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 (-49° F) because of dissolved calcium salts, and others live in Yellowstone Hot Springs at temperatures of 363 K (194° 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 to 523 K (482° 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 Africa that picked up tools and forged language skills. At each stage in the development of life there were dozens or even hundreds of branching points. The success or failure of each line did not follow a simple notion of "the survival of the fittest," but was influenced in large part by sudden and chance events. There have been nearly a billion species of animals on Earth and genuine intelligence has arisen in only one.
On the other hand — and you should not be surprised in science to find that there are competing explanations for most phenomnea — an argument can be made for convergence as well. The narrative told by Stephen Jay Gould is evocative but other paleontologists read the same fossil reconrd and draw different conclusions. They see a convergence in the the basic biochemical mechanisms of life. They note complex structures that develop in different creatures on very different branches of the tree of life, such as the eyes of mammals and the eyes of cephalopods. They argue that the development of a central nervous system and a brain offers a selective advantage so will be favored by evolution. Contingency should not be used to support a purely pessimistic view of the prospects for intelligent life elsewhere in the universe. Despite the apparent obstacles, humans did evolve on Earth. The arrow of evolution does lead in some cases to both complexity and intelligence. On other planets, the proliferation of multi-celled organisms and intelligent life may have been rapid once the basic biochemistry of life was in place.
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.