Research

The astrophysical and astrochemical processes within cold interstellar molecular clouds control the formation of protoplanetary disks and the subsequent assembly of planets. Exciting observations from ALMA and JWST are giving us fresh insight into these processes, but many puzzles remain. Closer to home, new ways of studying the compositions of asteroids and comets, including very exciting space missions and sample return missions, provide a complementary approach to deducing how our own solar system developed. Given the surprising diversity of exoplanet systems and the wide range of exoplanet parameters, we can now ask whether Earth-like planets that orbit sun-like stars are common or rare. Can only planets like ours give rise to life, or could life originate on a much broader spectrum of planetary types?

Soon after the moon-forming impact, the Earth cooled sufficiently to allow liquid water to accumulate. It is possible that life emerged shortly after surface conditions became clement. But did large Hadean impacts frustrate the origin of life and delay the emergence of a stable biosphere until the Archaean era? Or did large impacts generate transiently reducing atmospheres that enabled the prebiotic chemistry that gave rise to life? 

Ongoing exploration of Mars provides a vital case study on the capacity of planetary environments to support life. The absence of plate tectonics on Mars has preserved an extensive record of sedimentary rocks that chronicles the physical and chemical evolution of surface environments; this provides our Solar System’s only record of conditions similar to the prebiotic Earth. A key question is whether life ever got started on Mars. Did liquid water persist long enough for life to emerge? Did the redox state of the surface and atmosphere facilitate prebiotic synthesis? If so, are traces of prebiotic chemistry preserved in the ancient sedimentary record on Mars? If not, how did environments on Mars and their evolution differ from that of the Earth? Future studies of Mars may allow us to narrow the range of conditions that are compatible with the emergence of life on a young planet.

Surface land environments provide many opportunities for the concentration, accumulation and processing of the organic compounds important for the origin of life. But the availability and nature of the first land-based environments on Earth is poorly constrained. Did life start on volcanic island arcs, or was the later emergence of continental crust essential? Did surface environments such as hot springs or alkaline carbonate lakes play a key role in the origin of Life? 

The photoredox chemistry of cyanide, sulfide, sulfite and bicarbonate could in principle generate most of the building blocks of biology. However, as our understanding of prebiotic pathways advances, it is becoming increasingly important to understand how natural environments could have facilitated or frustrated these pathways. Could starting materials and intermediates accumulate and be purified by processes such as crystallization? Were different surface environments required for different synthetic steps, and if so what transport processes facilitated multi-step chemical syntheses? What physical processes led to the synthesis of homochiral pools of nucleotides? How did the surface UV flux influence both the synthesis and degradation of critical compounds? How were abundant sources of energy such as UV light converted into forms of chemical energy that could drive the synthesis and oligomerization of nucleotides and amino acids?

The first cells are thought to have been simpler versions of modern cells, consisting of a cell membrane that encapsulated the primordial genetic material. The processes that lead to the spontaneous assembly of such protocells are relatively well understood, but what is not well understood are the physical and chemical mechanisms that drove the growth and division of the earliest protocells. Did protocell reproduction occur in something like Darwin’s ‘warm little pond’, or did growth, division and RNA replication depend upon a dynamic and fluctuating environment? 

At a more detailed level, the composition of protocell membranes and of the primordial genetic material is not well constrained. While membranes composed largely of fatty acids have many of the properties appropriate for a primitive cell, more complex lipids may have been required to provide stability to the presence of the metal ions required for RNA replication. Although recent work suggests that the genomes of the first protocells were composed of more or less modern RNA, the possibility remains that some variant nucleic acid arose first and was later replaced by modern RNA. The key questions regarding replication of the genetic material are whether and how nonenzymatic replication can occur, and if so, can it occur with sufficient fidelity for the propagation of useful sequences?

Nonenzymatic RNA replication is inherently slow and inaccurate. How did this primitive process transition to more efficient and accurate ribozyme catalyzed replication? We still do not understand how the first ribozymes evolved from the small primordial genomes of the earliest protocells. How would RNA fitness landscapes constrain the evolution of ribozymes? How did phenomena such as molecular crowding, liquid-liquid phase separation, and encapsulation within protocell membranes affect ribozyme activity and evolution? 

As the replication apparatus improved, additional functions could be encoded in the genome. But given that each evolutionary step must confer an advantage, how did prebiotic chemical pathways morph into genetically encoded metabolic pathways? Did the complex process of genetically encoded peptide synthesis emerge very early as a result of inherent physical relationships between RNAs and amino acids, or did it emerge later by building on RNA encoded metabolic activities?

The most abundant planetary systems are around M dwarf stars, and they are also the easiest to study. But such planets suffer from two problems with respect to the origin of life: those in the habitable zone tend to be tidally locked, and the frequency of highly energetic stellar flares may strip away their atmosphere. Can a subset of such planets be defined, so that limited observational resources can be most productively directed? 

Observations of very young planetary systems are of great interest as this is our best hope for obtaining direct evidence of the nature of early planetary environments. However it is highly unlikely that life in its early stages will leave an observable signal. The existence of oxygen in an exoplanetary atmosphere is often considered as a necessary but not sufficient piece of evidence for life. Are there other biosignatures that could help to provide greater confidence in life detection criteria? It is critical to ask what aspects of planetary transformation by life would generate biosignatures that are suitable for the detection of life on exoplanets.

Studies of deep phylogeny, together with careful evaluation of the earliest fossil and isotopic records can place constraints on the time of the Origin of Life on Earth. But these signatures of early life are often ambiguous, and more must be done to improve the accurate interpretation of the biogeochemical records of the early Earth. What factors controlled the dominant biogeochemical cycles of ancient Earth and (bio?)geochemical cycles on Mars? What were the major steps in the early co-evolution of life and environments on Earth, up to and including the Great Oxidation Event? 

In order to detect life, life must not only originate, but also be sustained over planetary timescales - it would be near impossible to detect life if it only existed for a short time scales, or if it was only present in relatively small quantities. Therefore, the key to the search for life is understanding how life is maintained over planetary timescales, namely what physical, chemical and biological factors are needed to ensure long-term biosphere stability. On Earth, the emergence and expansion of oxygenic photosynthesis, animals and land plants are responsible for some of the largest changes to our planet. Understanding the timing at which these major biological and planetary transitions occurred as well as the mechanisms behind these evolutionary leaps are essential to investigations of the prevalence and search for life outside of Earth.