The Ediacaran world

The terminal period of the Proterozoic, called Ediacaran after a locality in the Flinders Ranges of South Australia (or Vendian in the Russian terminology), marks the first appearance of undoubted macrofossils. Because they are seemingly complex (and because they have been studied mainly by palaeozoologists), all Ediacaran macrofossils were interpreted originally as early multicellular animals (metazoans). The title of Martin Glaessner's 1984 monumental book, The Dawn of Animal Life, expresses this view. The discovery of similar fossils in approximately 30 localities all over the world further contributed to the assumption that the Ediacaran fauna represents simply a prelude to the Cambrian evolutionary explosion of metazoan phyla. Consequently, the Ediacaran period could be considered as the initial stage of the Palaeozoic.

Subsequently, this view has been challenged by the vendobiont hypothesis (Seilacher, 1984, 1992). The challenge started with the observation that most of these organisms represent hydrostatic ‘pneu’ structures, whose various shapes were maintained by a quilted skin (as in an air mattress) and the internal pressure of the living content. Another feature shared by all vendobionts is the allometric growth of the quilt patterns. No matter whether the addition of new ‘segments’ continued throughout life (serial mode) or stopped at a certain point, followed by the expansion and secondary subdivision of established quilts (fractal mode), compartments never exceed a certain millimetric diameter. Such allometry is common in oversized unicellular organisms, such as certain algae (Acetabularia) and larger Foraminifera.

Introduction

In recent years, there has been great interest among some particle physicists and astronomers in assessing the dependence of the gross structure of the universe on the values of its defining constants of nature. This agenda has been partly motivated by the recognition that many of the most important structures in the universe, and its most crucial evolutionary pathways over billions of years of cosmic history, are surprisingly sensitive to the values of some of those constants. Since we lack any fundamental understanding of why any of the constants of nature take the values that they do, this state of affairs appears surprisingly fortuitous. It provokes cosmologists to consider our observed universe in the context of a wider ensemble of possible universes in which the laws of nature remain the same but the constants of nature, or the boundary conditions that specify the overall expansion dynamics of the universe, are allowed to change. In effect, a type of “stability” analysis is performed to ascertain how large such changes in the structure of the universe and its defining constants could be and still give rise to a recognizable universe.

These considerations have attracted renewed attention with the realization that our universe may have a significant non-uniform structure in space, in which the values of many of the quantities that we dub “constants” may in fact be variable values of spacetime-dependent fields.

Introduction

Of the three big questions astrobiology addresses, none has captured the public imagination as much as the question, ‘Are we alone?’ As knowledge increases about the physical environments available in the universe, organisms on the Earth are used to gauge the minimum envelope for life. Organisms that live on the physical and chemical (and perhaps, biological) boundaries are called ‘extremophiles’. Members of the domain Archaea are undoubtedly the high-temperature champions on Earth, surviving at temperatures far above the boiling point of water. However, a wide phylogenetic variety of organisms are able to inhabit other extremes, from low temperature to high salinity, from desiccation to high levels of radiation. In some cases, the adaptation is as simple as keeping ions out of the cell (low pH), but in others more profound adaptations are required. Reactive oxygen species are highly toxic, and thus organisms that are aerobic are arguably extremophilic. Thus, extraterrestrial habitats previously thought to be uninhabitable have been shown to be, at least theoretically, habitable, thus informing the search for life elsewhere.

Extremophilic organisms, or ‘extremophiles’, are integral to understanding the three big questions in astrobiology: Where do we come from? Where are we going? Are we alone? We do not know the environmental conditions under which life arose, although speculation includes hydrothermal areas; nor do we know the environments in which the earliest life forms existed.

My contribution to this volume is that of a theologian interested in the relationship between science and religion. I will be asking whether what we are calling “fitness for life” and “biochemical fine-tuning” are consistent with, and perhaps even supportive of, the ageless religious convictions that the universe is here for a reason and that life is the intended consequence of divine love, wisdom, and creative power.

Today, it is particularly striking to many scientists that cosmic constants, physical laws, biochemical pathways, and terrestrial conditions are just right for the emergence and flourishing of life. It is not surprising, of course, that, as life exists, the cosmic and chemical conditions for it had to have been formatted for such an emergence. It would be remarkable, however, if the format could have been otherwise, and hence not right for life. During the universe's history, it now seems that only a very restricted set of physical conditions operative at several major junctures of emergence could have opened the gateways to life (Hogan, 2000). So, what principles lie behind the narrowing of the gateways that allowed only those conditions preparatory to life to flow through while excluding any cosmological principles, physical parameters, and chemical laws that would not have permitted such an outcome?

