What is Life?:How chemistry becomes biology (23 page)

This different topology for the two worlds has interesting consequences. It not only explains life’s diversity but it also explains how we are able to go back in time and seek our evolutionary roots. A divergent topology in the forward direction becomes a
convergent
one in the backward direction. It is that convergent topology in the
reverse
direction that enables us to utilize phylogenetic analysis and the fossil record to trace our evolutionary history going back in time, to deduce that all living things can be divided into three life kingdoms—Archaea, Bacteria, and Eukarya—to trace out the history of life on earth toward life’s so-called Last Universal Common Ancestor (LUCA). But that, of course, means that we can say nothing at all regarding where evolution may take us in the future. Set off on a divergent path and there’s no telling where you’ll get to. As Yogi Berra, the well-known sports celebrity, once put it: ‘If you don’t know where you are going you will wind up somewhere else.’ The different reactivity patterns of both ‘regular’ and replicative
systems as a function of time—forward or backward—is simply explained.
^63
,
64

We have remarked how life’s homochiral (single-handed) nature presents a puzzle at two levels. First, how did life’s single-handedness emerge from a universe that is inherently two-handed, and second, how is that homochirality maintained, given that homochirality is intrinsically less stable than heterochirality. We have seen in this book that one of the key ideas that can explain the emergence of life on earth is the enormous kinetic power of auto-catalysis. It is then remarkable to discover that the unexpected emergence of homochirality from a heterochiral environment can be explained in precisely the same terms! Normally when one carries out a chemical reaction that transforms a non-chiral substance (possessing no handedness) into a chiral one, the product is composed of equal amounts of left- and right-handed forms. But in 1995 the renowned Japanese chemist, Kenso Soai, made a remarkable discovery.
⁶⁵
In certain instances it is possible to obtain effectively just one homochiral product from a non-chiral starting material. Somehow the symmetry of the system is broken, which is quite extraordinary. It’s like tossing a coin a thousand times and observing 999 heads and one tail! No wonder Kenso Soai’s unexpected result caused a sensation. In other words it
is
possible to generate homochiral systems, starting from a non-chiral environment, even though for many years this was considered physically unreasonable. So what has this to do with the emergence of life?

Soai’s highly unexpected result is explained by the fact that the chemical reaction he studied proceeds
autocatalytically,
and therefore
product formation shows exponential growth. If the reaction mixture is initially seeded with a tiny excess of one of the chiral products, then the spectacular amplification that autocatalysis can generate results in that product reaching a level of purity very close to 100 per cent. In other words, just as replication is autocatalytic, so homochirality (single-handedness) can be induced in a system that shows autocatalytic behaviour. This reaction and its detailed explanation are somewhat technical but the bottom line is straightforward:
the kinetic power of replication which is responsible for the emergence of life could well have been responsible for one of life’s most striking features—its homochiral character.
The pieces of the life puzzle do fit together. How satisfying!

We have explained the
emergence
of homochirality from a non-chiral environment, but how is that homochirality maintained if homochirality is intrinsically less stable than heterochirality. Like several previous life dilemmas, this issue is also resolved through the DKS concept. Yes, systems that are of one chiral form
are
less stable than heterochiral mixtures, but that is only true in a thermodynamic sense. We have already seen that in the context of replicating systems, the stability that counts is DKS, and for this stability kind it turns out that homochiral systems are
more
stable than heterochiral ones. Life’s reactions require high specificity, meaning precise lock-and-key type interactions between reacting molecules and that can only be obtained in homochiral systems. Introduce heterochirality into such systems and you will end up with half the keys not fitting into their locks! Homochiral systems are therefore more effective replicators than heterochiral ones, and as a consequence homochiral systems exhibit greater stability in the crucial DKS sense.

We discussed this most amazing of life’s properties in some detail in
chapter 1
. To reiterate, both the structure and the behaviour of all living things lead to an unambiguous and unavoidable conclusion—living things have an ‘agenda’. Living things act on their own behalf. But how can that be? How can matter, when organized in the manner we classify as biological, seemingly follow different rules from those of inanimate systems? How can matter of any kind appear to have an agenda? Let us see how the DKS concept can help resolve this puzzle. Recall that the reactions of simple replicating systems—say, replicating molecules—would follow the thermodynamic directive, much like a car without an engine follows the gravitational directive—it can only roll downhill. But once a replicating entity has taken on an energy-gathering capability, the replicating entity is now ‘freed’ of thermodynamic constraints and can follow the
kinetic
directive—the drive toward greater DKS. As we discussed earlier, a replicating entity with an energy-gathering capability is now like a car
with
an engine—it can go uphill too. That means that a replicating system with an energy-gathering capability would
appear
to have an agenda. It would seem to be acting purposefully, as it would no longer need to be confined to the downhill thermodynamic path, which we interpret as
objective
behaviour, but rather the path toward systems of greater DKS, which could involve the equivalent of rolling some way uphill. In other words, once a replicator has taken on an energy-gathering capability (as part of the general process of complexification toward more complex and more stable replicating systems), we would interpret and understand its subsequent replicative behaviour as
purposeful.
⁶⁶
Monod’s
paradox—how a purposeful system can emerge from an objective universe, is seen to result from the interplay of kinetic and thermodynamic directives in chemical reactions. In the ‘regular’ chemical world, thermodynamics is the dominant directive and results in so-called
objective
behaviour. In the replicating world, kinetics is the dominant directive and so actions in that world
appear
purposeful.

