The Emergence of Life

THE EMERGENCE OF LIFE Life arose on the planet during periods of crustal formation and volcanic gases from the atmosphere and oceans (Table 1). It was also a time of heavy meteorite bombardment. Rocky asteroids and icy comets constantly rained down on early Earth, possibly providing the planet's main source of water. Interplanetary space is filled with debris hitting newborn planets. Some of this space junk may have supplied organic compounds, from which life could have evolved. Previous theories about the creation of life relied on the so-called primordial soup hypothesis. To test this theory, a spark release chamber (a device first designed in the 1950s to mimic the early atmosphere and oceans) was built to represent prebiotic conditions on the early Earth (Figure 12). As a result, all the precursors of life (the elements in the room: methane, ammonia and hydrogen) come together. With countless combinations and permutations, organic molecules evolved that could replicate themselves. However, the time taken for such a random event would involve billions of years. In addition, evidence collected from ancient rocks on Earth, the Moon, and meteorites suggests that the amount of ammonia and methane assumed to be in the primordial atmosphere was not nearly as much as originally thought. Biophysicists, who study the mechanisms of life, have found intriguing evidence in the interior of the 4.5-billion-year-old Murchison meteorite, named for a site in Western Australia where it fell to Earth in 1969.The meteorite contained organic compounds such as lipids capable of self-assembling into cell-like membranes—essential requirements for the first living cells. The meteorite, a carbonaceous chondrite, is believed to have broken an asteroid that formed around the same time and from the same material as Earth.The organic chemicals provided the first clear evidence of extraterrestrial amino acids. The materials in meteorites thus contain many essential components necessary to create life. Earth is still pelted by meteorites containing amino acids, protein precursors. Early meteorite impacts will also most likely make conditions very difficult for proteins to organize into living cells. The first cells may have been repeatedly annihilated, forcing life to originate over and over again. Whenever primitive organic molecules attempted to organize themselves into living matter, impacts often blasted them apart before they had a chance to reproduce. Some big naysayers may have generated enough heat to vaporize large parts of the ocean many times over. The evaporating oceans would increase surface pressure more than a hundred times greater than the current atmosphere, and the resulting high temperatures would sterilize the entire planet. Several thousand years will pass before steam condenses into rain and ocean basins repopulate, just to wait for the next impact of ocean evaporation. Such harsh conditions could re-emerge hundreds of millions of years of life. Perhaps the only safe place for life to evolve is 3 to 4 miles on the deep ocean floor, where high-density hydrothermal vents exist. Hydrothermal vents such as geysers on the seafloor (Figure 13) that dispense mineral-laden hot water heated by shallow magma chambers located just below the seafloor. The vents may have created an environment capable of producing large amounts of organic reactions and could have provided the materials and energy needed to create the planet's first life. They will also provide thriving life forms with all the essential nutrients needed to sustain themselves. Indeed, such environments exist today and are home to some of the strangest creatures found on Earth. In this environment, life could have originated as early as 4.2 billion years ago. From the very beginning, life had many common characteristics. No matter how diverse organisms are today, from the simplest bacteria to ourselves, the central molecular machinery is exactly the same. Each cell of every species is built from the same set of 20 amino acids. All life forms use the same energy transfer mechanism for growth. All strands of DNA are left-handed double helices, and the operation of the genetic code in protein synthesis is the same for all living things. With so much in common, all life inevitably arose from a common ancestor. Every alien form, of which there is no descendant today, became extinct early in the history of life. Moreover, no new life forms are being created today either because the current chemical environment is not conducive to the formation of life or living organisms prey on newly created organisms before they have a chance to evolve. Since life appeared within the first half billion years of Earth's existence, it must have evolved into a complex organism out of simple materials rather quickly. Primitive bacteria, descended from the earliest known life forms, remain by far the most abundant living things. Evidence that life began early in Earth's history when the planet was still hot enough exists today as thermophilic (heat-loving) bacteria, found in hot springs and other hot water environments around the world (Figure 14) as well as deep underground or deep beneath the ocean floor. The existence of these organisms is proof that thermophiles are the common ancestor of all life. Early conditions on Earth would have been ripe for the evolution of thermophilic organisms. Most have sulfur-based energy metabolism, and sulfur compounds would be abundant on hot, volcanically active planets. Fortunately for the early Earth, it had a lot of sulfur, which spewed from the abundance of volcanoes. As long as the surface temperature remains hot, the molecular rings of sulfur atoms in the atmosphere will block the sun's ultraviolet radiation. Otherwise, the first living cells will hiss in the deadly rays of the Sun. However, ultraviolet shielding may not have been necessary in the primordial atmosphere, as some primitive bacteria seemed to tolerate high levels of ultraviolet radiation. Evidence that life began quite early in Earth's history when the planet was steaming hot exists today as archaebacteria, or simply archaea. They range more widely than previously believed, and many parts of the ocean are full of them. One-third of the picoplankton biomass (the smallest plankton) in Antarctic waters is archaea. Such abundance could mean that archaea play an important role in global ecology and may have a major influence on ocean chemistry. The first living organisms were very small clumps of noncellular protoplasm. Selfreplicating organisms feed on a rich broth of organic molecules produced in the ancient ocean. Such an abundance of nutrients sets off a rapid chain reaction, resulting in phenomenal growth. Organisms float freely in ocean currents and spread to all parts of the world. Although the first simple organisms seem to have arrived as soon as conditions on Earth became favorable, nearly a billion years passed before life even remotely resembled anything alive today. After studying how Earth evolved, the next chapter will examine the Archean eon and the earliest life forms along with the earliest rocks.