The considerations on gravitational collapse so far have been with the motivation to address the physical questions such as the role of collapse in astrophysics and cosmology. Many of the cosmic processes, such as the birth of stars, the formation of galaxies, and others, are not well understood today, but it is clear that gravitational collapse will play a major role there. Hence, understanding the dynamics of the collapse is important, as has been attempted here in various cases.

The important question of the final fate of massive stars at the end of their life cycle, when they have used all their nuclear fuel, and when gravity becomes the sole and key governing force, has drawn much attention for many decades. The importance of this issue was highlighted by Chandrasekhar (1934), who pointed out that the life history of a star of small mass must be essentially different from that of a star of large mass, and that while a small mass star can pass into a white dwarf stage, a star of large mass cannot go to this state, and one is left speculating on other possibilities. The question as to what happens when a massive star, heavier than a few solar masses, collapses under its own gravity has been a fundamental key problem in astronomy and astrophysics. If the star is sufficiently massive, beyond the white dwarf or neutron star mass limits, then a continued gravitational collapse must ensue without achieving any equilibrium state, when the star has exhausted its nuclear fuel.

Introduction

We live in a biofriendly world. Were it otherwise, we wouldn't be around. The question is, therefore, how biofriendly is it? Physicists have addressed this question and have come to the conclusion that if any of the fundamental physical constants were a little smaller or a little larger than they are, the universe would be very different from what it is and unable to produce or harbor living organisms. Not everyone, however, subscribes to the concept of “fine-tuning” embodied in the so-called Anthropic Principle, some preferring instead the notion of a “multiverse,” in which our universe is only one in trillions of trillions, perhaps the only one that, by mere chance, happened to have the right combination of constants to enable it to serve as our birthplace and abode.

In contrast, biologists and other scientists interested in biology generally take the universe for granted and ask instead to what extent the manifestations of life, including humankind, fit within the existing physical and cosmic framework. Nothing could better illustrate the depth of their ignorance on this subject than the diversity of answers they have given, which cover virtually the whole array of possibilities. Many agree with the late Jacques Monod, who, in his best-seller Chance and Necessity (1971), expressed his skepticism in the oft-quoted sentence: “The Universe was not pregnant with life, nor the biosphere with man.”

Introduction

The robust formation of planets as well as abundant sources of water and organic molecules are likely to be important prerequisites for the wide-spread appearance of life in the cosmos. The nebular hypothesis of Kant and Laplace was the first to propose that the formation of planets occurs in gaseous disks around stars. The construction of new infrared and submillimetre observatories over the last decade and a half has resulted in the discovery of protoplanetary discs around most, if not all, forming stars regardless of their mass (e.g., reviews by Meyer et al. (2006), Dutrey et al. (2006)). The recent discoveries of extrasolar planets in over a hundred planetary systems provides good evidence that Jovian planets, at least, may be relatively abundant around solar-like stars (see Chapter 1). These results beg the question of whether protoplanetary disks are also natural settings for the manufacture of the molecular prerequisites for life. Life requires water and organic molecules such as amino acids, sugars, nucleobases, and lipids as building blocks out of which biological macromolecules and cellular structures are made, and many of these can be manufactured in protoplanetary disks.

In the first part of this chapter we review the properties of protoplanetary disks and how planets are believed to form within them. We then consider the evidence that these disks may be a major source of the water and biomolecules available for the earliest life, as on the Earth.

Four basic observations

Progress in cosmology in the past few decades has also led to new insights into the global question of the emergence of intelligent life in the universe. I am referring not to discoveries that are related to very localized regions, such as the detection of 200 extrasolar planetary systems (at the time of writing), but rather to properties of the universe at large.

In order to set the stage properly for the topics that follow, I would like to start by presenting four observations with which essentially all astronomers agree. These four observations define the cosmological context of our universe and form the basis for any theoretical discussion.

1. Ever since the observations of Vesto Slipher in 1912–22 (Slipher, 1917) and Edwin Hubble (1929), we have known that the spectra of distant galaxies are red shifted.

2. Observations with the Cosmic Background Explorer (COBE) have shown that, to a precision of better than 10−4, the cosmic microwave background (CMB) is thermal, with a temperature of 2.73 K (Mather et al., 1994).

3. Light elements, such as deuterium and helium, have been synthesized in a high-temperature phase in the past (see, for example, Gamow, 1946; Alpher et al., 1948; Hoyle and Tayler, 1964; Peebles, 1966; Wagoner et al., 1967).

4. Deep observations, such as the Hubble Deep Field, and the Hubble Ultra Deep Field, have shown that galaxies in the distant universe look younger. Specifically, they are smaller (see, for example, Roche et al., 1996; Ferguson et al., 2004), and they have a higher fraction of irregular morphologies (see, for example, Abraham et al., 1996). This is what one would expect from a higher rate of interactions and from the “building blocks” of today's galaxies.