There are other profound life issues that we have not touched upon—consciousness, for example. While consciousness is certainly a characteristic of life, it is not an essential one, as it is only associated with advanced life forms. Accordingly, we have not dealt with it. Nonetheless, the issue of consciousness should be mentioned, if only to demonstrate how limited our understanding of some life characteristics remains. Having said that, the phenomenon of consciousness can be explored through its evolutionary context. Evolution is the process by which all properties of matter are exploited in the evolutionary drive toward more effective replicating systems. Evolution exploits matter’s propensity for hardness when that is useful, as in bones. It exploits matter’s ability to be flexibly firm when that is needed, as in cartilage; matter’s ability to be liquid when that is needed, as in blood; matter’s ability to be transparent as in crystallin, the protein from which the lens of the eye is made; matter’s ability to conduct electric charge, and so on. But it turns out that matter in some particular organization has an even more remarkable characteristic—the remarkable property of consciousness. Indeed, an extraordinary characteristic—matter can be self-aware. Evolution has discovered that capability of matter, like all others that it has come across, and utilized it in the ongoing
search for stable replicating entities. If we want to understand consciousness and its basis, we should study its source—neural activity at its most rudimentary level, and then track the phenomenon, step by step, through to its more advanced manifestations, ultimately to us humans. So the approach would be the same as the one we have taken in addressing the problem of abiogenesis—start simple. A fascinating scientific journey awaits us.

Having explained life’s global characteristics in chemical terms, we can now pose the question: how would alien life look? Since we believe that life on Earth emerged from inanimate matter, it naturally follows that under appropriate conditions life could also emerge elsewhere in the universe. And while that life could also be based on the same molecular foundation—the nucleic acid–protein duo—other replicative combinations cannot be ruled out. We now understand that the basis of life consists of long-chain molecules capable of catalysing their own replication, which together with other chain-like molecules possessing catalytic capabilities would undergo a continual process of replication, mutation, and complexification. However, there is no reason at all to believe that in principle there would not be chemical combinations, other than that nucleic acid-protein duo, that could lead to that same general result. In fact, all of our experience in chemistry tells us that chemical characteristics are related to groups of substances, not to individual ones, so the expectation would be that, in principle at least, there would be a
group of materials
on which the processes of life could be based. So, if life did emerge on some other planet, one
based on a different biochemistry from that on earth, can our theory of life offer some insight into how such life would appear? I believe so. My short answer: life on other planets would look exactly like that on our own!

I write that partly tongue in cheek because life’s diversity has offered us an unimaginably large array of forms, from microscopic bacteria through to blue whales, so it is hard to see how life forms of any other kind would strike us as fundamentally different in their external appearance, and certainly no more alien looking than many of life’s existing forms. More to the point, however, is the fact that life’s morphology appears to be based on what living things require it to be, rather than some directive that comes from its underlying chemistry. Cars made from fibreglass, aluminium, or steel don’t look too different from one another because their appearance is based on the shape cars need to be in order to function as cars. All cars, regardless of the material from which they are made, require an external shell in which to house the motor and create a cabin for passengers to sit in. They all possess windows so the driver can see where he is going, and wheels to minimize friction. That is true whether the cars are made in the US or in China, whether the windows are glass or plastic, whether the engine is electric or gasoline. In the same way, life forms that emerged from some replicating entity that did not belong to the nucleic acid family, but were able to complexify and evolve toward replicating entities of greater DKS, would likely utilize the same universal concepts that nucleic acid-based biochemistry discovered. Depending on the extent to which that other life form had evolved, it would also express network characteristics, and may have discovered the replicative value of a cell structure, in which the cell’s
functional parts with its replicative and metabolic capabilities would be incorporated. The theory of life presented here is not one based on material, but one based on process, and therefore the nature of the material would be secondary, possibly even incidental, in governing life’s underlying characteristics.

Which brings us to the most intriguing of questions—how would one synthesize a simple living system? To this question there is no simple answer. If the theory of life presented here teaches us anything, it is that the synthesis of some entity that would possess the characteristics of a primitive life form, say a protocell, faces enormous difficulties. Let’s see what these are. I will begin with some observations.

The relationship between living and non-living systems is particularly fascinating in at least one respect. It is so easy to transform living systems into non-living ones, but, as we know all too well, the process is not reversible—life is so easy to destroy, but (chemically speaking) so hard to make. That simple fact in itself is highly informative. The problem with the synthesis of a living system is not one of material, but, as noted, one of organization. You can have all the components of a living cell available, but packaging it so that it behaves as a living entity is where the difficulty lies. So what is the problem? Life is a
dynamic
state of matter meaning that the biomolecules that make up the living cell are in a constant state of flux. A simple physical analogy that captures this dynamic character would be that of a juggler juggling several balls. That dynamic state is of course identical in a material sense to the one in
which a man stands next to those balls, which are resting on the ground. But the difference is profound. How easy it is to take a juggler juggling several balls and to convert him into the non-juggling state, one in which all the balls are lying on the ground. A hefty push and you are there! A man standing next to five balls would be the metaphor for death. Of course going in the other direction is not that simple. You cannot simply throw five balls in one go at a person and expect him to enter the juggling state. That won’t work. In the same way, if you take all the components of a living cell and mix them together, you won’t end up with a living cell. At very best, if all the bits and pieces end up in the right place, you’ll end up with the equivalent of a
dead
cell. You’ll end up with a clump of stuff—a thermodynamic aggregate. Recall, however, that the living cell is in a dynamic, far-from-equilibrium state, like that bird flapping its wings to stay airborne. Simply bringing together the components that can potentially make up an integrated and dynamic system that we would classify as alive won’t lead to that special organizational and dynamic character that we recognize as life.