Evolution is Contingent

As natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection.

Charles Darwin
On the Origin of Species 1859

 

Wind the tape of life back to Burgess times, and let it play again. If Pikaia does not survive in the replay, we are wiped out of future history—all of us, from shark to robin to orangutan.

Stephen Jay Gould
Wonderful Life: The Burgess Shale and the Nature of History

 

Charles Darwin
Charles Darwin (1802-1882)

Evolution, according to Charles Darwin, was slow, steady and inevitable — flowing smoothly from simple to complex and toward eventual superiority and perfection. The progression from fish to amphibians to reptiles to mammals and, finally, to us, reflected this orderly process of progress and purpose. This view held much in common with the Aristotelian religious concept of scala naturae, or “The Great Chain of Being,” in which all living things are arranged in a divine order of perfection. Man, the pinnacle of evolution, was closest to perfection among all animals; closest to the angels; closest to God. And the entire history of evolution was the story of our ascent towards this perfection.

In recent decades there has been movement away from the ideological paradigm of scala naturae. Most scientists now hold a different view. The late Stephen Jay Gould and fellow evolutionary biologist Niles Eldredge proposed the theory of punctuated equilibria in a landmark paper presented in 1972. The fossil record, they argued, shows long periods of static equilibrium—with little change in anatomy and very few new species—punctuated by sudden bursts of activity, with the appearance of many new species at once. Bacteria and single-celled organisms, for example, ruled the world for almost 3 billion years with little change. But the Cambrian Explosion upended this equilibrium. Multi-cellular organisms evolved, and scores of new species suddenly sprang to life within just a few million years. This was followed by another period of stasis with relatively few new species, until mass extinctions triggered a new surge of activity. There were rapid bursts of new species in the fossil record after both the End Permian and End Cretaceous extinctions. Evolution happens slowly until some catastrophe happens, and then the world changes overnight. There is no steady march to perfection.

While there remains debate about the causes of punctuated equilibrium, there is little doubt that mass extinctions contribute greatly to the phenomenon. There have been five mass extinction events over the past 500 million years documented in the fossil record. These mass extinctions not only reset the clock of evolution; they changed its course. They eliminate species and temporarily reduce biodiversity but create new space—both ecologic and geographic—for the design of new species via natural selection. Entirely new taxonomic orders, with never-before-seen anatomies—tend to occur after mass extinctions. The End Permian extinction 251 million years ago decimated 95% of Earth’s species including most reptiles but opened new space for the few survivors – including the mammal-like reptiles, which gave rise to the mammalian lineage, and archosaurs, which gave rise to the dinosaurs and their avian descendants. The asteroid collision that wiped out the dinosaurs and 75% of Earth’s species 66 million years ago, opened opportunities for the radiation of new groups including us mammals.

As David Raup describes in his provocative book Extinction: Bad Genes or Bad Luck, without mass extinctions, life can become rather static and unchanging. Over time the natural course of a species’ evolution reaches anatomic stability, with smaller and smaller changes in body design. Organisms do not start over from scratch, but must build on existing structures, and this limits innovation. As organisms evolve, their basic body structure becomes fundamental, and, after a certain point, mutational changes to that body structure are not compatible with survival. It would be difficult, for example, for mammals to evolve an entirely new circulatory system. In the absence of mass extinctions, evolution is constrained to minor variations on existing structures – a phenomenon called phylogenetic constraint.

About 540 million years ago, suddenly out of nowhere, there appeared a diversity of spectacular macroscopic life teeming in the seas. Where there had only been simple microorganisms for billions of years, the oceans were suddenly filled with a diversity of marine animals and algae the likes of which the world had never seen before. This was the Cambrian Explosion. Documentation of life during this period was captured in the spectacular fossils of Burgess Shale. These fossils held special significance not only because of their elegance of form, preservation, and beauty, but because they so completely revised our view of the natural history of life. According to Stephen Jay Gould’s interpretation in Wonderful Life, the number of phyla, or body plans, found in the fossils far exceeds the number of phyla today. Somehow, the roughly one hundred animal phyla identified in Burgess Shale have collapsed over the years, leaving us with less than 40 animal phyla today. Examples of modern phyla include chordates (mostly vertebrates), molluscs (snails, octopuses, mussels and others), arthropods (spiders, centipedes, crustaceans, insects) and annelids (earthworms, leeches). All the phyla present today were already present in the Burgess Shale—meaning that many have been lost but no new phyla have been created. But what we have lost in the number of phyla (disparity) we have gained in new groups within each phyla (Classes, Orders, Genus, and Species) and this has resulted in increased diversity. For example, mammals (Class), primates (order) and humans (Genus) have all evolved within the chordate phylum since the Cambrian explosion. Within each phylum there are far more species today than were present in the Burgess Shale. As Gould succinctly put it, “The history of life is a story of massive removal followed by differentiation within a few surviving stocks, not the conventional tale of steadily increasing excellence . . .”

