Life got off to a quick start, gaining a foothold almost as soon as it could on the cooling crust. For two thirds of the entire history of life on Earth, no organism evolved beyond the simplest, or prokaryotic, design. For the better part of another billion years, eukaryotic cells propagated without any great increase in complexity. After all this time, nothing beyond a single-celled organism existed on the face of the planet. About half a billion years ago, something extraordinary happened on Earth. Simple life forms proliferated and a couple of them eventually developed intelligence. The last half billion years of the Earth's evolution are thus dramatic because they gave rise to one species &mdash Homo sapiens — with the ability discover and understand the universe.
The evolution of intelligence occurred relatively recently in the Earth's history. To illustrate this, let us consider the history of the Earth in terms of the analogy of a single year. The Earth forms on January 1, and by the end of February the first simple cells have a tenacious hold. The atmosphere is enriched with oxygen during the spring and early summer, and by the end of October, cells with nuclei exist for the first time. From here on, life exhibits a startling acceleration in complexity. The oceans fill with multi-cellular life forms in the Cambrian era that begins in mid-November. Land animals do not appear until the end of November. Dinosaurs rule the Earth for the first part of December, and primates first appear on December 26. Our hominid ancestors do not arrive on the scene until 10 A.M. on December 31, and Homo sapiens does not develop until 3.5 minutes before midnight. The entire modern history of astronomy, including our ability to communicate through space with electromagnetic waves, occupies the last tenth of a second of this analog year: a time of 11:59:59.9 P.M on December 31.
There was a period when life made dramatic gains in complexity — October and November in the analogy where the history of Earth is a year. About 600 million years ago, the Earth's oceans witnessed an explosion of multi-celled organisms. This spurt in evolution was probably encouraged by the buildup of oxygen in the atmosphere, which allowed more complex metabolic processes, and by the breakup of the continents, which provided more niches for life. Before the surge in evolution, the eukaryotes had only three kingdoms: fungi, plants, and animals. After the surge, each of these three kingdoms generated scores of separate evolutionary lines. As with the origin of life itself, there is evidence for several failed experiments in multi-celled organisms. All higher-level animals, including humans, evolved from only one of these experiments. The rich diversity of life in the pre-Cambrian oceans has been chronicled by Harvard paleontologist Steven Jay Gould in the story of the Burgess Shale, a fossil site in Canada. Throughout this time, few organisms had "hard parts," leaving the fossil record very sketchy. So we rely on fortuitous events like those that led to the Burgess Shale, where entire ecosystems were gently entombed by a mudslide on the ocean floor.
The Cambrian fauna included the first hard-bodied sea creatures, the trilobites. Trilobites have two eyes and a complex body structure, but they are extremely primitive by human standards. The vertebrates in the oceans of 500 to 600 million years ago are loosely lumped together as "fishes," but they are strikingly different in function and form. Most did not survive to live in the oceans of today. About 450 million years ago, plants began to spread across the continents, their spores and seeds carried by the wind. Next came the extraordinary migration of certain types of fish to the land. The transition from a streamlined aquatic animal to one that dragged itself around on poorly formed limbs must have conveyed some adaptive advantage, but it is an unlikely and remarkable transition is retrospect. Equally remarkable is the fact that 350 million years ago insects first took to the air, followed 200 million years ago by the descendants of one type of reptile, birds. By this time, the fossil record shows that life had radiated into all possible environments.
It would be a mistake to see the progression towards greater sophistication and complexity as predictable or inevitable. About 225 million years ago, after dinosaurs became established on the land, the fossil record in the oceans shows that 95 percent of all marine species became extinct in a short period of time. The cause of this mass extinction is unknown. Then, 65 million years ago, another mass extinction led to the demise of the dinosaurs. This second catastrophe helped the evolution of the mammals, which were in competition with the dinosaurs for food. Thereafter, many mammals developed complex central nervous systems and large brains, but for many millions of years there is no sign of high intelligence.
Primates appeared on the scene only 4 or 5 million years ago, yet there are many more evolutionary branches and extinct lines before we arrive, some 500,000 years ago, at modern Homo sapiens. It took 99.99% of the time since life began on Earth to develop a human level of intelligence. Humans have only had the ability to explore and communicate with the cosmos for a few hundred years — a blink of the eye in the eons since the motor of life first turned over.
Once life begins, is the eventual development of intelligence inevitable? This question is central to the likelihood of communication with life beyond the Solar System. First, we need to define intelligence. Most people would agree on the relative intelligence of the occupants of this planet. For example, a sequence of species ranked in order of increasing intelligence might be the following: worms, frogs, birds, dogs, chimpanzees, humans. In general, intelligence corresponds to brain size, which correlates with body mass. Intelligence varies continuously among species, but we can set a high standard. Let's define intelligence as the capability of abstract thought, coupled with the ability to use tools or technology and to control the environment.
Life on Earth often evolves towards organisms of greater complexity. It might be supposed that intelligence carries with it an adaptive advantage and so is favored in natural selection. This optimistic view must be tempered by two facts. First, some life forms such as blue-green bacteria have remained essentially unchanged for 3.5 billion years. Tropical reefs have also been existence a long time, and the coral reefs of today's oceans are just the current version of a recurrent ecosystem. Life can certainly adapt to a changing environment without ever becoming smart. Second, most species become extinct, and do not reappear. There have been an estimated 500 million species of animal and plant life in the Earth's history; 99.8% of those are extinct. We can speculate that if humans disappeared, another intelligent species would evolve, but we cannot be sure.