ocean did not reach its present concentration of salts until about 500 million
years ago.The warm seas were heated from above by the Sun and from below
by active volcanoes on the ocean floor, which continually supplied seawater
with the elements of life.
THE EMERGENCE OF LIFE
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
PLANET EARTHTABLE 1 EVOL UTION OF THE BIOSPHEREBillions of Percent Biologic Event
Years Ago Oxygen Effects ResultsFull oxygen conditions 0.4 100 Fish, land plants, Approach present biologic
and animals environs
Appearance of 0.6 10 Cambrian fauna Burrowing habitats
shell-covered animals
Metazoans appear 0.7 7 Ediacaran fauna First metazoan fossils
and tracks
Eukaryotic cells appear 1.4 >1 Larger cells with Redbeds, multi-cellular
a nucleus organisms
Blue-green algae 2.0 1 Algal filaments Oxygen metabolism
Algal precursors 2.8 <1 Stromatolite mounds Initial photo-synthesis
Origin of life 4.0 0 Light carbon Evolution of the biosphere