Chance and purpose in the evolution of biospheres

Our living planet is orders of magnitude more biologically productive than what could be supported if life was solely reliant on the slow release of critical nutrients from Earth’s crust. Highly efficient biogeochemical cycling of these nutrients in the ocean and on land, with an extraordinary efficiency of 99.9%, supports this elevated productivity. The thermodynamic drive for biogeochemical cycling arises from the creation of far-from-equilibrium organic matter produced by chemo-, and especially, photosynthesisers. This organic matter provides the chemical energy to drive a series of chemical reactions energetically dependent on available reactants (largely oxidants, themselves the product of organic matter synthesis) and kinetically accelerated by microorganisms. These boundary conditions set the stage for the emergence of biogeochemical cycles. 

An in-depth focus on the establishment of an oxygen-rich biosphere nearly 2.5 billion years ago highlights how biogeochemical cycles evolve across such global and irreversible transitions. Interactions among external forcings of Earth’s planetary ecosystems, namely increasing solar luminosity, continental growth, and decreasing volcanism, and internal feedbacks regulating biogeochemical cycles, create a system in which biotic innovation (eg the origin of oxygenic photosynthesis) is not immediately expressed as a change in environmental state (eg atmospheric oxygenation). Counterintuitively, we find that earlier origins of oxygenic photosynthesis lead to longer delays in atmospheric oxygenation. These findings influence considerations of planetary habitability and life detection on exoplanets.

Lee R Kump, Pennsylvania State University, USA

Lee R Kump, Pennsylvania State University, USA

Lee Kump is a biogeochemist recognised for his contributions to our understanding of the factors that drove coupled biological and environmental evolution through Earth history, with a special focus on the carbon cycle, paleoclimates, mass extinctions, and the controls on atmospheric oxygen. He is Professor of Geosciences and John Leone Dean of the College of Earth and Mineral Sciences at Penn State. Kump is an elected member of the National Academy of Sciences, a fellow of the Geological Society of America, the American Geophysical Union, the Geochemical Society, the European Association of Geochemistry, and the Geological Society of London. He is the author of over 150 research publications and has co-authored three textbooks: The Earth System, Dire Predictions, and Mathematical Modeling of Earth’s Dynamic Systems. 

After briefly describing James Lovelock’s Gaia Hypothesis, Dr Doolittle will argue that Gaia does not reproduce, or rather that it has what Peter Godfrey-Smith would term “too many parents” to undergo natural selection according to Lewontin’s Recipe. So it does not make sense to most Darwinians. If that recipe were extended to include differential persistence as well as differential reproduction, or if the “gene’s-eye view” of Richard Dawkins as further extended by David Hull and us were adopted, then the Gaia Hypothesis, and lesser claims about some multispecies communities, holobionts, and ecosystems, would make sense. That’s what the It’s the song not the singer(s) theory aimed to do.

Dr W Ford Doolittle FRS, Dalhousie University, Canada

Dr W Ford Doolittle FRS, Dalhousie University, Canada

Doolittle obtained his BA from Harvard and his PhD from Stanford, and has been at Dalhousie for 53 years. He is known for the “introns early” hypothesis, the “selfish DNA” hypothesis, stressing the importance of lateral gene transfer, and the “It’s the song not the singer(s)” hypothesis. Also for proving the endosymbiont hypothesis for plastids, developing genetics for Archaea and documenting lateral gene transfer.

Studies on biospheric evolution often assume planktonic cyanobacteria were present in Precambrian oceans but that their productivity was limited by a variety of ecological and geological factors. However, available evidence suggests that ancestors of extant planktonic cyanobacteria only colonised the ocean in the late Neoproterozoic or early Phanerozoic, geologically close to the start of a major period of atmospheric oxygenation that ultimately triggered the rise of animals. It is an open question whether earlier planktonic marine cyanobacteria existed but went extinct, possibly due to snowball Earth glaciations, and that records of their existence were erased, or that cyanobacteria first colonised the open ocean for the first time during this period. If the latter is true, we lack a consensus explanation for why cyanobacteria did not colonise the open ocean sooner. In this talk, Dr Braakman will review evidence on the metabolic evolution of marine picocyanobacteria, a monophyletic group that dominates extant cyanobacterial primary production, performing around ~25% of oceanic CO2-fixation. Metabolic innovations in this group provide insights into how the marine biosphere may have overcome earlier constraints on productivity, and how the rise of arthropods may have been the key facilitating factor that allowed cyanobacteria to make the evolutionary leap from the benthos to the open ocean, helping transform the biosphere and Earth as a whole.

