Chapter 18: Life On Earth

A photograph of Charles Darwin by Julia Margaret Cameron, from 1868

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."

A diagram showing where quantum gravity sits in the hierarchy of physics theories.

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.

Repenomamus giganticus, a triconodont from the Early Cretaceous of China, pencil drawing, digital coloring

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 into 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 million 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.

Cross section of the quintic Calabi–Yau manifold. Produced using the methods described in Hanson (1994), "A construction for computer visualization of certain complex curves".

On the other hand — and you should not be surprised in science to find that there are competing explanations for most phenomena — 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 record and draw different conclusions. They see a convergence in 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 and 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.