Natural Selection
What governs the evolution of complex organisms? At the
molecular level, life evolves through slight imperfections in the replication
of DNA. A change in the sequence of bases in a DNA molecule is called a
mutation, and it can occur through a transcription error or through the
influence of an external agent such as a chemical, a cosmic ray, or a gamma
ray. Some mutations are neutral and do not affect the function of an organism.
Others can be either helpful or harmful to an organism's ability to survive in
its environment. Charles Darwin was unaware of the molecular mechanism of
evolution, but he witnessed the diversity and adaptation of species in response
to the environment. Mutations occasionally produce offspring with improved
survival traits. In turn, these offspring live longer and have more offspring
of their own, promoting retention of the new trait. This mechanism is called
natural selection. "The capacity to blunder slightly is the real marvel of
DNA," wrote physician Lewis Thomas, author of Lives of a Cell.
"Without this special attribute, we would still be anaerobic bacteria and
there would be no music."
Natural selection has led to life's rich diversity. Nature
has rolled the dice many times over billions of years, with results based on
the ability of each species to survive in a changing environment. Life on Earth
fills a remarkable range of evolutionary niches. Microorganisms can live in
Antarctic ponds at 228 K (-49º F) because of dissolved calcium salts, and
others live in Yellowstone Hot Springs at temperatures of 363 K (194º F). Some
bacteria exist at altitudes where the atmospheric pressure is only 10 percent
of the pressure at sea level; others are found several kilometers deep into the
Earth’s crust. Shrunken microbes can be found inside rocks where no sunlight
reaches. In 1980, oceanographers discovered entire colonies of sea animals
clustered in the darkness around volcanic vents deep on the seafloor. An entire
food web is based on bacteria that utilize volcanic heat and metabolize
hydrogen sulfide (H2S) gas. This all occurs at temperatures of up to523 K (482º
F) and pressures of 250 atmospheres, conditions as severe as those on Venus. We
consider this environment a bizarre evolutionary niche today, but these
conditions are reminiscent of the Earth just after life started.
An important aspect of evolution is the idea of contingency,
first presented by Alfred Russel Wallace, who developed the theory of natural
selection at the same time as Charles Darwin. Contingency says that the
development of intelligence and humans was the result of many evolutionary
branching points, where the progression was heavily influenced by external
events such as meteor impacts and climate change. The diversity of life forms
has occasionally been pruned back by mass extinctions.
The process of evolution is not random, but the details of
survival have the character of a lottery. It is therefore impossible to predict
the nature of higher organisms given their primitive ancestors. As Stephen Jay
Gould has put it, if you "replay life's tape" and watch the
development of organisms, you would be unlikely after 4 billion years to see
humans, primates, or perhaps even any mammals at all. Gould illustrates the expression
of contingency with a fine example — the Frank Capra movie, It's a Wonderful
Life. In the movie, a good but poor man (George Bailey, played by Jimmy
Stewart) is driven to despair and contemplates suicide. His guardian angel
pulls George back from the brink by showing him how the world would have been
if he had not existed. In George's absence, his town has been taken over by a
robber baron and has slid into grim destitution. Replaying life's tape has
yielded a different but sensible outcome, where small changes lead to cascades
of accumulating difference.
Life's history on Earth is rich with examples of
contingency. Despite a rapid start, it took 3 billion years for the first life
forms to evolve to the first and simplest multi-celled organisms. The Sun has
only 5 billion years more to live as a main-sequence star; its later giant
phase will engulf and destroy all life on Earth. If evolution were only a
factor of three slower, the pinnacle and the endpoint of life would be the
humble algae or plankton. Given all the errors and uncertainties, the branches
and pathways, we can have no confidence that intelligence will necessarily
evolve during the stable phase of a solar-type star. The Cambrian oceans
witnessed a rapid explosion of fauna. Two dozen quite different "floor
plans" for life evolved, yet only a couple survived to propagate their
descendants to the present day. Serendipity played a role in the survival of
these few lines; no paleontologist could look at the range of Cambrian fauna
and safely predict the winners. Move forward to the time of the dinosaurs.
Contrary to common perception, dinosaurs were not outwitted by the small and
agile mammals. Mammals coexisted with dinosaurs for 100 millions years without
any trends towards domination or larger brains. Rather they adapted to the
nooks and crannies of the dinosaur's world. Mammals gained ascendancy due to a
highly unpredictable event — the impact of an asteroid that caused the demise
of large land animals.
The contingency of evolution must logically extend to Homo Sapiens. There was nothing inevitable or inexorable about
the newly erect tribes on the plains of
Advanced life forms tend to be fragile. Humans can only
survive a 3% variation in body temperature, but our skills have nonetheless
allowed us to live in equatorial wet jungles and dry deserts and to travel to
arctic plains and mountain summits where air pressure is barely half that at
sea level. In summary, natural selection on Earth has produced species capable
of occupying most environments. Life may have evolved on other planets, but
this life may look very strange to us. After all, if mushrooms and corals and
woolly mammoths and Venus flytraps all evolved on one planet, how much greater
may the differences be between life forms on two different planets? Feathers
and fur and sex and seeds and symphonies may be the products of Earth only.
Recommendations:
THE ORIGIN OF LIFE ON EARTH
EVOLUTION AND INTELLIGENCE
FROM MOLECULES TO CELLS
THE NATURE OF LIFE
LIFE AS DIGITAL INFORMATION
EARLY COSMOLOGIES
STATES OF MATTER
PERIODIC PROCESSES