The Scope and Story of Our Science
A. The Formation of Planet Earth
We reconstruct key stages in the formation of the Earth, answering the questions where and how our planet formed, what determined its composition and the changes in its internal state over time, how crucial elements responsible for the atmosphere and oceans were delivered and why they were retained, and how surface conditions responsible for life depended on these earlier stages. We have developed the first integrated models of accretion and transport in stellar disks across scales from gas and dust, through crucial intermediates known as “pebbles” , to the migrating orbits of early planets, and effects of giant impacts. By combining computer models with cosmochemical evidence, we can account for the diversity seen in planetary systems, while constraining the possible sources for light elements (C, O, Si, H, S, N) on early Earth. We have discovered that as Earth’s core separated from its rocky mantle, it could have incorporated much more silicon and oxygen than had been believed, along with significant hydrogen which remains there to this day. Crucially, the early precipitation of silicon oxides as the core cooled created a vigorous buoyant stirring force increasing the likelihood to form an early magnetic field protecting Earth’s atmosphere from loss by solar wind. Finally, we have learned that the cooling rate of early planetary surfaces is highly sensitive to solar irradiance at certain thresholds, changing by more than a factor of ten over the small difference of orbital radius between Earth and Venus. We understand why, from similar starting compositions, Venus dried and fell into a runaway greenhouse, while Earth retained an ocean and sequestered most of its CO2 in the subsurface.
B. Early Earth-Life System
We seek to understand the origin of life as the emergence of a new geological system, accounting for the interactions between oceans, atmosphere, and solid Earth that are key ingredients for early chemical evolution. Concerning the oxidation state of Carbon and Nitrogen in early Earth’s atmosphere, we have discovered that a single late giant impact, which best explains the deposition of iron-loving elements in our mantle, would have produced a secondary hydrogen atmosphere that lasted for 200 million years at the beginning of the Hadean eon. Moreover, the complex stable isotope signatures of Sulfur indicate that Earth maintained a higher ratio of CO to CO2 than has been expected well into the Archean, a crucial difference for synthesis of complex organics. We find that diverse planetary surface conditions are essential to the emergence of life. We have studied roles of ocean bottoms and land as sources of nutrients, of the sun, atmosphere, rock/water interface, and radioactive subsurface as sources of energy, the timing of water delivery, and composition of the earliest oceans and crust. Through these parameters we can say in how far Earth is special in its capacity to originate life. We have shown how numerous energy sources can produce reactive C and N compounds, including voltages across hydrothermal chimneys, secondary electrons from ionizing γ-radiation, and atmospheric UV. Our models indicate that species such as nitrate, ammonia, and CO were available in the early oceans, and we have found reaction pathways that produce both precursors to key compounds such as nucleobases, and activators that can drive ligation of amino acids and nucleotides. We are moving the laboratory control of traditional Origins studies deeper into the domain of combinatorial complexity that characterizes real planetary surface geochemistry, combining new computational and analytic methods, and applying these to formation, structure, and properties of functional polymer families such as linear and branched polyesters.
C. Evolution of Life on Earth
We identify major transitions that shaped the evolving biosphere, its architecture and modes of evolution and its dependence on planetary conditions through time, and how biological evolution has fed back to shape Earth’s geological history. We integrate synthetic and evolutionary biology to understand elements of structure and function in early organisms. We have demonstrated functional proteins translated using simpler genetic codes, and enzymes to synthesize key amino acids that do not require those acids in their sequences. We have achieved increasingly complex functions in synthetic cell membranes, and coordination of molecular systems within and across them. We have reconstructed genomes and bioenergetic systems of ancient bacteria, showing their coevolution with planetary chemistry and Life’s own capacity to maintain ever larger and more reliable molecular systems. We have reconstructed isotopic and mineral signatures linking biological major transitions to the rock record, both for sulfur metabolism and for oxygenic photosynthesis, the biological innovation that has most impacted every surface environment on Earth. Genomes on Earth are carried by two kinds of lifecycles: one in free-living cells and the other in viruses. We have expanded worldwide knowledge of the diversity of viruses of thermophilic Archaea by nearly 100%, and are exploring the limiting conditions for single-stranded DNA and RNA viruses to understand constraints on an RNA world. ELSI now hosts one of the largest collections of Archaeal viruses of any institution in the world. Finally, we have demonstrated new evolutionary paradigms for the use of external constraints as scaffolds to create novel complexity. We have shown how catalytic imprecision can be the gateway to new functions – likely an essential mechanism in early eras of short genomes and unreliable replication. We have discovered the same paradigm repeated for the evolution of development, as animals in the age before complex regulatory programs used fractal growth rules to transduce feedbacks from nutrient limits into variable, adaptive phenotypes.
D. Life in the Universe
Our studies of the history of Earth and its Life reveal a unifying paradigm, of alternating stages of diversification and selection, which serves as a springboard to understand the habitability of planets around other stars and what principles may make life on them different or similar to our own. Our understanding of the importance of diverse environments for life on Earth extends to our study to ice-covered water-worlds of the kind that are most common in the solar system, and perhaps around other stars, to enrich the complex concept of habitability. Computational combinatorial methods allow us to ask which aspects of Earth-Life reflect inherent limits of chemistry, and thus may be universal. We have shown that the biological amino acids as a set cover a much wider range of properties essential to protein function than random sets of possible amino acids of similar size. Selection has made the biological set more unique because they are closer to the limits of chemical possibility. Principles of life are essential features that can be formalized independently of the way they are expressed in Earth’s biology. One such feature is heredity, without which selection cannot lead to adaptation. We have derived measures of capacity for heritable variation that do not depend on genes or genomes, and apply across a range of widely studied compositional models and even more general dynamical systems. Finally, the emergence of Life anywhere will be an outcome of bootstrapping. A universal biology must explain how simple patterns of dynamical complexity can create more complex patterns in a self-maintaining hierarchy. We have studied this problem for the emergence of lineages at the forming of the genetic code, showing how an era of innovation-sharing in which all components are exchanged independently can produce the error-buffering properties of the biological code, which then permit reliable protein synthesis, faithful molecular replication, and the emergence of vertical descent in a Darwinian world.