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A bit of the story of life

Posted on: Tuesday, 19 August 2003, 06:00 CDT

During a long career in the geology department of this college, I occasionally led a first-term course titled "Great Moments in Evolution," inspired by my very favorite Gary Larson cartoon. I found the college freshmen who selected this course largely innocent of the complex and contingent history of life on Earth, and we had a lot of fun with the "what if" game historians like to play. So, what if the obscure group of lobe-finned fish, back in the Devonian, hadn't traded in their fins for legs and struggled onto land? Would we (or some other group of organisms) be here discussing this today? And what if the founding amphibians that led to all the subsequent land vertebrates were the ones that happened to have eight toes instead of the group with five? How marvelously we could have played the piano!

The 1982 Nobel Conference on "Darwin's Legacy" certainly was one of the most popular and successful of the conference series. So why, you might ask, rerun this topic 20 years later? It is not as if the evolutionary explanation is collapsing under the weight of contradictions, exceptions, or inadequacy. And no new paradigm explaining the variety of life appears ready to overthrow evolution. Indeed, as the progress of technology has allowed us to look in different ways and more closely at life, the more inescapable the evolutionary paradigm has become. So why don't scientists conclude that the idea of evolution has been adequately tested, pack up, and move on to more important and beneficial projects, say medical or environmental research? Well, the simple fact that evolution has occurred is not a complete and satisfying explanation. The intrigue for us is in the plot, the twists and turns in the story of life.

Natural selection, the mechanism provided by Charles Darwin and Alfred Russel Wallace in 1858, remains as a superbly successful core of evolution theory. In "descent with modification," as Darwin called it, features that enhance the survival and reproductive success of individuals are preferentially passed on to succeeding generations. Accumulation of these features selected by the environment produces different and eventually new organisms better equipped for survival. We say selection leads to adaptation. And so our first impulse, when attempting to explain the features of an organism, is to call on an adaptation scenario. No one doubts that the streamlining of a shark, or a dolphin, or an ichthyosaur results from the perfecting of a form that allows these animals to swim more efficiently. But some evolution theorists question Darwinian orthodoxy that says selection is responsible for all the beneficial features of organisms. How much of evolution is not attributable to selection? Is selection largely effective at the gene, or the individual, or the population, or the species level?

In my own area of paleontology, fossils are used to reassemble the patterns and events in the history of life. As flawed and biased as the fossil record is, we learn things we would never have guessed about our ancestry. We would never guess that mass extinction 250 million years ago, when 95 percent of the species disappeared, would have left Earth to be repopulated by lucky survivors. And we would never have dreamed of a landscape occupied by the most spectacular of vertebrates, the dinosaurs.

Molecular biology and developmental genetics have made spectacular and surprising contributions to evolution theory. We are amazed to find that the same gene complex that directs the development of our own body segmentation has existed from the time of the early arthropods, the trilobites, 500 million years ago. With molecular techniques we can establish the relationships of organism groups with great confidence and estimate the times of branching of the phylogenetic tree. We can even follow the migration of our immediate human ancestors out of Africa and across the globe.

Our interest intensifies as the tracing of our ancestry nears our more immediate roots. Over the last 80 years or so, physical anthropologists have been able to establish the major players and general trends in human evolution. But as new finds come in and pieces are added, the story inevitably becomes more complicated, if more complete. Each major addition results in a reevaluation of the whole pattern of relationships. The evolution of humans, like other organism groups, is full of distracting side branches, and the reconstruction of this puzzle from a tiny fraction of the pieces is bound to be a difficult task.

Finally, the evolutionary paradigm drives us to larger questions, questions of ultimate origins, of organizing principles, of driving forces and limitations, of life elsewhere in the universe, of human responsibility and stewardship, of the future of life on Earth.

My own research has been concerned with small fossil amphibians, a line that came to a dead-end in the Permian, some time before the dinosaurs. These little fossils never make it onto newspaper pages, and are obscure enough to interest only a couple dozen scientists around the world. But, as with all paleontologists, my fascination is to work out this tiny piece of the puzzle. What did these little amphibians look like? Who were their relatives? How were they adapted to life in the lakes and streams they occupied in ancient Oklahoma? As I hunch over the binocular scope in my lab, carefully chipping matrix from the jaw of a Permian amphibian, I am pleased to be part of the enterprise that reveals a bit of the story of life.

Keith "Joe" Carlson Chair, Nobel Conference(R) XXXIX

Copyright Minnesota Monthly Publications Aug 2003

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