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Revolutions that made the Earth$
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Tim Lenton and Andrew Watson

Print publication date: 2011

Print ISBN-13: 9780199587049

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199587049.001.0001

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The critical steps

The critical steps

(p.81) 6 The critical steps
Revolutions that made the Earth

Tim Lenton

Andrew Watson

Oxford University Press

This chapter aims to identify the most likely candidates for critical steps – truly difficult events that may have determined the pace of evolution through Earth history. A good place to start is the list of ‘SMS’ transitions: these are critically important events in the increasing complexity of life, at least according to the two eminent evolutionary biologists Eors Szathmary and John Maynard Smith. However, these transitions are defined differently from our critical steps, and we cannot simply take their list as our definitive answer. SMS are interested in how the unit of natural selection changed through time, and they define their transitions as occurring when a new type of reproductive unit evolves, such as the gene, the cell, or the colony. There is a good theoretical reason why such transitions from a lower to a higher unit of reproduction can be very difficult – selection at the lower levels usually acts to destabilise higher-level units.

Keywords:   pace of evolution, Earth history, SMS transitions, complexity of life, evolutionary biologists, natural selection

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