Date Published: May 8, 2012
Publisher: Public Library of Science
Author(s): Gerald F. Joyce
Abstract: Why do we have so much trouble distinguishing life from non-life, and distinguishing our biology from another? When in doubt, one should count the “bits” of heritable information.
Partial Text: Thanks to a combination of ground- and space-based astronomical observations, the number of confirmed extrasolar planets will soon exceed 1,000. An increasing number of these will be said to lie within the “habitable zone” and even be pronounced as “Earth-like.” Within a decade there will be observational data regarding the atmospheric composition of some of those planets, and just maybe those data will indicate something funny going on—something well outside the state of chemical equilibrium—on a potentially hospitable planet. Perhaps our astronomy colleagues should be forgiven for their enthusiasm in declaring that humanity is on the brink of discovering alien life.
What, in fact, is the probability that a temperate, rocky planet will generate life? Science cannot say. That is because, based on the one known example of obscure origins, even a Bayesian would not want to assign a probability to such an event. The probability assessment would be more meaningful if there were even one more genuine example of life, whether discovered in space, on Earth, or in a test tube. If that entity had all of the properties of terrestrial life described above, then one would conclude that, indeed, we are not alone. But what if the entity had only some of those properties? What if it could self-reproduce, directing the assembly of progeny of identical composition, but could not evolve new functions? What if it consisted of complex chemical processes within a cellular compartment but had no basis for maintaining heritable genetic information? What if it had all of the properties of life but was descended from our own life form rather than derived from an independent origin?
In principle, there are two pathways by which a new life form can arise: either directly from chemistry or spun off from some other biology. If life arises from chemistry, as is thought to have occurred on the primitive Earth (Figure 2), then it begins with zero heritable bits and organizes into a bit-generating system. Following an era of prebiotic chemistry, perhaps reaching a high level of chemical complexity, molecular memory arises. Molecules of variable composition begin to replicate, mutate, and evolve in a Darwinian manner. If, instead, life arises from another life form, then it may have a privileged beginning, benefitting from a chemical environment that has been shaped by preexisting life. A new life form may grow on the spoils of prior life with no carryover of bits, or it may descend from prior life with some carryover of bits into a different genetic system. The latter type of transition is thought to have occurred during the early history of life on Earth when RNA-based life gave way to DNA/protein-based life . That transition likely involved a substantial transfer of bits that had accrued within RNA and were ported over to DNA through what might be termed The Great Reverse Transcription. We see echoes of those ancestral bits in the sequences of ribosomal RNA, tRNA, and possibly other contemporary RNAs that are present across all three kingdoms of life.
The above discussion emphasizes that there are many possible routes by which one life form can give rise to another, but it does not address the question of how an initial life form arises in the first place. A key question to ask is: What is the minimum number of bits it takes to provide a replicating, evolving system that has the ongoing capacity to accrue more bits? That will depend on the chemical nature of the first self-replicating molecules and the resources that are available in its environment. When astronomers speak of habitable zones they refer to a planetary orbit that is of appropriate distance from a star to maintain liquid water on the planetary surface. This may be too restrictive or too generous a definition, depending on your point of view . Perhaps life can exist in a non-aqueous environment, although there is little data to support this conjecture. Conversely, even a temperate little pond of water, salts, and dilute organics may be insufficient for life to originate. The pond would need to accumulate heteropolymers of variable composition, including some that could replicate and provide the basis for molecular memory, and it is not clear whether this is a common or exceedingly rare occurrence.
My laboratory recently described an example, outside of biology, of a chemical system that can undergo Darwinian evolution in a self-sustained manner . “Self-sustained” in this context means that all of the bits necessary for the system to undergo Darwinian evolution are part of the system that is evolving. The chemical system involves pairs of RNA enzymes that catalyze each other’s synthesis by joining together two oligonucleotide building blocks (Figure 3A). Each of the two building blocks can adopt thousands of potential alternative compositions and thus can be joined to form millions of different combinations. Each of the many possible combinations can self-replicate and transmit compositional information to its progeny. Variants that replicate most efficiently grow to dominate the population until new, more advantageous variants arise to supplant their predecessors in a never-ending Darwinian battle for survival.
Someday the threshold may be crossed in which an alternative genetic system contains more heritable bits than the number of bits required to initiate its operation. Crossing that threshold is a reasonable criterion for what would constitute a new form of life. A life form that arises directly from bit-free chemistry would be considered “new” from the outset, while one that derives from a biological cell would have a long way to go before reaching the threshold. Between these two extremes lie the possibilities of starting with a modest number of bits, whether by the luck of combinatorial chemistry or derived from preexisting life, then accruing enough bits within the system to be regarded as new life. Perhaps the first true alternative to terrestrial biology will be found on an extrasolar planet, in a rock from Mars, or within an extreme environment on Earth. More likely, it will be the handiwork of an intelligent species that has discovered the principles of Darwinian evolution and learned to devise chemical systems that have the capacity to generate bits on their own.