Science is the midwife of metaphysics. However much it might protest that it confines itself to physical realities only, science cannot help but provoke metaphysical questions in the human mind. To be sure, when it confines itself to its own specialized sphere, each science is strictly physical, physical both in the scope of its investigation and in the results and data those investigations produce. But science's ultimate import is always metaphysical.

The problem of what exists: why this universe?

Einstein reportedly said: “What really interests me is whether God had any choice in the creation of the world”. What he meant by this informal remark was whether the physical universe must necessarily exist as it is or whether it could have been otherwise (or could have not existed at all). Today, almost all scientists believe that the universe could indeed have been otherwise; no logical reason exists why it has to be as it is. In fact, it is the job of the experimental scientist to determine which universe actually exists, from among the many universes that might possibly exist. And it is the job of the theoretician to construct alternative models of physical reality, perhaps to simplify or isolate a particular feature of interest. To be credible, these models must be mathematically and logically self-consistent. In other words, they represent possible worlds.

Let me give one example from my own research (Birrell and Davies, 1978). The equations of quantum field theory describing a system of interacting subatomic particles are often mathematically intractable. But several “toy models” exist, the equations for which may be solved exactly because of special mathematical features. One of these, known as the Thirring model, describes a two-spacetime-dimensional world inhabited by self-interacting fermions. This impoverished model of reality is designed to capture some features of interest to physicists in the real world. It is not, obviously, an attempt to describe the real world in its entirety.

Introduction

Any chemist looking at the molecular workings of a living cell from the vantage point of organic chemistry may have moments in which he desists from scientific business-as-usual and finds himself standing in awe before so much “molecular ingenuity” and sheer chemical beauty. If anyone, besides the biochemist, may be fit to recognize such marvels on the molecular level and put them into a proper perspective, it is the synthetic organic chemist, who tends to judge any new discovered molecular structure or process by the criterion of whether he could do such a thing himself: “If I had to, could I make this?” The question reflects a dichotomy, epistemological in nature, that has been with the science of organic chemistry from the very beginning: the two-fold task of studying molecules occurring in nature and creating by chemical synthesis molecules that have never existed before. Chemical synthesis has traditionally been the organic chemist's major tool for exploring the molecular world: the ability to synthesize molecules of ever-increasing complexity that mirror those produced by living nature has been a significant measure of progress in organic chemistry as a whole. Yet, the gap between what chemists are able to create by chemical synthesis and what nature achieves in biosynthesis remains immense.

Fortunately, it is inspiration rather than resignation that chemists are drawing from this gap, and they are encouraged to do so by considering how chemical thought and the chemist's ability to make molecules have changed in the course of the past two centuries.

How did life begin?

I (and most scientists) would answer, “By accident.” But what an absolutely unlikely accident it must have been! The earth on which life first appeared – prebiotic earth – was most inhospitable: a violent place, wracked by storms and volcanoes, wrenched by the pull of a moon that was much closer than the one we know now, still battered by cosmic impacts. On its surface and in its oceans were myriads of organic compounds, some formed in processes occurring on earth, some imported by infalls from space. Out of this universe of tumult and molecules, somehow a small subset of chemical processes emerged and accidentally replicated, thus stumbling toward what became the first cells. How could such a chaotic mixture of molecules have generated cells? Order usually decays toward disorder: Why do the tracks that led to life point in the opposite direction?

The origin of life is one of the biggest of the big questions about the nature of existence. Origin tends to occur frequently in these big questions: the origin of the universe, the origin of matter, the origin of life, the origin of sentience. We, scientists and non-scientists alike, have troubles with such “origins” – we were not there watching when the first events happened, we can never replicate them, and, when those first events happened, there was, in fact, no “we.”

Introduction

Kant (1755) and Laplace (1796) laid the foundations of our theory of planet formation, arguing that the Solar System formed in a flattened disk orbiting the Sun. Today we know such disks to be a part of the star formation process (see Chapter 4), and we have come a long way toward understanding the details of how a protostellar disk turns itself into planets. Most importantly, for ten years now we have been finding planets orbiting other stars; the current count of detected exoplanets exceeds 180, so that the Solar System now constitutes only a small minority of all known planets. The bad news is that Mercury, Venus, Earth, and Mars remain the only examples of terrestrial planets we know, since such bodies are still well below the mass threshold for detection around ordinary stars. Thus, apart from a pair of bodies orbiting the pulsar PSR1257 + 12 (Wolszczan and Frail, 1992), all known exoplanets are giants: gaseous bodies like Jupiter or Saturn with a couple of potentially Neptune-like examples recently added to the menagerie. However, impressive as the ‘king of the planets’ and its kind are, it is a terrestrial planet – a modest rocky body, three-thousandths Jupiter's mass – which actually harbours the life in our Solar System. And as we contemplate the possibility of life elsewhere in the Universe, it is inevitably upon terrestrial planets that we focus our attention.

Introduction

Every living organism is adapted to a specific growth temperature. In the case of humans, this is 37 °C and an increase by 5 °C becomes fatal. In the world of microbes, the growth temperature range is much more diverse: heat lovers (‘thermophiles’) grow optimally (fastest) at temperatures up to 65 °C (Brock, 1978; Castenholz, 1979). Since the time of Pasteur, it had been assumed generally that growing (vegetative) cells of bacteria were killed quickly by temperatures of 80 °C and above. The Pasteurization technology is based on this observation. In contrast, during the past few decades, hyperthermophiles (HT; Stetter, 1992) that exhibit unprecedented optimal growth temperatures in excess of 80 °C have been isolated (Stetter et al., 1981; Zillig et al., 1981; Stetter, 1982). HT turned out to be very common in hot terrestrial and submarine environments. In comparing the growth requirements of these present-day HT with the conditions on ancient Earth, similar organisms could or even should have existed already by Early Archaean times. Propelled by impact energy, microbes could have spread in between the planets and moons of the early Solar System. Is there any evidence for the existence of microbes at that time? Most likely, yes. But the recognition of ancient microfossils on the basis of morphology turned out to be difficult, leading to controversy. Nevertheless, there are chemical traces of life within rocks from Precambrian deep sea vents (Schopf et al., 1987; Brasier et al., 2002; van Zullen et al., 2002).