Dr Rogier Braakman, Massachusetts Institute of Technology, USA

Dr Rogier Braakman, Massachusetts Institute of Technology, USA

Dr Braakman studies metabolic evolution and the self-organisation of ocean microbial ecosystems over geologic time. His group takes an integrative, multi-scale approach to studying living systems using a variety of data and a combination of bioinformatic, experimental and theoretical approaches. He obtained his PhD at Caltech and completed postdoctoral fellowships at the Santa Fe Institute and MIT before moving into his current position as Research Scientist in the department of Earth, Atmospheric & Planetary Sciences at MIT. 

In this talk, Dr Falkowski will discuss the biotic evolution of metabolism and the origin of life on Earth.  Life derives energy from the external environment and, in so doing moves electrons, with or without protons; a process we call “metabolism”.  There is a remarkably small subset of reactions responsible for these reactions.  All are based on transition metals bound to a very limited subset of protein structures.  These molecules, called “oxidoreductases”, ultimately are responsible for altering the gas composition of a planet. Dr Falkowski will explain the evolution of the protein structures and how these molecules came to make life a thermodynamic non-equilibrium system.

Professor Paul Falkowski, Rutgers University, USA

Professor Paul Falkowski, Rutgers University, USA

Dr Paul Falkowski is a distinguished professor in the Departments of Marine and Coastal Sciences and Earth and Planetary Sciences at Rutgers University. He holds the Bennett L Smith Chair in Business and Natural Resources, served as founding director of the Rutgers Energy Institute, and is the 2018 recipient of the Tyler Prize for Environmental Achievement.

The distinction between adaptive significance (function) and “accidental by-product” is fundamental in evolutionary biology. A bird’s mating display may attract predators as well as mates, but the former is a by-product while the latter is the trait’s function, that is, the thing that it is “meant” to do. This distinction was central to the traditional neo-Darwinian objection to the Gaia hypothesis, which was precisely that the life-sustaining homeostatic mechanisms in question were at best a by-product, given the absence of any selection at the level of the whole biosphere. Proponents of the more recent “Darwinised Gaia” idea accept this point, but argue that nonetheless one can reconcile core aspects of the Gaia hypothesis with Darwinian theory.

In this paper we turn the spotlight on the distinction between adaptive function and accidental by-product which underlies the debates over Gaia, old and new. We argue that the distinction is clear enough in paradigm cases, but that there is an important class of cases in which it is less clear. These cases arise where there is hierarchical structure, such that selection at one level yields a by-product that is manifest at a higher biological level; and where there is also a second process of selection taking place at the higher level, typically over a different timescale, acting on a higher-level trait for which the lower-level by-product is partly causally responsible. Examples of this selective mechanism are readily found. Where such a mechanism exists, it is unclear whether the biological features in question should be considered adaptive or not. This observation is used to offer fresh light on the validity of the Darwinised Gaia idea.

Professor Samir Okasha FBA, University of Bristol, UK

Professor Samir Okasha FBA, University of Bristol, UK

Samir Okasha FBA is Professor of Philosophy at the University of Bristol. He has spent much of his career working on issues in the philosophy of biology, in particular the foundations of evolutionary theory. He also worked on issues in epistemology and metaphysics of science, including causation, induction and probability.

Samir has published widely in both philosophical and scientific journals. His 2006 book Evolution and the Levels of Selection was awarded the Lakatos Prize for an outstanding contribution to the philosophy of science. His latest book, Agents and Goals in Evolution, was published in 2018. He is also the author of Philosophy of Science: a very short introduction, and Philosophy of Biology: a very short introduction, both published by Oxford University Press.

The average long-term impact of Darwinian evolution on Earth’s habitability for life remains extremely uncertain, largely due to a separation of spatial and temporal scales between biological populations undergoing natural selection and planetary-climatic metrics for habitability. Recent attempts to bridge this gap by “Darwinising” non-replicating biogeochemical processes are unclear about the exact nature of the varying populations subject to putative persistence-selection. A model framework is presented to explicate persistence selection between distinct variants of global-scale biogeochemical cycles:

1. A “cycle-biota-variant” (CBV) can be defined non-arbitrarily as one biologically facilitated pathway for net recycling of an essential element, plus the genotypes driving the relevant interconversion reactions.
2. Distinct CBVs can “compete” by persistence selection if they are individualized by distinct climatic side-effects that affect relative persistence.
3. Geochemical or climatic feedback, operating over timescales separable from those of Darwinian dynamics, can render such climatic effects irreversible, giving rise to CBV-level “competitive exclusion”.

