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
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 varied organisms are today, from the simplest bacteria to ourselves,
its central molecular machinery is exactly the same. Every cell of every species
is constructed 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 helixes, and the operation of the genetic code in protein synthesis is
the same for all living things.
With so much similarity, all life must have sprung from a common
ancestor.Any alien forms, of which no descendants exist today, became extinct
PLANET EARTH