thought let us consider what the evolution of intelligence on the Earth has meant for the survival of life on Earth. The fragility of life relates directly to three factors: environmental variation, diversity/disparity and range.
The effects of the first two factors are simple to grasp. The greater the frequency and magnitude of the variation in environmental conditions, the more difficult it is to maintain a stable system. Likewise, the more diversity and disparity there is amongst organisms inhabiting a particular region, the more likely that one or more species will be able to deal with the environmental variations that do occur.
Range, however, is the most crucial. By spreading itself over a large territory, life cannot be extinguished easily by isolated events. This is the advantage of large animals. We cannot tolerate the extremes that bacteria can, but we can deal with inclement conditions by adapting or moving on. Migration is a particularly good example of an adaptation unavailable to simpler life which allows the organism to derive benefits from a much larger range.
Intelligence is by far the best known means of increasing the range of a species. Through our use of tools and clothing, human beings now inhabit the entire planet and can claim a range in pressure, temperature, salinity, pH, you name it - larger than that of any other organism, bacteria included. As such, the Intelligence habitable zone (IHZ) for a solar system housing intelligent life is limited only by the availability of raw materials and energy; aside from politics and economics, there is no reason why humans could not establish a permanent presence on Europa or even further out in the solar system.
As such, we are the first organism produced by our planet with the capability to outlive the death of our Sun, four billion years hence. Thus spaceflight represents the most important adaptation ever produced by life on Earth, and it is an adaptation that we must not lose if we are to preserve life in our corner of the universe.
With the emergence of intelligent life, the habitable zone (HZ) increases in size. This larger Intelligence Habitable Zone (IHZ) shows how through the use of spaceflight and nuclear energy generation, it is possible to spread life to any location with sufficient raw materials, mainly water ice. Discarding waste heat is a difficulty which corresponds to the left edge of the purple trapezoid, but the right edge has no well-defined boundary.
For an interesting introduction to some of the central questions posed by Astrobiology, I highly recommend the book Rare Earth (most recently, 2003) by Peter Ward and Donald Brownlee. For a more advanced read, try Lunine’s Astrobiology (2005), a tome well-worth close study. Those looking for background on questions surrounding the initial emergence and diversification of animals (more generally “complex metazonans”) are advised to consider Stephen J. Gould’s Wonderful Life (1990). As a note on the images, I have selected NASA or Wikipedia media wherever possible and have made an effort to attribute the base images. If I have missed something, please feel free to leave a comment or contact me and I will fix it! With the exception of the Phoenix and planetary images, assume all image content is covered under: http://en.wikipedia.org/wiki/GNU_Free_Documentation_License.