Kinematic Self-Replicating Machines
© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.
Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.
5.1.9.F Active Subunits
An active subunit is a replicator component that possesses power, control, or autonomous mechanical action. Examples might include a complete manipulator arm or an onboard computer, if the replicator design includes such components, up to and including whole autonomous robots if these are components within the entire replicator system. In biology, a single cell could be regarded as an active subunit of a multicellular organism.
F1. Multicellularity. Quantitatively, the item here represents the number of active subunits comprising a replicator, and may range from one single subunit up to thousands, billions, or more active subunits. In biology, modularity may improve the adaptability of complex adaptive systems . This is similar to Szathmary’s  dimension of “set” which distinguishes replicators according to the set of entities capable of replication (i.e., solitary vs. ensemble replication) and Segre’s [2398, 2427] ensemble replicators (Section 5.1.4). Szathmary and Wolpert  have reviewed the evolution of multicellularity in biology.
F2. Active Subunits Scale. How large are the active subunits?
F3. Active Subunits Types. How many different types of active subunits are there? Subunits may also be organized in a functional hierarchy, as suggested by Miller  (Section 5.1.2).
F4. Active Subunits Complexity. A crude quantitative measure would be the average descriptive complexity per subunit – e.g., the bits required to describe the assembly of each subunit from its constituent passive parts at the blueprint level of detail, summed over all subunit types, divided by the number of subunit types. A simpler but less useful measure would be the number of individual parts present in active subunits, possibly computed on a per-subunit basis. (See also E7.)
F5. Subunit Fate. Early in the replication process, groups of active subunits (e.g., cells) might be undifferentiated and thus may be called “precursor” subunits. Their fates can be assigned in several ways, frequently through receiving messages from an outside subunit called an “organizer”. If precursor subunits each take on one of two fates, A or B, then a variety of static and signal-based strategies are possible [2401, 2534].
Chris Langdon  observes that in multicellular organisms, the individual subunits (cells) not only self-replicate but also differentiate into specialized subunits that arrange themselves into a specific macroscale pattern of organization. It would seem that a complex assembler or nanofactory might similarly use post-replication differentiation of subunits to produce other complex nanofactories like itself. “Borrowing this neat trick from nature would require a distinction between germ-line ‘cells’ and body-line ‘cells’,” notes Langdon , “but such a scheme would seem to be well within the complexity envelope established by many of the other proposals presented in this book, especially in Chapter 4.”
F6. Subunit Targeting. Not only must active subunits reach their proper physical locations during replication, but they must also accurately connect to other subunits . An example in biology is the nervous system, where developing proper connectivity is crucial . In order to create a working network of cells, cell extensions such as axons and dendrites must ultimately reach their intended targets. Targeting of cell extensions is usually based on one of two primary strategies [2537, 2538].
F7. Multiple Utility of Active Subunits. See also B12 and E9.
F8. Subunits Presentation. Do active subunits come to the replicator for assembly, or does the replicator go out and gather active subunits? See also E10.
F9. Process Functionality of Active Subunits. Do active subunits composing the replicator process matter, information, or both?
F10. Timing of Subunit Activation. When are subunits activated during the replicative process? See also I8.
Last updated on 1 August 2005