Understanding the Early Major Transitions in Evolutionary History Part 1: Stages in the Emergence of Complex Life

Abstract

Early Major Transitions 1: Stages in the Emergence of Complex Life. Recommendation ​ We recommend that early evolution research, t​ he excavation of the historical record of biology, continue to be a high priority of the Astrobiology Program because of its role in understanding the origin and diversity of life on Earth, its importance in guiding prebiotic chemistry research, and contribution toward understanding the ability of life to adapt to diverse environments. Early evolution research is key to, and must be integrated into the design of, life detection strategies. Finally, there remain keys to alternate trajectories that life could have taken in early evolved clades, providing insights into the possibility of other life forms elsewhere. Motivation ​ All extant life on Earth shares a common biochemistry based on a relatively small set of organic molecules (Kluyver and Donker, 1926), the same mechanisms of information storage and inheritance (Woese, 1965), RNA, DNA and protein, enzymatic cofactors and ATP energy currency, a dependence on water, a related cellular organization, and a handful of core metabolic pathways with reactions performed by proteins with a shared ancestry. These central features of life as we know it all evolved in the period between the origin of life and the last universal common ancestor (LUCA) of all extant life. Understanding their evolutionary history has an important role in astrobiology research, and has increasing potential thanks to improved research strategies and cross-disciplinary collaborations, as well as advances in molecular and cellular biology, increasing power of computational resources, and breadth of bioinformatics databases. Extant biology contains detailed and interpretable records of pre-biological processes and molecules and of early biology. The early history of life on Earth provided by these "top-down" approaches provides unique information and important perspectives for NASA's astrobiology and exobiology programs. These approaches have revealed extensive information on the geochemistry of Earth, chemical evolution, pre-LUCA biological phenomena and early evolution of Bacteria, Archaea and Eukarya. Top-down approaches guide bottom-up approaches, and directly inform astrobiological questions such as planetary conditions for origins and habitability. Understanding how and under what circumstances life increased in complexity, became capable of populating new environments, and altering the planet and its atmosphere, can inform our expectations of when a planet or moon may be inhabited. Furthermore, in the search for life elsewhere, we only have one biosphere to inform our search for extraterrestrial life. That biosphere was very different in early evolutionary history. Thus, understanding the early biosphere gives us an extended set of features which may be informative of biological processes across a broader time window; currently, we cannot distinguish universal features of life in the universe from those that are quirks of evolutionary history as it occurred on Earth. But this distinction is essential for designing sufficiently broad life-detection methods. ​ Below are several examples of current research areas and future directions within the field of early evolution that center on the identification of ancient proteins and proteomes, the functions they performed in ancient life, and their implications for ancient organismal ancestors.