What is Life?:How chemistry becomes biology (6 page)

But then in the sixteenth century the beginnings of an intellectual stirring took place which before too long built up into a tsunami, an intellectual storm that transformed the scientific landscape of the time. What is now termed the modern scientific revolution, whose central figures include Copernicus, Descartes, Galileo, Newton, and Bacon, radically changed mankind’s perception of the universe and his proper place in it. Its major accomplishment: the long-standing teleological view of the universe underwent a dramatic reassessment and, in scientific quarters at least, was effectively discarded. In what was a dramatic turnaround from those 2,000 years of deeply entrenched and established thinking, that revolution dismissed the idea of an underlying purpose in nature, and replaced it by a view—indeed, the very essence of the modern scientific revolution—that
nature is objective,
that there is no underlying purpose to the natural order. The scientific and philosophic implications of that revolution cannot be overstated. Jacques Monod, in fact, considers that idea the
single most important idea
offered by man over the 150,000–200,000
years that he has inhabited the planet. That single idea propelled mankind into a new conceptual reality, one whose ultimate significance and impact we have yet to fully discover. But, paradoxically, that revolutionary idea, together with the accompanying change in man’s perception of the universe, only served to raise serious difficulties with regard to the life issue. Indeed the change in scientific perception ended up
accentuating
the life riddle by the creation of what appeared to be undeniable contradictions within the
new
scientific thinking. Prior to the modern scientific revolution a unity of sorts could be found in man’s view of the cosmos; teleology encompassed both the animate
and
inanimate worlds. But as a direct result of that revolution, the need to explain the existence of two worlds, and the nature of the relationship between those two worlds, necessarily arose. Remarkably then, the modern scientific revolution was not only unable to satisfy mankind’s relentless urge to find his proper place in the universe, but placed new and seemingly greater obstacles along the path to an improved understanding of the material world, a world that necessarily incorporates both animate and inanimate.

The next major step in this ongoing saga was the 1859 landmark publication of Charles Darwin’s
On the Origin of Species.
Remarkably, Darwin’s theory of evolution, though offering a grand unification of biology, only served to widen the chasm separating animate and inanimate. As previously mentioned, the scientific revolution of the seventeenth century was slow in coming about because Aristotle’s teleological argument was so persuasive, so logical, so empirically based—the world around us simply exudes endless examples of purposeful design, though, of course, that entire edifice of purpose rests on a biological foundation. In his paradigm-shattering thesis,
Darwin swept away the most compelling basis for believing in a teleological universe by the profound insight that a simple mechanistic explanation—natural selection—lay behind the emergence of purposeful design in living systems. Through his principle of natural selection, Darwin was able to extend and reinforce the scientific revolution, a revolution based on the axiomatic premiss of an objective universe, into that one area where it had seemed awkwardly inapplicable—into biology. Following that epoch-making contribution, cosmic teleology, at least in scientific circles, was finally laid to rest.

However, though Darwin did provide a ‘physical’ explanation as to how simple life evolved into increasingly complex life, Darwin did not explain, or even attempt to explain, the manner by which inanimate matter was transformed into simple life. Interestingly, that problematic omission was already obvious during Darwin’s time, notably by Darwin himself. In a letter to a botanist colleague he remarked: ‘it is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter’. Darwin deliberately side-stepped the challenge, recognizing that it could not be adequately addressed within the existing state of knowledge. Ernst Haeckel, one of Darwin’s contemporaries, put it rather less kindly with his comment: ‘the chief defect of the Darwinian theory is that it throws no light on the origin of the primitive organism—probably a simple cell—from which all the others have descended. When Darwin assumes a special creative act for this first species, he is not consistent, and, I think, not quite sincere …’
⁷
The central question of how life emerged—how design, function, and purpose were generated and incorporated into
non-living
matter, remained unresolved, a perpetual thorn in the side of the physical sciences.

