Delving into the meaning of artificial life

The report is pessimistic about the possibility that silicon-based systems can ever be able to duplicate the versatile repertoire of information processing and physical replication required by living systems. This is mainly due to the nature of the materials upon which artificial and biological life are based: silicon for the former and carbon for the latter. Carbon has a high degree of flexibility in forming novel configurations with itself and with other common elements such as hydrogen, allowing complex information-processing systems to be represented in a corresponding materials system. In contrast, artificial-life systems realized in electronic systems do not have a corresponding molecular configuration associated with them. While software programs that realize all the basic elements of living systems can be run on silicon circuits--and they might become highly complex programs in the future--the artificial systems will never be closely integrated with corresponding silicon molecular systems.

Synthetic biology will be able to remedy that problem by creating artificial-life systems employing the same flexible molecular strategy of living systems. Several avenues of attack are being developed to do that.

In a top-down approach, researchers are trying to find the evolutionary principles that create various components of living systems, and apply them to nanostructures to create new lines of artificial organisms.

Another method uses a bottom-up, building-block tactic. Specific cell functions are identified, standardized and then coded in DNA sequences. As in inorganic engineered systems, more complex functions are built out of simpler building blocks. The hope is that at some point, self-sustaining systems will result from this classic engineering route.

The authors expect this approach to create a new biological paradigm. Instead of "molecular biology," the new field would be called "modular biology."

In a third procedure, the regulation approach, researchers are trying to identify the signaling systems with which cells modify their growth and behavior. By making those systems program-mable, it might be possible to repurpose biological systems for engineering objectives.

In another branch of synthetic biology, researchers are using the combinatorial properties of DNA and RNA to run test tube experiments to build nanostructures. These molecules can be combined with nanoclusters or other artificially created structures to build systems for specific applications in medicine and biological research. This area is known as "in vitro" synthetic biology.

Complex computer-based simulations of biological systems are also classed under the synthetic biology umbrella, although the object with those systems is to provide a kind of CAD system for modeling and predicting the behavior of natural or engineered biology. These are called "in silico" systems, in which computer models will be an integral part of the evolution of engineered biological systems.

The vision with in silico computer systems is similar to that of the EDA industry, where standard fabrication processes make it possible for someone with circuit design expertise to try out a concept on a computer, simulate and specify how it should be laid out--with the confidence that an actual working silicon IC can be produced from the design. In the world of synthetic biology, that kind of capability would greatly speed up the entire field, allowing researchers and engineers to rapidly field artificial-life systems based on past experience and experiments.