In the long, unfolding story of nature's development, any conceivable series of physical conditions or constants other than those that would lead to life have been tossed aside.

Crane Brinton, Harvard historian, friend of Lawrence J. Henderson, and fellow member of The Saturday Club, wrote the obituary for Henderson in the Club's third commemorative volume (Brinton, 1958, p. 207). Noting that Henderson was somewhat out of the ordinary – crossing the Charles River on several occasions to keep appointments at the Medical School (Boston) and the College (Cambridge) and then recrossing it to get to the Business School (Boston) – Brinton went on to note Henderson's other non-traditional characteristics: “Ticketed as a biological chemist, he later took the title physiologist and, although he would not have liked the name, at the end of his career he was a sociologist [emphasis added].”

Brinton went on: “A cross section of his publications may indeed be so drawn up as to seem an academic scandal.” Brinton ran through the publications, from the well-known The Fitness of the Environment (1913) and The Order of Nature (1917); the more esoteric On the Excretion of Acid from the Animal Organism (1910, 1911); the simple volume Blood: A Study in General Physiology (1928); the unexpected transcript of an interview on the experiments in the Liberty Bread Shop (Brinton, 1958, p. 208); in his later life, The Study of Man (1941); to Pareto's General Sociology: A Physiologist's Interpretation (1935). Brinton jocularly added that a piece by Henderson – a biographical memoir on the life of the poet Edwin Arlington Robinson (a close friend from his student days) written as a memoir for the American Academy of Arts and Sciences – is to be found in the Woodberry Poetry Room of Harvard's Lamont Library.

The chapters in this volume, written from a wide variety of perspectives, explore the possibility of extending the theme of “fine-tuning” beyond the domain of cosmology, where it first entered into serious discussion in the mid-1970s, to other sciences such as biochemistry and biology. As a prelude to this investigation, it seems worthwhile to explore the theme of fine-tuning itself in some detail, given the ambiguities that still surround it and the vigor of the continuing disagreement as to what its implications are. How did fine-tuning make its way into the cosmological discussion? What precisely did – and does – it amount to? What were – and still are – the responses to it? How is one to evaluate those responses? Achieving a measure of clarity on these issues should make it easier to appreciate the search for fine-tuning or its analogs elsewhere in the sciences.

The infinities of space and time in Newtonian mechanics were not propitious to the formulation of a cosmology, a theory of the cosmic whole, although the notion of gravity gave a hint, at least, as to how material complexity could form. The unification of space and time by Einstein's general theory in a non-Euclidean geometrical framework offered new possibilities, and Hubble's subsequent confirmation of galactic expansion pointed Lemaître to a universe model that would, from a “primeval atom,” expand into the universe we know.

This is a book about whether our universe is “biocentric.” The Oxford English Dictionary defines this term as “treating life as a central fact” [1]; thus a biocentric universe is one predisposed towards producing life (life's centrality is implicit if “the fitness of the environment [for life] far precedes the existence of the living organisms” [2]). To date, this unusual idea has been most thoroughly explored (and most widely publicized) under the umbrella term “Anthropic Principle” in physics [3]. In essence, this principle refers to a suite of fundamental physical parameters, dimensionless constants that interact to imbue our universe with such interrelated phenomena as a diverse periodic table of elements, a preponderance of carbon and water, stars that emit energy, and planets that orbit them [4]. It asserts that, without clear explanation at present, the constants responsible for this state of affairs appear finely tuned in our universe to values peculiarly sympathetic with life's emergence.

Even if we accept this view of physics at face value, we remain a long logical leap from establishing truly biocentric credentials for our universe. Understanding “what is” versus “what might have been” for physics must be met by an equivalent understanding in biology. Thus the interface of biochemistry, where physics becomes biology, deserves especially close scrutiny. In this context, the first and perhaps most important point of this chapter is to emphasize that in considering physics and biology, two fundamentally different sets of expectations collide.

Like others (see for instance Oparin, 1968; Monod, 1970), I too am convinced that the origin of life will be eventually understood in scientific, rational terms. However, I recognize, of course, that so far it remains an unsolved problem, and even an unfathomable one. We scientists are playing with partial explanatory hypotheses and have few hard facts at our disposal. Therefore, we must remain humble in attempting to describe each scenario and be careful to discuss each one's constraints, recognizing where the scenarios may be flexible to synthesis with other hypotheses. Thus, my present discussion explores some areas in which there may indeed be some flexibility in terms of what solutions could potentially lead to living systems – in other words, where some “coarse-tuning” might be tolerable.

