Life arose on this planet during a period of crustal formation and volcanic outgassing of an atmosphere and ocean (Table 1). It was also a time of heavy meteorite bombardment. Rocky asteroids and icy comets constantly showered the early Earth, possibly providing the main source of the planet’s water. Inter-planetary space was littered with debris that pounded the newborn planets.
Some of this space junk might have supplied organic compounds, from which life could evolve.
Earlier theories on the creation of life relied on the so-called primordial soup hypothesis. To test the theory, a spark discharge chamber (a device first designed in the 1950s to replicate the early atmosphere and ocean) was built to represent prebiotic conditions on the early Earth (Fig. 12). As a result, all the precursors of life (the elements within the chamber: methane, ammonia, and hydrogen) came together. By countless combinations and permutations, an organic molecule evolved that could replicate itself. However, the time
TABLE 1 EVOLUTION OF THE BIOSPHERE
Billions of Percent Biologic Event
Years Ago Oxygen Effects Results
Full oxygen conditions 0.4 100 Fish, land plants, Approach present biologic
required for such a random event would involve billions of years. Moreover, evidence gathered from ancient rocks on Earth, the Moon, and meteorites suggests that the amount of ammonia and methane assumed to be in the pri-mordial atmosphere was not nearly as abundant as originally thought.
Biophysicists, who study the mechanics of life, have discovered intriguing evidence in the interior of the 4.5-billion-year-old Murchison meteorite, named Figure 12 A spark
discharge chamber represents prebiotic conditions on the early Earth.
*6 dpi-10%*6 dpi-10%*6 dpi-10%
*6 dpi-10%*6 dpi-10%*6 dpi-10%
*6 dpi-10%*6 dpi-10%*6 dpi-10%*6 dpi-10%
Water Amino acids
Heat
Circulating cool water to condense steam Electrodes to produce spark Methane, ammonia, and hydrogen
Steam
for a site in Western Australia where it fell to Earth in 1969.The meteorite held lipidlike organic compounds able to self-assemble into cell-like membranes—an essential requirement for the first living cells. The meteorite, a carbonaceous chondrite, is believed to have broken off an asteroid that formed about the same time and from similar materials as Earth.The organic chemicals provided the first unambiguous evidence of extraterrestrial amino acids.The material in the mete-orite thus contains many essential components necessary for creating life.
Earth is still pelted by meteorites that contain amino acids, the precur-sors of proteins.The early meteorite impacts would also most likely have made conditions very difficult for proteins to organize into living cells.The first cells might have been repeatedly exterminated, forcing life to originate over and over.Whenever primitive organic molecules attempted to arrange themselves into living matter, frequent impacts blasted them apart before they had a chance to reproduce.
Some large impactors might have generated enough heat to evaporate most of the ocean many times. The vaporized ocean would have raised sur-face pressures more than a hundred times greater than the present atmosphere, and the resulting high temperatures would have sterilized the entire planet.
Several thousand years would elapse before the steam condensed into rain and the ocean basins refilled again, only to await the next ocean-evaporating impact. Such harsh conditions could have set back the emergence of life hun-dreds of millions of years.
Perhaps the only safe place for life to evolve was 3 to 4 miles down on the deep ocean floor, where a high density of hydrothermal vents existed.
Hydrothermal vents are like geysers on the bottom of the sea (Fig. 13) that expel mineral-laden hot water heated by shallow magma chambers resting just beneath the ocean floor.The vents might have created an environment capa-ble of generating an immense number of organic reactions and could have provided the ingredients and energy needed to create the planet’s first life.
They would also have given evolving life-forms all the essential nutrients needed to sustain themselves. Indeed, such an environment exists today and is
early in the history of life. Furthermore, no new life-forms are being created today either because the present chemical environment is not conducive to the formation of life or living organisms prey upon the newly created organ-isms before they have any chance of evolving.
Since life appeared within the first half-billion years of Earth’s existence, it must have evolved into complex organisms from simple materials rather quickly. Primitive bacteria, which descended from the earliest known form of life, remain by far the most abundant living beings. Evidence that life began early in Earth’s history when the planet was still quite hot exists today as ther-mophilic (heat-loving) bacteria, found in thermal springs and other hot-water environments throughout the world (Fig. 14) as well as deep underground or far below the ocean floor.
The existence of these organisms is evidence that thermophiles were the common ancestors of all life.The early conditions on Earth would have been ripe for the evolution of thermophilic organisms. Most of these have a sulfur-based energy metabolism, and sulfur compounds would have been plentiful on the hot, volcanically active planet.
Fortunately for the early Earth, it had an abundance of sulfur, which spewed out of a profusion of volcanoes. As long as surface temperatures Figure 13 An active
hydrothermal vent and sulfide mineral deposits at the East Pacific Rise.
(Photo courtesy USGS)
remained hot, ring molecules of sulfur atoms in the atmosphere would block out solar ultraviolet radiation. Otherwise, the first living cells would have siz-zled in the deadly rays of the Sun. However, an ultraviolet shield might not have been necessary in the primordial atmosphere, because some primitive bacteria appear to tolerate high levels of ultraviolet radiation.
Evidence that life began quite early in Earth 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 teem-ing with them. A third of the biomass of picoplankton (the tiniest plankton) in Antarctic waters were archaea. Such abundance could mean that archaea play an important role in the global ecology and might have a major influence on the chemistry of the ocean.
The first living organisms were extremely small noncellular blobs of pro-toplasm.The self-duplicating organisms fed on a rich broth of organic mole-cules generated in the primordial sea. Such a nutritional abundance set off a rapid chain reaction, resulting in phenomenal growth. The organisms drifted freely in the ocean currents and dispersed to all parts of the world. Although
Figure 14 Hot carbonated springwater undercuts bedded travertine deposits at Yellowstone National Park,Wyoming.
(Photo by K. E. Barger, courtesy USGS)
the first simple organisms appeared to have arrived soon after conditions on Earth became favorable, almost another billion years passed before life even remotely resembled anything living today.
After learning how Earth evolved, the next chapter will examine the Archean eon and the earliest life-forms along with the earliest rocks.