WG3: Tracing the origin and evolution of life on Earth and finding its limits

Leaders:

 - E. Javaux (Belgium): _
 - P. Lopez-Garcia  (France): _ 

To date terrestrial life is the only unambiguous example of life in our universe. Understanding how life evolved on Earth and diversified to colonize all available habitats will help to build a reference to look for potential forms of extraterrestrial life. In  this Action, we will take a bidirectional approach by i) studying the oldest traces of life on Earth  and establishing realistic environmental constraints for the earliest life forms and ii) studying extant diversity to try to infer information about early biological diversification and the limits of life.

a) Early traces of life.

When, where, and how life emerged on Earth remains controversial and these three questions are the most challenging ones this theme will have to face. Learning more about earliest microbial ecosystems is crucial to understand how life shaped and intertwined with abiotic processes on Earth. Interdisciplinary expertise will be used to study traces of life in past environments at different temporal and spatial scales by:

1. Investigating the nature, dynamics and diversity of Archean environments, to unravel realistic physico-chemical constraints imposed on the evolution of early life. This will involve geological mapping, geochemical-geophysical-petrographic characterization of target environments, dating, studies of sedimentary basins to constrain ecological gradients (pH, redox, temperature, nutrients), and investigations of hydrothermal circulation driven by magmatic and possibly meteorite impact heating. Target environments will include shorelines, shallow-marine sediments, cherts, banded-iron formations, carbonates, interbedded volcanic strata, and marine and continental hydrothermal environments.
2. Developing new microscopic and microchemical tools and criteria to test candidate textural, geochemical, biomineral traces in the rock record, and microbial fossilization mechanisms through analysis of morphology, ultrastructure, chemical, elemental and isotopic composition
3. Modeling Archean biotic and abiotic processes in order to discriminate them.

b) The extent and limits of life.

Substantial progress has been made in the exploration of microbial diversity using molecular tools that document  i) the greater genetic diversity of microorganisms compared to eukaryotic animals and plants, and ii) that a substantial fraction of that microbial diversity thrives in extreme environments thought to be previously inhospitable to life. Amazing adaptations characterize microorganisms from very hot to very cold habitats, from hypersaline to highly acidic or alkaline environments and from the deep, cold sea to the subsurface enviironments  in the oceanic and continental crusts.

Despite significant methodological molecular and metagenomic advances, we are however still far from understanding the extent of biological diversity and its limits. The most challenging part for this theme will be to:

Extend and better define the physico-chemical limits within which life is expected to occur. This may help to orient the search for extraterrestrial life and to redefine habitable zones.
Infer, via comparative phylogenomics, common features present in the last common ancestor of terrestrial organisms. This should help to impose biological constraints on the nature of even earlier living organisms and primordial entities before subsequent diversification of life in three domains (Archaea, Bacteria and Eucarya).
Interdisciplinary collaborations between biologists, geologists, chemists, astrophysicists and philosophers fostered by the Action, should provide a coherent and critical set of boundaries for the distribution of life across the broad spectrum of terrestrial ecosystems and possibly on other planetary systems.