CBV-level persistence selection thus operates on quasi-Darwinian individuals defined by the existence of a connection between (1) and (2). The timescale separation in (3) distinguishes this process from a generic byproducts-based null hypothesis. Regions of parameter space supporting the above results are characterised by covariance between these two sets of properties, analogous to the covariance between fitness and traits required by conventional Darwinian selection. These general ideas broadly conform with aspects of the historical geochemical and phylogenetic records. 

Dr Richard Boyle, University of Exeter, UK

Dr Richard Boyle, University of Exeter, UK

Richard Boyle is interested in the long-term coevolution of life and the “Earth system”. He initially studied evolutionary biology before gaining a PhD in Earth system modelling. His research involves mathematical modelling of changes in biogeochemical feedbacks introduced by life, and the effect of climatic context on biological evolution, particularly transitions in the level at which selection operates. He is also interested in the feasibility and coherence of the various attempts to reconcile the “Gaia” hypothesis with Darwinian evolution.

The search for signs of life beyond Earth is a key motivator in exoplanet research.  A suitable “biosignature gas” is one that: can accumulate in an atmosphere against atmospheric radicals and other sinks; has strong atmospheric spectral features; and has limited abiological false positives. We now have a long list of potential biosignature gases including isoprene which is produced by life in as high quantities as methane, and phosphine which on Earth is only associated with life. Despite promise from the successfully operational JWST, we are now confronted with practical challenges of tiny signals and M dwarf star contamination. Another severe challenge is the unknown exoplanetary environments likely vastly different from Solar System planets. We review the thousands of molecules produced by life on Earth in context with the phosphine on Venus and methane on Mars prescient backdrop for a reality check on the future of exoplanet biosignature gases. Our pace and history of milestone discovery in exoplanets in the last three decades, combined with new telescope paradigms, promises to eventually deliver on finding signs of life beyond Earth.

Professor Sara Seager OC FRGSC, Massachusetts Institute of Technology, USA

Professor Sara Seager OC FRGSC, Massachusetts Institute of Technology, USA

Professor Sara Seager is a Professor of Physics, Planetary Science, and Aeronautics and Astronautics at the Massachusetts Institute of Technology where she holds the Class of 1941 Professor Chair. Her ground-breaking research ranges from the foundation of exoplanet atmospheres to innovative theories about life on other worlds to development of novel space mission concepts. She was the Deputy Science Director of the NASA mission TESS; PI of the JPL-MIT CubeSat ASTERIA; and has had numerous leadership roles in concept development for space-based direct imaging missions to discover another Earth. She currently leads the Morning Star Missions to Venus to search for signs of life or life itself in the Venus clouds. Her many accolades include a MacArthur Foundation “genius” grant, the Kavli Prize in Astrophysics, and appointment as an Officer of the Order of Canada. She is the author of The Smallest Lights in the Universe: A Memoir.

The hard steps model is a simple formulation of the evolution towards complex life on Earth and, by implication, on other planets. It assumes that, at the longest time scale, the rate at which evolution occurs is paced by the occurrence of difficult “critical” transitions that happen randomly and rarely. Applied to Earth, the model suggests there have been just a small number of such transitions (three or four, widely spaced in time) in the pathway to “intelligent observers”: humans. While the transitions in the model are usually envisaged as purely biological evolutionary innovations, this misses the reality that, on Earth, the evolution of life and the planetary environment have been inextricably linked. The scale of the transformations that have occurred on Earth suggests that the critical steps should be seen as whole-system transitions. Specifically, the Paleoproterozoic and Neoproterozoic glacial episodes are candidates for such transitions: they were both caused by, and drivers of, fundamental and wide-ranging biological evolutionary advances that were necessary for complex life to arise. The Paleoproterozoic glacial periods were probably triggered by the appearance of free oxygen in the atmosphere (itself the delayed result of oxygenic photosynthesis that originated hundreds of millions of years earlier). These glaciations set the stage and may have been necessary for the evolution of eukaryotes. Much later, the Neoproterozoic glaciations appear to have triggered the radiation of Metazoa, and the inception of the modern Earth inhabited by large and complex organisms. If the rise of a technologically adept species such as humans is considered another such transition, it is perhaps no surprise that it too is being accompanied by climate change, that could be as catastrophic as these past crises unless we act decisively to prevent it.

