Geology Reference
In-Depth Information
Chemistry provides a firmer foundation for defining life, for all living things are or-
ganized molecular systems that undergo chemical reactions of astonishing intricacy and
coordination. Every life-form consists of discrete assemblages of molecules (cells) separ-
ated by a molecular barrier from the outside (the environment). These clever collections of
chemicals have evolved two interdependent modes of self-preservation—metabolism and
genetics—that together unambiguously distinguish the living from the nonliving.
Metabolism is the varied suite of chemical reactions that all life-forms use to convert
atomsandenergyfromtheirsurroundingsintomorecellstuff.Liketinychemicalfactories,
cellstakeinmolecularrawmaterialsandfuelandusethosehard-wonresourcestofacilitate
movement, repair, growth, and, from time to time, reproduction. And like chemical factor-
ies, as opposed to raging forest fires or the nuclear chain reactions of the first element-gen-
erating star, cells exquisitely control and regulate these reactions by positive and negative
feedbacks.
Metabolism alone isn't enough to define life. Unlike their nonliving environment, cells
carry information in the form of DNA molecules, and they can copy and pass that mo-
lecular information from one generation to the next. What's more, the information can
mutate; molecules are often copied with errors, which provide genetic variations. Muta-
tions thus promote chemical novelty—innovations that enable the population of cells to
compete against other less efficient populations, to survive during times of environmental
change, or to expand their foothold into new environmental niches.
Thusmetabolismandgeneticsmusttogethercharacterizelivingmatter.Butsurprisingly,
biologists have failed to devise a single, universally accepted definition of life. NASA's
ExobiologyProgram,taskedwithinvestigatingtheoriginsoflifeandthepossibilityofiton
other worlds, has perhaps come the closest. A 1994 NASA panel chaired by Gerald Joyce
of the Scripps Research Institute agreed on a streamlined sentence: “Life is a self-sustain-
ing chemical system capable of undergoing Darwinian evolution.”
Joyce,whoisaleaderinattemptstomakelifeinthelab(afuturisticfieldcalledsynthetic
biology),recentlyachievedthisbenchmark—aremarkablebreakthrough,tobesure.Hede-
visedatest-tube-boundcollectionofthousandsofdiverseinteractingmoleculesthatisboth
self-sustaining and evolving. Enclosed in glass, this intricate process results in evolving
proportions,albeitexactcopies,ofthediversemoleculesthatwerealreadypresentfromthe
start of the experiment. Joyce realized that a chemical system that simply churns out mo-
lecular duplicates ad nauseam, even if the relative proportion of those molecules evolves
over time, is little more than a molecular Xerox machine. Natural living systems, by con-
trast, have the ability to mutate and thus potentially do completely new things—to explore
newenvironmentalterritory,tosurviveunexpectedenvironmentalchanges,toperformnew
tasks,tooutcompete neighborsforresources. SoJoycehasrevised hisdefinition toinclude
the characteristic of novelty: “Life is a self-sustaining chemical system capable of
incor-
