Talk title: Slow, energy-efficient, and mysterious life deep within Earth’s crust

The subsurface contains a vast microbial ecosystem whose metabolic processes drive key biogeochemical cycles. These microbes are often from evolutionary branches on the tree of life that have never been characterized or described in a laboratory. Therefore, new methods have had to be developed to learn about these elusive communities in deep subsurface sediments. Methods such as direct identification of a suite of biomolecules (DNA, RNA, proteins, lipids, and metabolites), coupled to isotopic tracer incorporation and geochemical measurements have produced a more in-depth view of the role that these microbes play in Earth systems. From this work, a picture has emerged of a diverse ecosystem that may be growing orders of magnitude more slowly than life at the surface. Although it is slow, this life is metabolically active, contributing to the breakdown of ancient organic matter in ways that differ from those found in surface organisms. Recent work has identified zones where microbes have enough energetic resources to grow faster than the cellular decay rate. This suggests a model for adaptive selection to ultra-slow growth in long-term burial in marine sediments. Rather than simply being accidentally alive for thousands of years while riding a one-way conveyer belt to their deaths, it is possible that sediment microbes are adapted to this subsurface niche space. The payoff for long-term ultra-low activity is that rare events such as slumping, turbidite flows, or mud volcanoes enable a return to shallow sediments where higher quality substrates enable natural selection to occur.

This talk concerns two high-level questions that arise in astrobiology.  First, are there extant universal dynamical signatures of early life, preceding even the last universal common ancestor of all life on Earth? Second, are there universal laws of life, which can be deduced by abstracting what we know about life on Earth? I will show that the answer to both questions is almost certainly "yes", and that there are important relics of early life present in the structure of the modern day canonical genetic code --- the map between DNA sequence and amino acids that form proteins.  The code is not random, as often assumed, but instead is now known to have certain error minimisation properties.  How could such a code evolve, when it would seem that mutations to the code itself would cause the wrong proteins to be translated, thus killing the organism?  Using digital life simulations, I show how a unique and optimal genetic code can emerge over evolutionary time, but only if early life was dominated by collective effects, very different from the present era where individuals and species are well-defined concepts.  If time permits, I may also discuss a second universal signature of life that could be used as a biosignature: the breaking of chiral symmetry in biological amino acids and sugars.  The second question will be discussed with reference to universal phenomena in living matter today, and how function is emergent and can become liberated from the specific chemical substrate that instantiates complex living communities.  I will end by explaining why these seemingly philosophical questions are relevant to astrobiology but also to attempts to engineer, manipulate and control living systems for human health, agricultural engineering, and of course epidemic mitigation.