About 540 million years ago, virtually all the basic types of animals appeared in a geographic eyeblink known as the Cambrian explosion.
The late Stephen Jay Gould used this amazing event (if a period of millions of years can be called an event) in his 1989 bestseller Wonderful Life: The Burgess Shale and the Nature of History to debunk two popular myths about evolution. First, evolution is not a steady march toward more and more advanced forms (presumably culminating in us). Second, diversity does not steadily increase. The Cambrian was populated by many types of creature no longer around today, some so exotic as to be worthy of a science fiction film. Rather than growing steadily bushier, the tree of life was later brutally pruned, and we grew from the remaining twigs.
In The Plausibility of Life, Marc Kirschner and John Gerhart highlight another critical facet of this amazing period: since that period of innovation, no more than one new animal type has appeared. The diversity we enjoy today is built from basic parts that were "invented" in the Cambrian explosion.
Rather than get into how and why this happened, for now let's just regard it as an observational fact from the fossil record:
True innovation is rare.
We know this is true in human affairs. Producers of movies and TV shows, for example, often play the odds by re-using and recombining proven concepts. The same goes for technology, where many innovations combine familiar elements in new ways. It's faster, it's cheaper, and it's safer.
For whatever reason, the evolutionary history of life is a series of one-time innovations. After they are adopted, these "core processes" change very little, even though they have eons of time to do so. That doesn't mean that the organisms themselves stay the same--far from it. But they use the core processes in different ways, just as a bat wing is built in the same way as the human hand.
Kirschner and Gerhart discuss several examples of conserved core process, starting with the fundamental chemistry of DNA, RNA, proteins and the genetic code that connects them. Every living thing on earth uses the same chemistry. The appearance of the eukaryotic cell is defined by the presence of the nucleus, but a host of other innovations occurred at the same time. All eukaryotes from amoebae to people share these features even now. The joining of cells into multicellular organisms was also accompanied by innovations that helped the cells stick together and cooperate. Every plant and animal has retained these features largely intact.
Like the animal body plans of the Cambrian explosion, these burst of innovation occurred over relatively short periods, and were permanently added to the toolkit. All of the animals we know, from cockroaches to cockatoos, from squirrels to squids, arose by applying those tools in new ways.
But what is it about the core processes that makes them so resistant to change? What makes them so useful? And are the answers to these questions related?