The boundaries of philosophy, science, and theology

Some of the deeper questions that humans have raised are not always answerable within the boundaries of science. Instead, philosophers and theologians have approached such questions within their own domains of competence. One such example is provided by the question of purpose in evolution (see the discussion below). Indeed, the concept of purpose in a general sense may be understood as something that one sets before oneself as an object to be attained, an aim to be kept, a plan to be formulated. In attempting to give an answer to the question of “purpose in nature,” we should discuss the main components of human knowledge in an integrated way, so as to ask the right questions in the right field of knowledge. This approach should encourage us to provide appropriate answers that are reasonable within philosophy, science, or theology. At this juncture, it may also be argued that the task of a scientist should be independent of those of the other areas of human culture (Russell, 1991, p. 13). On the other hand, it is surely useful to be aware that this view of the role of science that is “divorced” from both philosophy and natural theology can also be seen from a different point of view (Townes, 1995, p. 166): because science and religion are evolving and are similar in their search for truth, convergence of these independent searches for truth may occur in the future.

Our knowledge of the universe is steadily expanding. In large measure, this has been a result of radioastronomical observations. Among the most important is that the majority of the radiation of the universe is almost uniform and follows the spectral distribution of a thermal source at a temperature of 2.725 K. This cosmic background radiation is the remnant of the initial event, the Big Bang. Although this radiation is essentially uniformly distributed in the universe, the distribution of matter is highly non-uniform.

Matter, 99% hydrogen and helium, is found virtually entirely within galaxies. Galaxies occupy 10−7 of the volume of the universe, but contain most of the known matter. The origin of this separation of radiation and matter is a topic of much current study, as is the question of galaxy formation and the abundance and distribution of intergalactic matter. Although they are clearly fundamental to the question of the chemistry that occurs, it is not my intent or capability to discuss these most interesting questions (see Peebles, 1993). The heterogeneous distributions of matter occur universally. The average density within galaxies is 1 atom of hydrogen cm3, whereas outside of galaxies estimates are of less than 1 atom of hydrogen m3 in intergalactic regions. This sharp aggregation of matter means that the chemistry is occurring within galaxies. It is sensible to focus the discussion, therefore, primarily on the molecular abundances and the chemistry occurring in our galaxy, the Milky Way, because observations are much easier in view of the decrease of radiation intensity as the inverse square of the distance.

Introduction

Tracing organismal (species) histories on large evolutionary timescales remains a big challenge in evolutionary biology. Darwin metaphorically labelled these relationships the ‘tree of life’, but in his notebook he expressed unhappiness with this label, because in the ‘tree of life’ depicting species evolution only the tips of the branches are alive; this layer of living organisms rests on dead ancestors and extinct relatives. Darwin mused that therefore the term ‘coral of life’ would be more appropriate (B26 in Darwin (1987)); however, this alternative label did not gain popularity. Different markers have been utilized to elucidate the relationship among different lineages: from morphological characters to complete genomes. Since complete genomes are now available for organisms from all three domains of life, it is possible to use large amounts of data to attempt to decipher the relationships between all known organisms.

Many comparative genome analyses have shown that different genes in genomes often have different evolutionary histories (e.g., Hilario and Gogarten (1993), Nesbo et al. (2001), and Zhaxybayeva et al. (2004)), which implies that the tree of life metaphor (and a bifurcating tree as a model for evolutionary relationships in general) might be no longer adequate (Doolittle, 1999). The incongruence between gene histories can be attributed to many factors, one of which is horizontal gene transfer (HGT). Simulations based on coalescence have shown that HGT can affect not only the topology of an inferred phylogeny (and therefore inferences of last common ancestors), but also divergence times (Zhaxybayeva and Gogarten, 2004; Zhaxybayeva et al., 2005).

Learning from our own existence

Hoyle's [1] successful prediction of the 7.6 MeV resonance of the carbon-12 nucleus, based on observation of his own carbon-based existence, established the scientific usefulness of anthropic principles. These principles have become common, if not yet standard, tools in cosmology, where theories of initial conditions may not yet exist – or, if they do exist, may admit a range of values [2, 3, 4, 5, 6]. At the same time, anthropic principles have retained a traditional role in religion and philosophy, where sensitive dependence of human existence on laws of nature that could imaginably have been otherwise is interpreted as evidence for human significance in the creation of the universe.

The tendency of life forms to make universal use of, and seemingly to depend on, specific details of natural law or historical circumstance does not end with nuclear abundances. Following decades of studying the ways in which mammalian blood achieves homeostasis by exploiting favorable regions in carbonic-acid chemistry and similar adaptations, Henderson compiled a list of such dependences in physiology [7]. Inverting Darwin's description of selective fine-tuning of organisms for “fit” to their environments, Henderson characterized his physiological sensitivities anthropically as evidence of “the fitness of the environment” for life.

The rapid growth in understanding of biology, from structures to systems, seems likely to expose many more sensitivities of life to details of chemistry, physics, and history.

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.