Life's Origin: The Beginnings of Biological Evolution

2. From Big Bang to Primordial Planet: Setting the Stage for the Origin of Life

chapter 2 From Big Bang to Primordial Planet Setting the Stage for the Origin of Life ALAN W. SCHWARTZ AND SHERWOOD CHANG introduction According to modern theory, life arose on the primitive Earth by a process of prebiotic chemical evolution. This process began with syntheses of organic chemical precursors of proteins, nucleic acids, and membranes in the early atmosphere and ocean, and ended with the emergence of life forms capable of self-replication—forms that could undergo Darwinian evolution through mutation and natural selection. Although most researchers accept this view of prebiotic evolution, opinions diverge on the nature and sequence of chemical events between the first and last stages. Many theoretical paths meander through this murky middle ground, but the track culminating in biological evolution remains unclear. While other chapters illuminate current progress in tracing this pathway, this chapter sets the stage, showing how life on Earth is related to—and a product of—a long sequence of cosmic events that preceded the formation of our planet. Biochemistry must surely have grown out of geochemistry, that is, out of chemical reactions occurring among inorganic components derived from Earth’s atmosphere, ocean, or crust. Even if we were to accept the tenets of panspermia—the idea that life was carried to Earth in meteorites from other bodies or in dust particles from interstellar space—we would still have to trace its beginnings to planetary processes elsewhere. And the nature of such processes is critical. Chemical evolution and planetary evolution are inextricably intertwined, so progress 46 toward the origin of life may end at any stage if a host planet’s physical or chemical development takes a wrong turn. Since the early 1950s, when research into life’s beginnings began in earnest, astronomers, astrophysicists, and cosmochemists have been making discoveries relevant to a general theory for the origin of life. Organic compounds in increasing numbers and complexity have been detected in the interstellar medium and comets. Many organic compounds of the sort that make up living systems—but unequivocally nonbiological and extraterrestrial in origin—have been identified in carbonaceous meteorites. Spectroscopic observations in the Solar System have revealed organic compounds on asteroids as well as on the giant planets and their satellites. Minute grains of organic matter similar to those found in meteorites have also been detected throughout our galaxy and beyond. Clearly, organic matter occurs throughout the Universe as an integral component of cosmic evolution. And, with the detection of increasing numbers of planets around other stars, the prospect of identifying other life-harboring solar systems seems inevitable. Throughout history, cultural concepts have shaped human perceptions of the Universe and humanity’s place within it. Today, our very perceptions have changed, expanding beyond the reaches of the Solar System to the stars and interstellar clouds that populate limitless space. And our concepts have changed as well. Just as biological evolution implies that all organisms diverged from a common ancestor, cosmic evolution implies that all matter in the Solar System had a common origin. Life, we can now argue, is the product of countless changes in the form of primordial stellar matter, and these changes were wrought by astrophysical and cosmochemical, as well as geological and biological, evolutionary processes. In the context of cosmic evolution, the chain of events that led to life’s beginnings extends back through planetary history to the origin of the Solar System, to the star-spawning turmoil of interstellar clouds, to the birth in stars of the biogenic elements that make up living systems. Indeed, the links in this chain constitute a cosmic history full of promise that life is widespread in the Universe.* From Big Bang to Primordial Planet 47 *Throughout this chapter, the reader is directed to images of astrophysical phenomena taken with the Hubble Space Telescope (HST). These pictures are made available by the Space Telescope Science Institute on the World Wide Web through links at http:// oposite.stsci.edu/pubinfo/SubjectT.html. To access a specific image, replace “SubjectT” in this web address with the bracketed code listed adjacent to the name of the phenomenon in the text. For example, the image referred to in the text as “Crab nebula [PR/2000/15/index.html]” can be retrieved at http://oposite.stsci.edu/pubinfo/PR/2000/ 15/index.html. origin of the biogenic elements Soon after the Big Bang, the expanding universe contained only three kinds of atoms—hydrogen...