Specifically, I address the following series of questions:

1. Is life necessarily based on carbon?

2. Must the “bricks of life” have originated by some process closely related to Miller's “spark tube” experiment, or do other possibilities exist?

3. Is it necessary that any kind of life be associated with water from the start?

4. Is it necessary for any kind of life to be cellular and for the cells to be bounded in water by membranes?

5. Are the n-acyl phospholipids of “classical” membranes, or the more recently discovered archaeal lipids, plausible constituents of primitive membranes?

6. Could early membrane-forming amphiphiles have simply been polyprenyl phosphates, following the “Strasbourg scenario” (Birault et al., 1996)?

7. Are there automatic consequences of the self-organization of amphiphiles (such as polyprenyl phosphates) in water into membranes, and does this lead to novel properties? Specifically, I consider the available evidence for the following features of membranes:

8. […]

Introduction

Mars is the world that has generated the most interest in life beyond the Earth. There are three reasons why Mars is the prime target for a search for signs of life. First, there is direct evidence that Mars had liquid water on its surface in the past, and there is the possibility that there is liquid water in the subsurface at the present time. Second, Mars has an atmosphere, albeit a thin one, that contains CO2 and N2. Third, conditions on Mars are cold and dry and thus are favourable for the preservation of evidence of organic remains of life that may have formed under more clement past conditions.

Mars may be cold and dry today but there is compelling evidence that earlier in its history Mars did have liquid water. This evidence comes primarily from the images taken from orbital spacecraft. Figure 12.1 from Malin and Carr (1999) shows an image of a canyon on Mars and represents probably the best evidence for extended and repeated, if not continuous, flows of liquid water on Mars.Water is the common ecological requirement for life on Earth. No organisms are known that can grow or reproduce without liquid water. Thus, the evidence that sometime in its early history Mars had liquid water is the primary motivation for the search for evidence of life (McKay, 1997).

Introduction

A decade has passed since the first discovery of an extrasolar planet by Mayor and Queloz (1995) and its confirmation by Marcy et al. (1997). Since this groundbreaking discovery, about 190 planets have been found around nearby stars, as of May 2006 (Schneider, 2006). Here we shall review the main methods astronomers use to detect extrasolar planets, and the data we can derive from those observations.

Aitken (1938) examined the observational problem of detecting extrasolar planets. He showed that their detection, either directly or indirectly, lay beyond the technical horizon of his era. The basic difficulty in directly detecting planets is the brightness ratio between a typical planet, which shines mainly by reflecting the light of its host star, and the star itself. In the case of Jupiter and the Sun, this ratio is 2.5 × 10–9. If we keep using Jupiter as a typical example, we expect a planet to orbit at a distance of the order of 5 astronomical units (AU, where 1 AU = 1.5 × 1011 m = distance from Earth to Sun) from its host star. At a relatively small distance of 5 parsecs from the Solar System, this translates into a mean angular separation of the sources on the sky of 1 arcsecond. Therefore, with present technology, it is extremely demanding to directly image any extrasolar planet inside the overpowering glare of its host star, particularly from the ground, where the Earth's atmosphere seriously affects the observations.

Introduction

The main themes of this chapter concern the phenomenon of emergence, the origin of life as a primary example of emergence, and how evolution begins with the inception of cellular life. The physical properties of certain molecular species are relevant to life's origins, because these properties lead to the emergence of more complex structures by self-assembly. One such property is the capacity of amphiphilic molecules such as soap to form membranous boundary structures, familiar examples being soap bubbles and cell membranes. A second example is the chemical bonding that allows biopolymers such as nucleic acids and proteins to assemble into functional sequences. Self-assembly processes can produce complex supramolecular structures with certain properties of the living state. Such structures are able to capture energy available in the environment and initiate primitive reactions associated with metabolism, growth, and replication. At some point approximately 4 billion years ago, cellular compartments appeared that contained macromolecular systems capable of catalysed growth and replication. Because each cellular structure would be slightly different from all others, Darwinian evolution by natural selection could begin, with the primary selective factor being competition for energy and nutrients.

Defining emergence

Researchers increasingly use the term emergence to describe processes by which more complex systems arise from seemingly simpler systems, typically in an unpredictable fashion. This usage is just the opposite of reductionism, the belief that any phenomenon can be explained by understanding the parts of that system.