In 1913, long after Charles Darwin had argued for the fitness of organisms for their environment, the Harvard chemist Lawrence J. Henderson pointed out that the organisms would not exist at all except for the fitness of the environment itself. “Fitness there must be, in environment as well as in organism,” he declared near the outset of his classic work, The Fitness of the Environment (1913, p. 6). While most of Henderson's contemporaries ignored the philosophical implications of this work, as John Barrow and Frank Tipler have noted, it “still comprises the foundation of the Anthropic Principle as applied to biochemical systems” (1986, p. 143).

Henderson pointed out the uniqueness of hydrogen, carbon, and oxygen in the chemistry of living organisms. Another two decades would pass before astronomers would establish that these were three of the four most abundant elements in the cosmos; but Henderson was at least aware that these atoms were commonly found in the stars and planets. In his treatise, he began with the properties of water, just as William Whewell had done eight decades earlier in his far more teleologically oriented Bridgewater Treatise (1833).

Henderson grouped the notable qualities of water under two headings: (1) thermal properties and (2) interaction with other substances. As far as he was concerned, these were empirical, observed properties with minimal theoretical explanation. (Remember that Rutherford's nuclear atom was still a future concept, while quantum mechanics and the nature of the hydrogen bond lay many more years ahead.)

Introduction

It is now believed that our universe was created around 13.8 billion years ago. Our planet earth came into existence around 4.5 billion years ago. For nearly a billion years or so after it was formed, the earth was stark and bereft of life. The matter contained on earth was inorganic with relatively small molecules. There were endless rock formations, oceans, and an atmosphere.

And then there was life.

The problem of how life was created is a fascinating one. Our focus is on looking at life on earth and asking how it works. The lessons we learn provide hints to the answer to the deep and fundamental question pondered by our ancients: Was life on earth inevitable? Then there are the questions posed by Henderson [1]: Is the nature of our physical world biocentric? Is there a need for fine-tuning in biochemistry to provide for the fitness of life in the cosmos – or, even less ambitiously, for life here on earth? Surprisingly, as we will show, a physics approach turns out to be valuable for thinking about these questions.

All living organisms have a genetic map consisting of a one-dimensional string of information encoded in the DNA molecule. An essential question that one seeks to answer is how an organism converts that information into a three-dimensional living being.

Life has many common patterns. All living cells follow certain simple “universal” themes.

Introduction

Life as we know it today completely depends on water. Without water, life would be either impossible or totally different. Thus, a deep understanding of the relationship between water and other simple building blocks of life is crucial to gain insight into how prebiotic life forms could have originated and evolved and whether the physical laws of this universe are in any way predisposed to the emergence of life (Henderson, 1913, 1917; Eisenberg and Kauzmann, 1985; Ball, 2001).

It is unlikely that under prebiotic conditions the complex and sophisticated biomacromolecules commonplace in modern biochemistry would have existed. Thus, research into the origin of life is intimately associated with the search for plausible systems that are much simpler than those we see today. However, it is also plausible that these simple building blocks of life might have been amphiphilic molecules in which water could have had an enormous influence on their prebiotic molecular selection and evolution, because water can either form clathrate structures or drive these simplest molecules together (Ball, 2001).

Structure of water

Water is both simple and complex. It is simple because it consists of only one oxygen atom and two hydrogen atoms (see Figure 20.1). But at the same time it exhibits highly complex molecular behavior (far exceeding the multibody problem in mathematics, planetary science, and astrophysics) wherever numerous water molecules interact dynamically (Eisenberg and Kauzmann, 1985; Ball, 2001).

Introduction

If we inspect our surroundings on earth, we will see a myriad of materials, objects, natural and artificial constructions (many resulting from the activities of the human species), and, of course, an enormous variety of living organisms, from the simplest bacteria to the most complex animals. We can also inspect the sky and observe other planets, stars, galaxies, and clusters of galaxies far away in our expanding universe, leaving us wondering whether life can also be found elsewhere. Is it all an inevitable product of such a finely tuned construct as the universe seems to be, given appropriate local conditions? At a very basic level, it must be. We are an evolved species co-existing with many other simpler species on which we depend. All are made of the same chemical elements resulting from that fine-tuning that allowed their kinetically controlled formation in big stars and created our planet and the fields to which all life is exposed. Life must be a possibility included in a finely tuned cosmos – we sense it in our minds. We also know that it has evolved and diversified here on earth, although the reasons for this – that is, the factors that determined evolution – are not so obvious. This question has intrigued philosophers and scientists through the ages, and it reached public awareness toward the end of the eighteenth century.

This book is part of a two-part program focused on the broad theme of “biochemistry and fine-tuning.” Fitness of the Cosmos for Life began with a symposium held at Harvard University in October 2003 in honor of the 90th anniversary of the publication of Lawrence J. Henderson's The Fitness of the Environment. The symposium was an interdisciplinary, exploratory research meeting of scientists and other scholars that served as a stimulus for the creative thinking process used in developing the content of this book. The chapters in this volume were developed following the symposium and take advantage of the rich technical and interdisciplinary exchange of ideas that occurred during the in-person discussions.

