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Life Before Its Origin on Earth: Implications of a Late Emergence of Terrestrial Life

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Life Before Its Origin on Earth: Implications of a Late Emergence of Terrestrial Life LIFE BEFORE ITS ORIGIN ON EARTH: IMPLICATIONS OF A LATE EMERGENCE OF TERRESTRIAL LIFE Julian Chela-Flores*,{ *The Abdus Salam ICTP, Trieste, Italy {IDEA, Caracas, Venezuela CHAPTER OUTLINE 1 Introduction.................................................................................................................................... 343 1.1 Time Available Before the Emergence of Life on Earth...................................................... 344 1.2 Rationalizing Our Origins in Terms of Thermodynamics..................................................... 345 2 How Would We See Ourselves if Early Origins are Identified?............................................................ 345 2.1 Approaching the End of Biocentrism if Life on Earth is a Latecomer .................................. 346 2.2 Anthropocentrism .......................................................................................................... 346 3 Philosophical Comments on An Early “Forest of Life” ........................................................................ 346 3.1 Process Philosophy........................................................................................................ 346 3.2 Stellar Evolution............................................................................................................ 347 3.3 Cultural Comments on An Early forest of Life................................................................... 347 4 Terrestrial Life as a Latecomer in Cosmic Evolution .......................................................................... 348 5 Conclusion ..................................................................................................................................... 349 References ..........................................................................................................................................349 Further Reading ...................................................................................................................................351 1 INTRODUCTION The established view that life on Earth is represented by a single-rooted phylogenetic tree with three domains (Woese et al., 1990) is confronted by alternative scenarios that view life in the cosmos, not as a tree of life, but better described by the metaphor of a universe with a “cosmic forest of life” (cf., Section 3). We attempt to follow-up the possibility that life on Earth is a relatively recent phenom- enon in geologic terms. There have been some suggestions that this may be the case, for instance, Habitability of the Universe Before Earth, editors: Richard Gordon & Alexei Sharov, Volume 1 in the series: 343 Astrobiology: Exploring Life on Earth and Beyond, series editors: Pabulo Henrique Rampelotto, Joseph Seckbach & Richard Gordon. ISSN 2468-6352. https://doi.org/10.1016/B978-0-12-811940-2.00015-0 © 2018 Elsevier Inc. All rights reserved. 344 LIFE BEFORE ITS ORIGIN ON EARTH Lineweaver (2001) has supported this view with his estimate on the age distribution of terrestrial planets in the Milky Way. His calculations show that Earth-like planets began forming more than 9 9.2 billion years ago and that their median age is tmed ¼(6.4Æ0.9)Â10 years. This estimate is much greater than the age of the Solar System itself. Living phenomena that emerged prior to terrestrial life has been suggested implicitly, though it has only been clearly stated in fiction. For example, Carl Sagan, a distinguished astronomer, astrobiologist, and confirmed enthusiast for science communication, militated in favor of life elsewhere being more ancient than life on Earth: This opinion is evident in his novel Cosmos. After a first contact had been made with another intelligent species, the more evolved beings assert (Sagan, 1985): In our case, we emerged a long time ago on many different worlds in the Milky Way. The first of us developed interstellar space-flight. In the same context, the English writer, Sir Arthur C. Clarke, remarkable for his writings on science fiction and his vision on the progress of science, advocated that life on Earth was a late cosmic phe- nomenon. This is especially clear in the short story The Sentinel (Clarke, 1951): “When our world was half its present age, something from the stars swept through the Solar System...and went again upon its way.” In terms of a traditional scientific approach that is more appropriate for the present work, theoretical hypotheses have only gone as far as arguing in favor of terrestrial life being a rare case in a galactic scale (Carter, 1983). Additional theoretical discussions support the rare life hypothesis, but accept that the “final scientific assessment on life in the universe will only come from biologists and observers” (Livio, 2008). The question of how ancient life in the cosmos really emerged can be tested, in principle, with pre- sent instrumentation and with future feasible technologies that are now being developed. These open questions force us to reconsider life before its origin on Earth. Some implications of the existence of extraterrestrial life as an astrobiology phenomenon have been covered in the cases of theology (Haught, 1998; Lovin, 2015) and philosophy (Lupisella, 2015). 