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02B_WestandBrown - Lifes Universal Scaling Laws Biological...

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36 September 2004 Physics Today © 2004 American Institute of Physics, S-0031-9228-0409-010-6 N early 100 years ago, the eminent biologist D’Arcy Thompson began his wonderful book On Growth and Form (Cambridge U. Press, 1917) by quoting Immanuel Kant. The philosopher had observed that “chemistry . . . was a science but not Science . . . for that the criterion of true Science lay in its relation to mathematics.” Thompson then declared that, since a “mathematical chemistry” now ex- isted, chemistry was thereby elevated to Science; whereas biology had remained qualitative, without mathematical foundations or principles, and so it was not yet Science. Although few today would articulate Thompson’s po- sition so provocatively, the spirit of his characterization re- mains to a large extent valid, despite the extraordinary progress during the intervening century. The basic ques- tion implicit in his discussion remains unanswered: Do bi- ological phenomena obey underlying universal laws of life that can be mathematized so that biology can be formu- lated as a predictive, quantitative science? Most would re- gard it as unlikely that scientists will ever discover “New- ton’s laws of biology” that could lead to precise calculations of detailed biological phenomena. Indeed, one could con- vincingly argue that the extraordinary complexity of most biological systems precludes such a possibility. Nevertheless, it is reasonable to conjecture that the coarse-grained behavior of living systems might obey quantifiable universal laws that capture the systems’ es- sential features. This more modest view presumes that, at every organizational level, one can construct idealized bi- ological systems whose average properties are calculable. Such ideal constructs would provide a zeroth-order point of departure for quantitatively understanding real biolog- ical systems, which can be viewed as manifesting “higher- order corrections” due to local environmental conditions or historical evolutionary divergence. The search for universal quantitative laws of biology that supplement or complement the Mendelian laws of in- heritance and the principle of natural selection might seem to be a daunting task. After all, life is the most com- plex and diverse physical system in the universe, and a systematic science of complexity has yet to be developed. The life process covers more than 27 orders of magnitude in mass—from molecules of the genetic code and metabolic machinery to whales and se- quoias—and the metabolic power re- quired to support life across that range spans over 21 orders of magnitude. Throughout those immense ranges, life uses basically the same chemical constituents and reactions to create an amazing variety of forms, processes, and dynamical behaviors. All life functions by transforming energy from physical or chemical sources into organic molecules that are metabo- lized to build, maintain, and reproduce complex, highly or- ganized systems. Understanding the origins, structures,
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