Goal 4: Understand how past life on Earth interacted with its changing planetary and Solar System environment
Investigate the historical relationship between Earth and its biota by integrating evidence from both the geologic and biomolecular records of ancient life and its environments.
Understand how the planetary environment has influenced the evolution of life and how biological processes changed the environment. An improved knowledge of how life has altered diverse environments throughout Earth history will improve our ability to detect remnant biosignatures, even in cases where life has become extinct (see Goal 7: biosignatures). Correlations and cause-and-effect relationships should be sought between biological evolution and both long-term and episodic environmental changes. Insights that emerge from syntheses of these perspectives will guide our search for life elsewhere.
Background
A full understanding of the historical relationships between life and the environment requires a synthesis that draws from many different fields of science. For example, our knowledge of long-term environmental change is largely inferred from research in geophysics, geochemistry and sedimentology. The ongoing reconstruction of the phylogenetic tree of life and the time scale of evolution derive from morphology, fossils, and especially, information stored in the genomes of living organisms. Molecular biomarkers help to link biological evolution to past environments. Likewise, biogeochemical cycles of carbon and its redox partners oxygen, sulfur, and iron, are integral to Earth's biosphere, and their isotopic records help us understand how the biosphere evolved. Knowledge of chemistry, physics, and Solar System dynamics places constraints on Earth's history of environmental change. With these tools and methodological framework, astrobiologists can study the reciprocal interactions of organisms and their planetary environment and address the following questions: What was the chemical and physical environment like when the earliest life (microbes) covered the Earth? Was this environment similar to the early environment of Mars? How, why, and when did the composition of the atmosphere change through time, including the step-wise increase in the oxidation state of the biosphere, and how did these changes impact Earth's biota? How did life respond to major planetary disturbances, such as bolide impacts, sudden atmospheric changes, and global glaciations, and were some disturbances caused by life? How has the planetary environment influenced the evolution of complex, multicellular, eukaryotic life, and what environmental changes were associated with the appearance of intelligent life? These and other questions are tied to this overarching goal that seeks to understand the historical interconnections between Earth and its biota to help guide our search for life elsewhere. All of this research requires a deeper understanding of evolutionary mechanisms at the levels of molecules, organisms and ecosystems (Goal 5). The results contribute directly to the identification of biosignatures (Goal 7).
Objective
4.1
Earth's early biosphere
Investigate the development of key biological processes and their environmental consequences during the early history of Earth through molecular, stratigraphic, geochemical, and paleontological studies.
Example investigations
- Examine the earliest sedimentary rocks for
biosignatures, such as microfossils and chemical fossils.
- Search for biosignatures of key microorganisms
and metabolic processes (e.g., photosynthesis) in rocks of Archean
age.
- Analyze genomic sequence data of prokaryotes and identify correlations between lineage divergence and events in the history of the biosphere.
Objective 4.2
Foundations of complex life
Study the origins and evolution of life forms that eventually led to more complex multi-cellular biota that appear in the fossil record or exist today.
Example investigations
- Study carbon isotopes and other proxies of
environmental change in Neoproterozoic rocks to better understand the
history of global climatic perturbations that may have influenced the
early evolution of complex life.
- Search for fossil evidence of eukaryotes in
rocks of Proterozoic age to determine the morphology, ecology, and diversity
of early eukaryotes.
- Analyze genomic sequence data of unicellular eukaryotes to gain insights into the early evolution of eukaryotic complexity, including the acquisition of cellular organelles.
Objective
4.3
Effects of extraterrestrial events upon
the biosphere
Examine the records of the response of Earth's biosphere (both the habitable environment and biota) to extraterrestrial events, including asteroid and comet impacts.
Example investigations
- Examine the evolutionary, ecological, and taxonomic
changes in Earth's biota following a known asteroid impact event.
- Investigate a known mass extinction event in the fossil record to determine whether it was caused or intensified by an extraterrestrial event, such as an impact or a nearby supernova.