Wonderful Life: The Burgess Shale and the Nature of History
by Stephen Jay Gould
Seven Possible Worlds
The collapse of the cone* and the ladder** open the floodgates to alternative worlds that didn't emerge, but might have arisen with slight and sensible changes in early events. The unrealised universes would have been every bit as ordered and explainable as the world we know, but ever so different in ways that we can never specify in detail. The enumeration of unrealised worlds is a parlor game without end, for who can count the possibilities? The universe is not so tightly interconnected that the fall of a petal disrupts a distant star, whatever our poets sing. But most quirky changes of topograpy and environment, most appearances and disappearances of groups (if not of single species), can irrevocably alter the pathways of life in substantial ways. The playground of contingency is immeasurable. Let us consider just seven alternate scenarios, arranged in chronological order to home in on the biological object that most excites our parochial fancy -- Homo sapiens.
I. Evolution of the Eukaryotic Cell
Life arose at least 3.5 billion years ago, about as soon as the earth became cool enough for stability of the chief chemical components. (I do not, by the way, view the origin of life itself as a chancy or unpredictable event. I suspect that given the composition of early atmospheres and oceans, life's origin was a chemical necessity. Contingency arises later, when historical complexity enters the picture of evolution.)
With respect to the old belief in steady progress, nothing could be stranger than the evolution of life -- for nothing much happened for ever so long. The oldest fossils are prokaryotic cells some 3.5 billion years old. The fossil record of this time also includes the highest form of macroscopic complexity evolved by these prokaryotes -- stromatolites. These are layers of sediment trapped and bound by prokaryotic cells. The layers may pile up one atop the other, as tides bury and re-form the mats -- and the whole structure may come to resemble a cabbage in cross section (also in size).
Stromatolites and their prokaryotic builders dominated the fossil record throughout the world for more than 2 billion years. The first eukaryotic cells (the complex textbook variety, complete with nucleus and numerous structures of the cytoplasm) appeared some 1.4 billion years ago. The conventional argument holds that eukaryotic cells are a prerequisite for multicellular complexity, if only because sexual reproduction required paired chromosomes, and only sex can supply the variation that natural selection needs as raw material for further complexity.
But multicellular animals did not rise soon after the origin of eukaryotic cells; they first appeared just before the Cambrian explosion some 570 million years ago. Hence, a good deal more than half the histor of life is a tale of prokaryotic cells alone, and only the last one-sixth of life's time on earth has included multicellular animals.
Such delays and long lead times strongly suggest contingency and a vast realm of unrealized possibilities. If prokaryotes had to advance toward eukaryotic complexity, they certainly took their time about it. Moreover, when we consider the favored hypotheses for the origin of the eukaryotic cell, we enter the realm of the quirky and incidental side consequences as unpredictable sources of change. Our best best theory identifies at least some major organelles -- the mitochondria and cytoplasts almost surely, and others with less confidence -- as descendants of entire prokaryotic cells that evolved to live symbiotically within other cells (Margulis, 1981). In this view, each eukaryotic cell is, by descent, a colony that later achieved tighter integration. Surely, the mitochondrion that first entered another cell was not thinking about the future benefits of cooperation and integration; it was merely trying to make its own living in a tough Darwinian world. Accordingly, this fundamental step in the evolution of multicellular life arose for an immediate reason quite unrelated to its eventual effect upon organic complexity. This scenario seems to imply fortunate contingency rather than cause and effect. And if you wish nevertheless to view the organelles and the transition from symbiosis to integration as predictable in some orderly fashion, then tell me why more that half the history of life passed before the process got started.
One final point that I find chilling with respect to the possibility of something like human evolution in an alternative world: Even though this first event took more than half the known history of life, I might be prepared to accept the probability of an eventual origin of higher intelligence if the earth were slated to endure for hundreds of billions of years -- so that this initial step took but a tiny fraction of the potential time. But cosmologists tell us that the sun is just about at the halfway point of existence in its current state; and that some five billion years from now, it will explode, expanding in diameter beyond the orbit of Jupiter and engulfing the earth. Life will end unless it can move elsewhere; and life on earth will terminate in any case.
Since human intelligence arose just a geological second ago, we face the stunning fact that the evolution of self-consciousness required about half the earth's potential time. Given the errors and uncertainties, the variations of rates and pathways in the other runs of tape, what possible confidence can we have in the eventual origin of out distinctive mental abilities? Run the tape again, and even if the same general pathways emerge, it might take twenty billion years to reach self-consciousness this time -- except that the earth would be incinerated billions of years before. Run the tape again, and the first step from prokaryotic to eukaryotic might take twelve billion instead of two billion years -- and stromatolites, never awarded the time needed to move on, might be the highest mute witnesses to Armageddon.*The term "cone" refers to the idea of "the cone of increasing diversity" of life. Many illustrations of life and evolution are based on such an idea -- for example, diagrams of an evolutionary tree. The idea implies low is primitive and few, high is better and more.
** The term "ladder" refers to the idea of the "ladder of upward progress of life", also known, incorrectly, as the "evolutionary ladder". Gould disproves both the idea of the cone and ladder elsewhere in the book -- life is not "improving" or "progressing". It simply is.