Passage One
Between 5,000 million and 4,000 million years ago the Earth
was formed, By 3,000 million years ago life had arisen and we have fossils of
microscopic bacteria-like creatures to prove it. (66)
Nobody knows what happened, but theorists agree that the key was the
spontaneous arising of self-replicating entities, i. e. something equivalent to
"genes" in the general sense. The atmosphere of the early Earth
probably contained gases still abundant today on other planets in the solar
system. Chemists have experimentally reconstructed these ancient conditions in
the laboratory. If plausible gases are mixed in a flask with water, and energy
is added by an electric discharge (simulated lightning), organic sub-stances are
spontaneously synthesized. These include the building blocks of RNA and DNA. It
seems probable that something like this happened on the early Earth.
Consequently, the sea would have become a "soup" of prebiological organic
compounds. (67) Today the most famous
self-replicating molecule is DNA (deoxyribonucleic acid), but it is widely
thought that DNA itself could not have been present at the origin of life
because its replication is too dependent on support from specialized machinery,
which could not have been available before evolution itself began. DNA has been
described as a" high-tech" molecule which probably arose some time after the
origin of life itself. Perhaps the related molecule RNA, which still plays
various vital roles in living cells, was the original self-replicating molecule.
Or perhaps the primordial replicator was a different kind of molecule
altogether. (68) Variants that were particularly good at
replication would automatically have come to predominate in the primeval soup.
Varieties that did not replicate, or that did so inaccurately, would have become
relatively less numerous. This led to ever-increasing efficiency among
replicating molecules. As the competition between replicating
molecules warmed up, success must have gone to the ones: hat happened to hit
upon special tricks or devices for their own self-preservation and their own
rapid replication. The rest of evolution may be regarded as a continuation of
the natural selection of replicator molecules, now called genes, by virtue of
their capacity to build for themselves efficient devices (cells and
multicellular bodies) for their own preservation and reproduction. (69)
Fossils were not laid down on more than a small scale
until the Cambrian era, nearly 600 million years ago. The first vertebrates may
date back 530 million years, according to fossil evidence--primitive, lawless
fishes with fins, gills, and fish-like muscle patterns--found in China in 1999.
Vertebrates appear abundantly in fossil beds between 300 and 400 million years
ago. (70) Mammals and, later, birds, arose
from two different branches of reptiles. The rapid divergence of mammals into
the rich variety of types that we see today, from opossums to elephants, from
anteaters to monkeys, seems to have been unleashed into the vacuum left by the
catastrophic extinction of the dinosaurs, 65 million years ago.
A. Among vertebrates, the land was first colonized by lobe-finned and
lung-bearing fish about 250 million years ago, then by amphibians and, in more
thoroughgoing fashion, by various kinds of animals that we loosely lump together
as reptiles. B. Once self-replicating molecules had been formed
by chance, something like Darwinian natural selection could have begun:
variation would have come into the population because of random errors in
copying. C. It is not enough, of course, that organic molecules
appeared in the primeval soup. The crucial step, as noted above, was the origin
of self-replicating molecules, molecules capable of copying
themselves. D. Although we naturally emphasize the evolution of
our own kind--the vertebrates, the mammals, and the primates--these constitute
only a small branch of the great tree of life. E. Three thousand
million years is a long time, and it seems to have been long enough to have
produced such astonishingly complex contrivances as the vertebrate body and the
insect body. F. Some time between these two dates--independent
molecular evidence suggests about 4,000 million years ago--that mysterious
event, the origin of life, must have occurred.