Professor Andrew Watson FRS, University of Exeter, UK

Professor Andrew Watson FRS, University of Exeter, UK

Andy Watson is Royal Society Research Professor at the University of Exeter. He is an Earth System scientist, with a special interest in the processes controlling the composition of the atmosphere, especially carbon dioxide and oxygen concentrations, and their connection to the Earth’s climate. He has contributed to a wide variety of topics, including the atmospheres of other planets, the physics and biogeochemistry of the oceans, paleoclimatology and astrobiology. His research group at the University of Exeter specialises in making and interpreting ocean and atmospheric measurements to high accuracy. He was made a Fellow of the Royal Society in 2003.

When searching for life in the galaxy we need to know what remotely detectable signatures life might leave for us to detect. The evolutionary pathway life takes on a planet will be specific to each planet, impacted by its bulk properties, and will dictate the planet’s biosignatures. As distance prevents us from directly observing any life on an exo-planet, instead we can use computer models informed by Earth history to reduce the number of assumptions needed to contemplate alien biospheres, and to make informed biosignature predictions.

Using a 0D model based on early-Earth, this talk will demonstrate that, for biosignatures resulting from energy extraction by organisms, the metabolism of the organism is the most important factor in determining the biosignature 'strength' (eg the level of oxygen in Earth’s atmosphere). The abundance of the organism itself does not largely impact the resulting biosignature. This allows us to consider alien-life in terms of metabolisms that are possible on the planet.

Then, using an abstract biological model of co-evolution, this talk will then demonstrate that the expected trajectory for any biosphere is to become more complex over time. This allows us to make the predictions on the complexity of exo-biospheres and, thus, paves the way to assembling models of likely ecosystems for specific planets.

Detailed biosignature predictions will need to be bespoke for each planet. The models presented in this talk present a way to construct these predictions using fewer assumptions.

Dr Arwen Nicholson, University of Exeter, UK

Dr Arwen Nicholson, University of Exeter, UK

Arwen’s PhD focused on developing models of Gaian regulation. She developed abstract models of life-environment coupled-systems, informed by Earth’s history, to study the influence of life on climate evolution. Since her PhD she has worked in the Physics Department at Exeter applying Gaia Theory to the search for alien life. This work has included determining the most important parameters required to quantify the abundance of biosignatures gases of nutrient limited biospheres, and developing models of how the presence of Gaian regulation might influence the habitable zone around planets. Arwen has also co-written public outreach articles on the science behind the slogan: There’s no planet B. 

The energy balance of a planet is set by the sunlight entering and the heat radiating back from it. Earth’s biosphere adds new complexity as energy flows through global marine and terrestrial ecosystems altering physical and chemical systems, including global climate. Energy is essential for life, and the main constraint on abundance and productivity is the amount of energy an organism can harvest. 

When the first lifeforms evolved fire-making culture (there is evidence in our hominin ancestors, some 1 million years ago) it sparked a novel form of energy relationship: deliberate combustion, which would change the planet. The innate drive to harvest ever more energy changed the evolution of our species and transformed the world’s environment. Humans became the dominant species globally.

In recent decades, however, the impact of humanity’s burning of ancient fossil life has altered the energy balance of the entire planet. As humans become ever more successful and ingenious at harnessing energy, our scope grows extraterrestrial: there are plans to colonise the Moon and Mars.

Meanwhile, the destabilising impact of humanity’s energy exploits on Earth’s unique biosphere risk the survival of multiple ecosystems, and the future of our own species. As the dominant, and consciously capable, species, does this imbue us with a responsibility over other life on earth.

Gaia Vince, UK

Gaia Vince, UK

Gaia Vince is a science writer and broadcaster exploring the interplay between human systems and the planetary environment. She is an Honorary Senior Research Fellow at the Anthropocene Institute at UCL and a host of BBC Inside Science. Her first book, Adventures In The Anthropocene, won the Royal Society Science Book of the Year Prize. Her latest book, Nomad Century: How To Survive The Climate Upheaval, explores global migration and planetary restoration in a radical call to arms.