The Fitness of the Cosmos program has provided a high-level forum in which innovative research leaders could present their ideas. In the spirit of multidisciplinarity, the fields represented by the meeting participants and book contributors are diverse. From the sciences, the fields of physics, astronomy, astrophysics, cosmology, organic and inorganic chemistry, biology, biochemistry, earth science, medicine, and biomedical engineering are represented; the humanistic disciplines represented include the history of science, philosophy, and theology.

This volume explores in greater depth issues around which the 2003 meeting was convened. It addresses the broad inquiry Is the cosmos “biocentric” and “fitted” for life? Keeping this question in mind, the authors presented their thoughts in the context of their own research and knowledge of others' writings on topics of “fitness” and “fine-tuning.” This work pays tribute to the groundbreaking inquiry of L. J. Henderson.

Introduction

The Earth is certainly, so far, the most interesting planetary body for astrobiology since it is still the only one where we are sure that life is present. However, there are many other bodies of astrobiological interest in the Solar System. There are planetary bodies where extraterrestrial life (extinct or extant) may be present, and which thus would offer the possibility of discovering a second genesis, the nature and properties of these extraterrestrial living systems, and the environmental conditions which allowed its development and persistence. Mars and Europa seem to be the best places for such a quest. On the other hand, there are planetary bodies where a complex organic chemistry is going on. The study of such chemistry can help us to better understand the general chemical evolution in the Universe and more precisely the prebiotic chemical evolution on the primitive Earth. Comets are probably the best example, especially considering that their organic content may have also been involved in the prebiotic chemistry on the primitive Earth.

Titan, which is the largest satellite of Saturn, may cover these two complementary aspects and is thus an interesting body for astrobiological research. Moreover, with an environment very rich in organics, it is one of the best targets on which to look for prebiotic chemistry at a full planetary scale. This is particularly important when considering that Titan's environment presents many analogies with the Earth.

Introduction

A glance at a bacterium, and a humpback whale, will reveal not only two immensely complex organisms, but also two very different life forms. Each is, in its respective way, constrained by a whole series of physical and chemical factors. One of the most obvious aspects is the fluid environment in which they live, albeit at scales that in being separated by about eight orders of magnitude are determinative of radically different behaviors. The bacterium's world is submillimetric, and accordingly it is dominated by constraints of viscosity. Motion involves the remarkable method of flagellar propulsion (the nearest thing to a wheel in biology; see, for example, Berg, 2003). When its flagella stop beating, the bacterium ceases its movement in a distance equivalent to the diameter of a hydrogen atom. The humpback whale, by contrast, occupies a liquid environment with which we are somewhat more familiar, although our swimming ability is feeble compared with the whale's oceanic travel range of thousands of kilometers. Fully aquatic, the humpback occupies a world that to us is both alien, with its complex system of echolocation, and familiar, with its ability to communicate – which includes singing.

Despite such wide divergences, the basic point of commonality is that both bacteria and whales live in environments where the physical controls imposed by the physico-chemical properties of water – be they viscosity or acoustic transmission – predetermine what is biologically possible. To this simple example could be added many other physical and chemical constraints.

Introduction

A famous metaphor in integrative biology refers to the relationship between environmental constraint and evolutionary change as “the ecological theater and the evolutionary play” (Hutchinson, 1965). In the forty years since the penning of that phrase, the relationship between play and stage has been a matter of vigorous and fascinating debate. The issues have profound implications, not only for our scientific account of the evolutionary process, but also for our expectation of what life might look like on other planets and, indeed, for our philosophical and theological understandings of what it might mean on this one.

The controversy involves differing conclusions about the roles of contingency and constraint in evolutionary history, including, among other things, the way in which fundamental regularities of the physico-chemical environment, or “stage,” influence the unfolding of the evolutionary drama. On the one hand, many prevalent expositions of the evolutionary play suggest that the fundamental or ultimate actors are genes, not organismic (much less mental) agents (Dawkins, 1976, 1998; Dennett, 1995). The drama itself is a theater of the absurd, a plotless improvisation using whatever props are contingently provided by the environment (Gould, 1989, 1996). Contingency is held to exert determinative influence on history, and, according to Stephen Gould's widely cited metaphor, “we would probably never arise again even if life's tape could be replayed a thousand times” (1989, p. 234).

Introduction

In his great classic The Fitness of the Environment (1913), Lawrence J. Henderson examined the fitness of the basic chemical constituents and chemical processes used by living organisms on earth and of the general environment, including the hydrosphere and atmosphere of the earth, and argued that the laws of nature and the properties of matter appear uniquely and maximally fit for life as it exists on earth.