Accounting for heat burdens: As climate warming becomes more and more apparent and influential, there is an increasing desire to predict its long-term impacts on species. Classically, this has been done by extrapolating lethal limits based on those observed in the laboratory. In the real world, however, organisms do not experience a single high temperature that then returns to a comfortable temperature, but rather a series of high temperatures during the hot season. Rezende et al. accounted for these accumulative effects in a dynamic model that accurately predicted empirical patterns in wild fruit fly populations, showing that cumulative effects of warming temperatures can be included in future estimates (see the Perspective by Huey and Kearney). Science , this issue p. 1242 ; see also p. 1163

Beating the curse of the parasite: The evolution of molecular replicators was a critical step in the origin of life. Such replicators would have suffered from faster-replicating “molecular parasites” outcompeting the parental replicator. Compartmentalization of replicators inside protocells would have helped ameliorate the effect of parasites. Matsumura et al. show that transient compartmentalization in nonbiological materials is sufficient to tame the problem of parasite takeover. They analyzed viral replication in a droplet-based microfluidic system, which revealed that as long as there is selection for a functional replicator, the population is not overwhelmed by the faster-replicating parasite genomes. Science , this issue p. 1293

As of today, the most credible scientific paradigm pertaining to the origin of life on Earth is undoubtedly the RNA World scenario. It is built on the assumption that catalytically active replicators (most probably RNA-like macromolecules) may have been responsible for booting up life almost four billion years ago. The many different incarnations of nucleotide sequence (string) replicator models proposed recently are all attempts to explain on this basis how the genetic information transfer and the functional diversity of prebiotic replicator systems may have emerged, persisted and evolved into the first living cell. We have postulated three necessary conditions for an RNA World model system to be a dynamically feasible representation of prebiotic chemical evolution: (1) it must maintain and transfer a sufficient diversity of information reliably and indefinitely, (2) it must be ecologically stable and (3) it must be evolutionarily stable. In this review, we discuss the best-known prebiotic scenarios and the corresponding models of string-replicator dynamics and assess them against these criteria. We suggest that the most popular of prebiotic replicator systems, the hypercycle, is probably the worst performer in almost all of these respects, whereas a few other model concepts (parabolic replicator, open chaotic flows, stochastic corrector, metabolically coupled replicator system) are promising candidates for development into coherent models that may become experimentally accessible in the future.

The RNA world is a very likely interim stage of the evolution after the first replicators and before the advent of the genetic code and translated proteins. Ribozymes are known to be able to catalyze many reaction types, including cofactor-aided metabolic transformations. In a metabolically complex RNA world, early division of labor between genes and enzymes could have evolved, where the ribozymes would have been transcribed from the genes more often than the other way round, benefiting the encapsulating cells through this dosage effect. Here we show, by computer simulations of protocells harboring unlinked RNA replicators, that the origin of replicational asymmetry producing more ribozymes from a gene template than gene strands from a ribozyme template is feasible and robust. Enzymatic activities of the two modeled ribozymes are in trade-off with their replication rates, and the relative replication rates compared to those of complementary strands are evolvable traits of the ribozymes. The degree of trade-off is shown to have the strongest effect in favor of the division of labor. Although some asymmetry between gene and enzymatic strands could have evolved even in earlier, surface-bound systems, the shown mechanism in protocells seems inevitable and under strong positive selection. This could have preadapted the genetic system for transcription after the subsequent origin of chromosomes and DNA. The RNA world refers to the stage of early evolution when RNA macromolecules were responsible both for storing hereditary information and performing enzymatic activities. Conflict arises between these two functions, however, as enzymatic activities of the ribozymes are in tradeoff with their replication rates. Here we address this problem by investigating the evolutionary emergence of a primordial transcription-like system in model protocells inhabited by unlinked replicators. Our numerical analysis demonstrates that division of labor between genes and enzymes could have emerged, given that there was a moderate to strong tradeoff between the enzymatic and template efficiency of one strand of the ribozymes. This division of labor results in a strong asymmetry in the numbers of the enzymatic and genetic strands of the macromolecules, in favor of the former. We offer insight into the emergence of the first transcription-like system, which is today characteristic of all known life forms.