Introduction: life beyond the habitable zone

As the horizons of the field of astrobiology have extended to the rapidly growing population of known extrasolar planetary systems, the concept of a habitable zone around each star has been of particular interest. With each discovery, the question is raised of whether the new planet is in the so-called habitable zone, or whether hypothetical orbits within that zone would be stable. In this context, the habitable zone is commonly defined as a range of distances from the star where temperatures would be in a range for life to be viable and for water to exist near the surface in a phase able to support living organisms (Kasting et al., 1993; Menou and Tabachnik, 2003; Raymond and Barnes, 2005). This restrictive definition can contribute to a relatively pessimistic view of the potential for extra-terrestrial life (e.g., Ward and Brownlee (2000)). It has even been used to advocate planetary exploration objectives, purportedly motivated by the search for life, that are restricted to the terrestrial planets in our Solar System (e.g., Hubbard (2005)).

Unless corrected, this widespread semantic usage of the term habitable zone would leave out one of the most likely places for exploration to reveal extraterrestrial life: Jupiter's moon Europa. The analyses cited above neglect tidal friction, which is known in the case of Jupiter's Galilean satellites to provide significant heating, comparable in fact to the solar heating in the habitable (and inhabited) terrestrial-planet zone.

Introduction

This chapter presents one of the very rare exobiological hypotheses. The main thesis is that there could be life in the dark dune spots (DDSs) of the southern polar region of Mars, at latitudes between –60° and –80°. The spots have a characteristic annual morphological cycle and it is suspected that liquid water forms in them every year. We propose that a consortium of simple organisms (similar to bacteria) comes to life each year, driven by sunlight absorbed by the photosynthetic members of the consortium. A crucial feature of the proposed habitat is that life processes take place only under the cover of water ice/frost/snow. By the time this frost disappears from the dunes, the putative microbes, named Mars surface organisms (MSOs) must revert to a dormant state. The hypothesis has been worked out in considerable detail, it has not been convincingly refuted so far, and it is certainly testable by available scientific methods. We survey some of the history of, the logic behind, the testable predictions of, and the main challenges to the DDS-MSO hypothesis.

History

The spots in question were observed on images made by the Mars orbiter camera (MOC) on board the Mars Global Surveyor (MGS) spacecraft between 1998 and 1999 (images are credited to NASA/JPL/Malin Space Science Systems). These features appear in the southern and northern polar regions of the planet in the spring, and range in diameter from a few dozen to a few hundred metres.

Perhaps the biggest disappointment to any beginning astrophotographer is finding out how hard it is to focus the camera accurately. With DSLRs, the problem is even worse than with film SLRs because the viewfinder is smaller and dimmer, and also because standards are higher.

We want to focus DSLRs more precisely than film SLRs because we can. Unlike film, the DSLR sensor doesn't bend. Nor does light diffuse sideways in the image. It's easy to view the image tremendously magnified on the computer, so we are much less tolerant of focusing errors than we used to be (to our detriment) back in the film era.

Viewfinder focusing

Many astrophotographers find it unduly hard to focus an SLR manually by looking through the viewfinder. If you're one of them, it's a good idea to investigate the problem and try to build your skill. Now that we have several means of confirming focus electronically, I don't think optical focusing should stand alone, but let's get as much use out of it as we can.

The viewfinder eyepiece

The eyepiece on a DSLR is commonly out of focus. Most DSLRs have an adjustment called the eyepiece diopter (Figure 8.1) which you are expected to adjust to suit your eyes. This is rarely done, because for daytime photography with an autofocus camera, it doesn't matter. To someone who always uses autofocus, the viewfinder is just for sighting, not for focusing, and the image in it need not be sharp.

This chapter is an overview of the main technical issues that affect DSLR astrophotography. Many of these topics will be covered again, at greater length, later in the book.

Image files

File size

Compared to earlier digital astrocameras, the images produced by DSLRs are enormous. Traditional amateur CCD cameras produce images less than 1 megapixel in size; DSLR images are 6–12 megapixels, and still growing.

This has several consequences. First, you can shrink a DSLR image to a quarter of its linear size (1/16 its area) and still put a decent-sized picture on a Web page. That's an easy way to hide hot pixels and other defects without doing any other processing.

Second, you're going to need plenty of file storage space. It's easy to come home from a single evening's work with a gigabyte of image files, more than will fit on a CD-R. Invest in a relatively large memory card for the camera, a fast card reader for your PC, and perhaps a DVD burner for storing backup files.

Third, older astronomical image processing software may have trouble with large DSLR images. Regardless of what software you use, you'll need a computer with ample RAM (at least 1–2 GB recommended) and an up-to-date operating system (but not too new for your older DSLR). Smaller astronomical CCDs work well with older, smaller laptops, but DSLRs do not.

Raw vs. compressed files

Most digital photographs are saved in JPEG compressed format.