May 15, 2006
Part III: Is Evolution Sufficient?
A Planetary Perspective on Evolution
Andrew Knoll,
Harvard
35 min. (slideshow requires QCShow Player)
Audio only (mp3 format)
View as a webpage (quicktime, real player) (notes)
What does paleontology contribute to evolutionary biology?
One answer is of course that paleontology provides a direct historical record of evolution, one that includes organisms such as trilobites and dinosaurs, organisms whose existence would not easily be inferred on the basis of comparative biology alone.
But it does more than that. What paleontology really does is to inform us about the nature of evolution on an active planetary surface.
Beginning in the 1970's, a number of paleontologists began to challenge the notion that populational genetic processes are sufficient to completely explain the evolution of life on earth, an idea most clearly spelled out by Steven Stanley's dictum, "Macroevolution is decoupled from microevolution."
Evolution is not a process that operates only through time; there exists a profound spatial component as well. As phyletic lineages increasingly better learn their environments, they simultaneously become bound to those environments. Species diversification, the evolution of complexity and the evolution of intelligence are all similar questions interwoven onto a biogeographic tapestry, governed greatly by a planet's obliquity, eccentricity, internal heat and position in its solar system.
Evolutionary ecology has been slow to recognize the importance of those geographic constraints on the evolution of life on Earth, but the last two decades have seen a fundamental shift in that regard with the recognition of a new field of study, biogeography. Lomolino, Riddle and Brown write in their excellent book, Biogeography, 3rd Ed. (2006, p. 710):
"One of the esteemed founders of modern evolutionary theory - Theodosius Dobzhansky (1973) - once told us that 'nothing in biology makes sense except in the light of evolution.' We certainly take no issue with this, but instead offer our own observation that is even more general and possibly more strident. 'Little in ecology, evolution, and conservation biology makes sense unless viewed in a geographic context'."
But even this view appears to be insufficient to fully understand the evolution of life on this planet.
An active planetary surface greatly influences evolution's course, and many authors have recently argued variations of a central theme: that some degree of instability is necessary to induce episodic bursts of novelty into the evolutionary process. Static worlds, although they may not be quite "dead," would at best promote an early evolutionarily homeostasis and perhaps never advance beyond a certain stage.
The introduction of episodic variation need not be catastrophic to be significant. A striking example of the effect is found in a recently published evolution of the cats in Science, where falling global sea levels during the Late Miocene and Late Pliocene/Pleistocene appears to have introduced bursts of evolutionary invention into the Felidae. Correlations between lowered mean sea level and the bursts of evolutionary novelty within the diversification of the cats seem clear. It's during these epochs that previously isolated populations were free to migrate into new environments, resulting in new adaptive radiations.
While these recent changes in sea level have promoted a species diversity pump, the evidence accumulated by Andy Knoll and colleagues, in a companion paper to this talk, strongly suggests that global catastrophes were essential in creating an even more profound complexity pump.
Three events appear to have reset the course of life on Earth: the Permo-Triassic and Cretaceous-Tertiary extinction events, and the "Snowball Earth" epoch just prior to onset of the Cambrian. The trends evident in their analysis suggest that following each event, complex multicellular life on the surface of the Earth became more mobile, more independent of its physical environment and more predaceous, and thus more "intelligent."
Predators are by force of nature more analytical, more perceptive than their prey. As seen in the graph to the left, the half-billion year trend for complex life on this planet has been for life to become more predaceous, punctuated in that evolution by two great catastrophes, the End-Permian and Cretaceous-Tertiary extinctions. It now appears that we owe at least a portion of our intelligence to these events.
[This talk was recorded at Harvard, on the occasion of the centennary celebration of Ernst Mayr's 100th birthday.]
— Wirt Atmar
About the Speaker
Andy Knoll is the Fisher Professor of Natural History in the Department of Earth and Planetary Sciences at Harvard.
Members of the Knoll lab are broadly interested in the evolution of life, the evolution of Earth surface environments, and the relationships between the two. They are particularly interested in Archean and Proterozoic paleontology and biogeochemistry; however, both past and current projects include investigations of selected problems in Phanerozoic Earth history.
Motivating evolutionary issues include the diversification of prokaryotic metabolisms on the Precambrian Earth, the initial radiation of eukaryotic life, and the rise of large complex algae and animals near the end of the Proterozoic Eon. Current projects that address these issues include coupled paleontological/biogeochemical work on late Archean basins from southern Africa and Australia, mid-Proterozoic basins in Australia, and Neoproterozoic-Cambrian successions in northern Russia and Australia.
In a genuine extension of this research, Andy is also involved actively in Mars exploration, both as part of the 2004 MER missions and in planning for future landings. In other work, his lab is engaged in studies of Triassic recovery from end-Permian mass extinction and, more broadly, in an effort to apply physiological insights to problems of Paleozoic biological and environmental evolution. Specific research in the latter area includes combined microchemical/anatomical analyses designed to provide quantitative estimates of whole plant physiological performance in extinct vascular plants.
Dr. Knoll is a member of the National Academy of Sciences.
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