One of the more daunting questions related to astrobiology\u2014the search for life in the cosmos\u2014concerns the nature of life itself. For over a century, biologists have known that life on Earth comes down to the basic building blocks of DNA, RNA, and amino acids. What\u2019s more, studies of the fossil record have shown that life has been subject to many evolutionary pathways leading to diverse organisms. At the same time, there is ample evidence that convergence and constraints play a strong role in limiting the types of evolutionary domains life can achieve. <\/p>\n
For astrobiologists, this naturally raises questions about extraterrestrial life, which is currently constrained by our limited frame of reference. For instance, can scientists predict what life may be like on other planets based on what is known about life here on Earth? An international team led by researchers from the Santa Fe Institute (SFI) addressed these and other questions in a recent paper. After considering case studies across various fields, they conclude that certain fundamental limits prevent some life forms from existing.<\/p>\n
<\/p>\n The research team was led by Ricard Sol\u00e9, the head of the ICREA-Complex Systems Lab at the Universitat Pompeu Fabra and an External Professor at the Santa Fe Institute (SFI). He was joined by multiple SFI colleagues and researchers from the Institute of Biology at the University of Graz, the Complex Multilayer Networks Lab, the Padua Center for Network Medicine (PCNM), Ume\u00e5 University, the Massachusetts Institute of Technology (MIT), the Georgia Institute of Technology, the Tokyo Institute of Technology, and the European Centre for Living Technology (ECLT). <\/p>\n The team considered what an interstellar probe might find if it landed on an exoplanet and began looking for signs of life. How might such a mission recognize life that evolved in a biosphere different from what exists here on Earth? Assuming physical and chemical pre-conditions are required for life to emerge, the odds would likely be much greater. However, the issue becomes far more complex when one looks beyond evolutionary biology and astrobiology to consider synthetic biology and bioengineering.<\/p>\n According to Sol\u00e9 and his team, all of these considerations (taken together) come down to one question: can scientists predict what possible living forms of organization exist beyond what we know from Earth\u2019s biosphere? Between not knowing what to look for and the challenge of synthetic biology, said Sol\u00e9, this presents a major challenge for astrobiologists:<\/p>\n \u201cThe big issue is the detection of biosignatures. Detecting exoplanet atmospheres with the proper resolution\u00a0is becoming a reality and will improve over the following decades. But how do we define a solid\u00a0criterion to say that a measured chemical composition is connected to life?\u00a0<\/p>\n \u201c[Synthetic biology] will be a parallel thread in this adventure. Synthetic life can provide\u00a0profound clues on what to expect and how likely it is\u00a0under given conditions.\u00a0To us, synthetic biology is a powerful way to interrogate nature about\u00a0the possible.\u201d<\/p>\n<\/blockquote>\n To investigate these fundamental questions, the team considered case studies from thermodynamics, computation, genetics, cellular development, brain science, ecology, and evolution. They also consider previous research attempting to model evolution based on convergent evolution (different species independently evolve similar traits or behaviors), natural selection, and the limits imposed by a biosphere. From this, said Sol\u00e9, they identified certain requirements that all lifeforms exhibit:<\/p>\n \u201cWe have looked at the most fundamental level: the logic of life across sales,\u00a0given several informational, physical, and chemical boundaries that\u00a0seem to be inescapable. Cells as fundamental units, for example, seem to be\u00a0an expected attractor in terms of structure: vesicles and micelles are automatically\u00a0formed and allow for the emergence of discrete units.\u201d<\/p>\n The authors also point to historical examples where people predicted some complex features of life that biologists later confirmed. A major example is Erwin Schr\u00f6dinger\u2019s 1944 book What is Life?<\/em> in which he predicted that genetic material is an aperiodic crystal\u2014a non-repeating structure that still has a precise arrangement\u2014that encodes information that guides the development of an organism. This proposal inspired James Watson and Francis Crick to conduct research that would lead them to discover the structure of DNA in 1953.<\/p>\n However, said Sol\u00e9, there is also the work of John von Neumann that was years ahead of the molecular biology revolution. He and his team refer to von Neumann\u2019s \u201cuniversal constructor\u201d concept, a model for a self-replicating machine based on the logic of cellular life and reproduction. \u201cLife could, in principle, adopt very diverse configurations, but we claim that\u00a0all life forms will share some inevitable features, such as linear information polymers or\u00a0the presence of parasites,\u201d Sol\u00e9 summarized. <\/p>\n In the meantime, he added, much needs to be done before astrobiology can confidently predict what forms life could take in our Universe:<\/p>\n \u201cWe propose a set of case studies that cover a\u00a0broad range of life complexity properties. This provides a well-defined road map to\u00a0developing the fundamentals. In some cases, such as the inevitability of parasites,\u00a0the observation is enormously strong, and we have some intuitions about why\u00a0this happens, but not yet a theoretical argument that\u00a0is universal. Developing and proving these\u00a0ideas will require novel connections among diverse fields, from computation and synthetic biology\u00a0to ecology and evolution.\u201d<\/p>\n<\/blockquote>\n The team\u2019s paper, \u201cFundamental constraints to the logic of living systems,\u201d appeared in Interface Focus<\/em> (a Royal Society publication).<\/p>\n Further Reading: Santa Fe Institute<\/em>, Interface Focus<\/em><\/p>\n\n
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