The dramatic advances in physics that took place in the first decades of the twentieth century failed in their turn to clarify the issue. Indeed, in 1933, Niels Bohr, one of the fathers of atomic theory, in a famous ‘Light and Life’ lecture, went as far as to propose ‘that life is consistent with, but undecidable or unknowable by human reasoning from physics and chemistry’.
⁸
Effectively, Bohr extended what he perceived as the ‘irrationality’ of quantum theory, one that physicists were forced to accept and accommodate, to biological systems as well. A kind of intrinsic biological irrationality! Living and non-living things can exist in two kinds of material form, and that is that. Erwin Schrödinger, the father of quantum mechanics, whose provocative little book,
What is Life?,
⁹
we mentioned earlier, was particularly puzzled by life’s strange thermodynamic behaviour. Simply, modern physics and biology appeared quite at odds—fundamentally incompatible. Schrödinger found himself following Bohr’s line of reasoning, and concluded, rather enigmatically, that living matter, while not eluding the established laws of physics, was likely to involve ‘other laws of physics’ hitherto unknown.

A generation later Jacques Monod, the Nobel biologist, in his classic 1971 monograph
Chance and Necessity,
¹⁰
lucidly reaffirmed the existence of a deep physics–biology divide, a divide only widened by the scientific revolution. The main issue that troubled Monod was life’s teleonomic nature. The very existence of that teleonomic character appeared to violate one of the fundamental principles of modern science—the objectivity of nature. Monod summarized the problem as follows:

Here therefore, at least in appearance, lies a profound epistemological contradiction. In fact the central problem of biology lies
with this very contradiction, which, if it is only apparent, must be resolved; or else proven to be utterly insoluble, if that should indeed turn out to be the case.

Simply put, how could function and purpose have emerged from an objective universe devoid of function and purpose? So though Aristotelian teleology had been vanquished by the new scientific order, its elimination left a troublesome vacuum. The scientific reality of teleonomy, so evident in every facet of the biological world, was undeniable. No cosmic implications there, just down-to-earth biological empiricism. But what is the source of this teleonomic character? How could purpose of
any
kind emerge from an objective universe? The conclusion seems inescapable: understanding life will require that we understand teleonomy—the two are necessarily and inexorably linked. But there is a positive aspect to this analysis. If we are able to explain the physical basis of teleonomy, it might provide mechanistic insight into the means by which life itself emerged. We will argue for such a connection in
chapters 7
and
8
.

In retrospect one might be tempted to say that part of the difficulty that physicists, such as Bohr and Schrödinger, had in addressing the life problem lay with the fact that the problem of what is life and how it emerged is fundamentally a
chemical
problem. After all, both the processes that govern the function of living systems, as well as the ones that presumably led to the emergence of living systems from inanimate matter, primarily take place at the scientific level of enquiry we call chemistry. But if one might consider that ignorance with regard to the chemical mechanisms of life was the missing element needed to properly address Schrödinger’s question, the dramatic developments within molecular
biology over the half-century following Schrödinger’s work proved otherwise. Watson and Crick’s 1953 landmark DNA study
¹¹
signalled the beginnings of a true revolution in our understanding of the cell-based machinery, the machinery of life. Major discoveries quickly followed—the mechanisms of DNA replication, protein synthesis, energy transduction, and central metabolic cycles, to name just a few. Truly dramatic advances in our understanding of many of the molecular mechanisms of life took place in rapid succession. Yet, paradoxically, our digging deeper and deeper into the mechanisms of life did not seem to lead us any closer to being able to address Schrödinger’s basic ‘what is life’ question, or the related question—how did life emerge? In fact, in 1974, twenty years after the discovery of DNA, Karl Popper, the iconic philosopher of science, supported the Bohr–Schrödinger view with his assertion that the origin of life problem was ‘an impenetrable barrier to science and a residue to all attempts to reduce biology to chemistry and physics’.
¹²
And the very same Francis Crick of DNA fame, in a 1981 text,
Life Itself
considered the emergence of life so miraculous an event that he even entertained the possibility of ‘directed panspermia’, the extreme idea that life on earth originated from outer space by the deliberate seeding of the earth by some alien life form!
¹³