Stephen Jay Gould
Stephen Jay Gould (1941-2002)

The late Stephen Jay Gould argued that the findings from Burgess Shale have truly profound implications for the natural history of life). The fact that there have been no new body plans since the Cambrian probably means that new phyla—new designs—do not inevitably evolve. This is in sharp contrast to traditional thinking, which holds there is an inevitable progression towards better and better body designs, progressing toward animals that are supremely adapted to their environments. No matter what disruptions might derail the train of evolution, the train will get back on track and produce animals with “perfect” fits to their environments. Gould agreed that phyla with some of the best body designs may have survived by natural selection, because superior anatomy has made them better adapted—but many others have survived by sheer dumb luck, avoiding destruction by the random catastrophic events that ruined others. Gould championed the concept of contingent evolution. Survival of species in this view is indeed a consequence of their adaptations but is also contingent upon the freak accidents that may or may not happen around them. If a catastrophe is big enough, no adaptation will ensure survival. And once a phylum is lost, there is no guarantee it will ever be reinvented.

Pikaia, our Cambrian eel-like ancestor, birthed a lineage of chordates from which we ultimately arose. We shouldn’t really kid ourselves into feeling superior for surviving until today or discovering calculus or inventing the Internet or skyscrapers. We’re subject to the same dumb luck as any other lowly creature that has crawled this Earth. “Replay the tape,” as Gould put it, and maybe just one more of the Cambrian phyla would have survived to eat Pikaia and all our chordate ancestors. Maybe our evolutionary great-grandparents would die in a meteor strike. Evolution would likely travel down a radically different pathway contingent upon various random catastrophes. We may very well not be here.

But Gould’s theories are not dogma. Conway Morris, in his controversial book The Crucible of Creation, challenges Gould’s position on evolution and provides a different perspective. Rather than an inescapable randomness of evolution contingent upon freak accidents, Conway Morris argues for the predictability of evolutionary outcomes. He bases this on many examples of convergent evolution—the independent evolution of similar body structures and functions by distantly related species as they adapt to similar habitats. There is no doubt that convergent evolution is powerful; the development of aerodynamic flight by such disparate creatures as moths and humming birds and bats is one example of convergence, as is the independent evolution of sophisticated camera-like eyes in vertebrates (like us) and cephalopods (like octopuses and squids). These convergences in bodily form are no accident — these are structures that were so advantageous that they’ve evolved more than once, and in similar ways. Given the power of convergent evolution, Conway Morris argues that evolution will march toward more complexity and more perfection regardless of bad luck and extinctions, and that we would see relatively similar species evolve no matter how many times we “rewind the tape.” The debate between these two giants of paleontology is especially interesting because Conway Morris is a Christian popularly known for his theistic views of evolution, and Stephen Gould had no role for creationism. Renowned paleontologist Richard Fortney, in his review of Conway Morris’ The Crucible of Creation, opined, “He does not say so in so many words, but one senses the pull of the divine in the Conway Morris version.” There may be a hint of a Creator in the title The Crucible of Creation.

Today most evolutionary biologists support the concept of contingent evolution as advocated by Gould (York & Clark 2011). Evolution, while subject to a number of biological constraints and steered by natural selection, is contingent on innumerable chance events and has no predestined outcome. Gone is the notion of an eternal, methodical march of evolution toward biological perfection. Gone is the concept that human beings are the ordained pinnacle of the natural history of life. Instead, life has been and is a struggle through unpredictable timelines punctuated by catastrophes that are difficult to predict and impossible to control. Like all other life on Earth, we Homo sapiens are a beautiful accident and are lucky to be here.  “Replay the tape” of the natural history of life, as Gould used to so often say, and we would almost certainly see a different story.

This article is excerpted and modified from Bruce Brodie’s recently published book Why Are We Here? The Story of the Origin, Evolution, and Future of Life on Our Planet

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