1.1 TIME AVAILABLE BEFORE THE EMERGENCE OF LIFE ON EARTH The age of the Solar System is about half the age of our galaxy. But the Sun itself is about half way through its lifetime. Consistent with past estimates, Earth was formed after 80% of Earth-like planets already existed (Behroozi & Peeples, 2015). Hence, the time available for life to have evolved before Earth is considerable. The afterglow radiation of the cosmic microwave background (CMB) has steadily decreased in temperature as the universe expands. Today, the expansion has induced a CMB temperature close to the absolute zero. This is equivalent to a red shift of z 100. Yet going back in time, we can estimate the time after the Big Bang as being 15 Myr when the temperature was close to room temperature, a condition that is favorable to life (Loeb, 2014). The implication is that in such circumstances any planet, or satellite around an early star, could have sustained oceans of liquid water. This phenomenon would be independent of the particular orbit, and it may even happen on planets that have escaped from their original stellar orbit. The only question that remains open is whether the early star system would have generated sufficient basic chemical elements (produced from earlier supernovae) for supporting biosynthesis. In the absence of sufficient data, such an early onset of life in the cosmos remains an open possibility. 2 HOW WOULD WE SEE OURSELVES IF EARLY ORIGINS ARE IDENTIFIED? 345 Thermal gradients are needed for life. These can be supplied by geological variations on the surface of rocky planets. Examples for sources of free energy are geothermal energy powered by the planet’s gravitational energy at the time of its formation and radioactive energy from unstable elements pro- duced by the earliest supernova. These internal heat sources (in addition to possible heating by a nearby star) may have kept planets warm even without the CMB, extending the habitable epoch from such an early age to later times. The CMB temperature at a time well before the moment of cosmic decoupling may have allowed ice to form on objects that delivered water to a planet’s surface. This phenomenon may have helped to maintain the cold trap of water in the planet’s stratosphere. Planets could have kept a blanket of mo- lecular hydrogen that maintained their warmth (Stevenson, 1999; Pierrehumbert & Gaidos, 2011), allowing life to persist on internally warmed planets at late cosmic times. If life persisted at z 100, it could have been transported to newly formed objects through panspermia (McNichol & Gordon, 2012). Under the assumption that interstellar panspermia is plausible, the redshift of z 100 can be regarded as the earliest cosmic epoch after which life was possible in our Universe. Searching for atmospheric biomarkers in planets, or satellites around low-metallicity stars in the Milky Way galaxy, or its dwarf galaxy satellites can test the feasibility of life in the early universe. Such stars represent the closest analogs to the first generation of stars at early cosmic times. 1.2 RATIONALIZING OUR ORIGINS IN TERMS OF THERMODYNAMICS A recent original approach to the origin of life is due to Jeremy England, which applies to any terrestrial- like planet. He emphasizes rationalization of the origin of life in terms of thermodynamics (England, 2013), rather than on chemical evolution, as we have persevered exhaustively in the past (Ponnamperuma & Chela-Flores, 1995; Chela-Flores et al., 1995, 2001). We assume only the occurrence of biological phenomena that are independent of any contingent facts here on Earth. We further assume that such conditions may hold in other places, such as in exoplanets, where life might develop, since the expected number of Earth-like planets is sufficiently large (cf., Section 1.1). We will refer to certain biological phenomena that make no reference to any contingent facts here on Earth. In addition, such special conditions could be expected to hold in other places where life might develop. As a consequence of the two previous assumptions, universal biology may be a robust hypothesis (Mariscal, 2015). To a certain extent, such an insight into a universal biology may be testable, especially concerning the case of evolutionary convergence (Chela-Flores, 2003). This is in principle possible to test in the
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