The conclusion is quite striking. In the broadest sense we have made surprisingly little progress regarding the ‘what is life’ question since Charles Darwin. Yes, we now know that all life is cell based, that genetic information is coded in the DNA molecule, that the proteins of life so critical to all of life’s functionality are expressed through a universal code that relates the DNA sequence to particular amino acids, that there is a universal energy storage facility based on the ATP molecule. But that detailed molecular understanding, of
enormous significance in its own right, has only served to substantiate Darwin’s original claim—that all life is derived from some early common ancestor, that life is one thing. Darwin, of course, was lacking the plethora of mechanistic details that modern molecular biology has generously bestowed on us, but the belief in the unity of life, the insight that all life is related through physical law, was the essence of his contribution and the basis of the Darwinian revolution. Quite remarkably then, the molecular insights showered upon us by sixty years of extraordinary discoveries in molecular biology do not seem to have brought us any closer to resolving the ‘what is life’ question. Yes, as we have already noted, we can see many, many trees in the forest of life, but the view of the forest itself remains frustratingly obscure.

Enormous effort has gone into attempts to define life over the years and we will end this section by considering some of the more recent ones. That brief survey will only serve to reaffirm how confused the life topic has become. Literally hundreds of definitions have been proposed over the years and there are few signs that the flow is abating. In
Searching for the Definition and Origin of Life,
¹⁴
Radu Popa lists forty definitions that were proposed in 2002 alone, the last full year before his book was published, suggesting that the process of defining life has within it streaks of autocatalytic character. And therein lies the problem—the plethora of different definitions of life, many incompatible, if not outright contradictory, make it clear there is some inherent difficulty with the ‘definition of life’ endeavour. Stepping back and reflecting on this expanding literature from
a distance brings to mind the metaphor of a dog chasing its tail. Let’s consider several recent examples of life definitions arbitrarily chosen from Popa’s list to illustrate the problem first hand.

Life is defined as a material system that can acquire, store, process, and use information to organize its activities.
¹⁵

Life is defined as a system of nucleic acid and protein polymerases with a constant supply of monomers, energy and protection.
¹⁶

Life is defined as a system capable of 1. self-organization; 2.self-replication; 3. evolution through mutation; 4. metabolism; and 5. concentrative encapsulation.
¹⁷

Life is simply a particular state of organized instability.
¹⁸

The above definitions, all relatively recent and all insightful in their own way, show almost no overlap. If all of the definitions hadn’t begun with the two words ‘life is…’, we would be excused for believing that these definitions were about totally different concepts. The first, by Freeman Dyson, focuses on information (software); the second, by Victor Kunin, on the nucleic acid and protein infrastructure (hardware) and the energy required to drive the process; the third, by Gustaf Arrhenius, attempts to specify several of the characteristics that living things share; while the fourth, by Remy Hennet, addresses life’s thermodynamic aspect. And had we been willing to list other definitions from the many others on offer, we would have been able to come up with more definitional variety. Life is indeed many things, yet none alone is life.

Finally let us consider the most common and generally accepted definition of life, the one proposed within the NASA Exobiology Program in 1992, and generally referred to as the NASA definition of life:
Life is a self-sustained chemical system capable of undergoing Darwinian evolution.
Though attractive in some respects, it also suffers from
certain deficiencies. The first might be considered a technical one. The NASA definition could be understood to refer to
individual
life forms, say, a bacterium, an elephant, or a human. However individual life forms cannot undergo evolution; they can only reproduce and die. It is only
populations
of living things that are able to undergo Darwinian evolution. But even ignoring that technical aspect, the definition remains problematic as it has obvious exceptions. A mule, the offspring from the mating of a horse and a donkey, is sterile, so it clearly cannot reproduce. That of course means that a population of mules cannot undergo Darwinian evolution, even though we all agree that mules are alive. The same goes for solitary rabbits—unable to reproduce, yet very much alive. This criticism, based on mules and single rabbits, has been expressed quite frequently in recent years and through repetition seems to have lost some of its force. However familiarity should in no way undermine its relevance and validity. The criticism is soundly based and cannot be ignored. Like so many life definitions, it is too easy to cite exceptions. Invariably living things are either
excluded
from the various definitions or non-living things are improperly
included
in them.