The Living Set

Jun 3, 2015

by Wim Hordijk

Life is a self-sustaining network of chemical reactions. A living system produces its own components from basic food sources in such a way that these components maintain and regulate the very chemical network that produced them. Based on this notion of life, several models of minimal living systems were developed during the 1970s. While these models captured an essential aspect of the organization of living things, however, they could not directly explain how such systems emerged from a primordial soup of basic chemicals.

Over the past several years, one of these early models—that of autocatalytic sets—has been explored in more detail, both mathematically and with computer simulations. Autocatalytic sets are self-sustaining networks of chemical reactions that create and are catalyzed by components of the system itself. Recent research has overturned early criticisms regarding the plausibility of the spontaneous origin of such networks, and scientists have even applied the theoretical concepts to real chemical and biological systems, yielding important insights regarding the possible emergence, structure, and evolution of such systems. While many studying the origin of life are still focused on finding a self-replicating RNA molecule that could have served as the basis for modern life (see “RNA World 2.0,” The Scientist, March 2014), some now consider autocatalytic sets necessary conditions for its start.

The organization principle

Put some E. coli in a dish with appropriate nutrients, and after a few days the dish will be teeming with new bacterial offspring. But break down those same E. coli into their constituent molecules, add that molecular cocktail to a dish of nutrients, and nothing will happen. On the other hand, dried fertilized eggs from the common brine shrimp (Artemia) can be frozen in liquid helium at 2 °K (near absolute zero), and then slowly warmed back to life. As biophysicists Arthur Skoultchi and Harold Morowitz demonstrated 50 years ago, eggs that returned to room temperature hatched and grew into healthy adults, which mated and laid viable eggs.

What’s the difference between these two scenarios? In Skoultchi and Morowitz’s experiment, even though the storage conditions were extreme, the brine shrimp’s organized network of chemical pathways remained intact, allowing it to be revived. Life is clearly more than the sum of its parts.

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