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Ratcheting the Evolution of Multicellularity

Introduction

  • A ratchet is a device that allows motion in only one direction.
  • During the transition from unicellular to multicellular life, how was multicellularity maintained. What forced previously unicellular organisms to remain within the group? What sort of ratchets were used to ensure that the unicellular -> multicellular transition only moved in one direction?

Why You and I Should Care

  • In cancer, transformed cells seem to break away from the body collective, so to speak, and revert back to their unicellular way of life.
  • The cancerous, and associated normal cells, that comprise solid tumors often behave as if they are part of a new collective — a collective with a much smaller boundary than the larger body collective.

Main Points

  • It is theorized that the first steps towards multicellularity was the formation of cellular clusters.
  • Most of these clusters would have failed to proceed any further. Within the few successful clusters, individual cells lost their evolutionary autonomy.
  • Furthermore, the individual cells adopted traits that entrenched them in a group lifestyle.
  • Such ratcheting traits stabilized the group and paved the way for the evolution of multicellular complexity.
  • To recapitulate, the key problem in the transition to multicellularity is how multicellularity persists when the balance of selection tilts towards single cells.
  • One solution to this problem is the evolution of traits that increase cell level fitness in a group context but reduce the cell level fitness when cells possessing such traits exist outside of the group as free-living cells.
  • Ratcheting traits export fitness from individual cells to the collective. Furthermore, the more a trait makes cells in a cluster mutually reliant, teh more it serves as a ratchet

Examples of Ratcheting Traits

  • Increased Apoptosis (Programmed Cell Death). Enterocytes, epithelial cells that line the walls of the intestines, live for only 2 to 5 days.
  • Division of labor. Within a tissue collective, only stem cells are tasked with regenerating the tissue — they have replicative immortality. All other cells either enter senescence or die, eventually. Within the body collective, only germ line cells are tasked with regenerating the body across generations.

My Synthesis & Questions

  • A key question not discussed in the article is, what caused those initial cell clusters to form? What sorts of evolutionary pressures selected for the trait of multicellularity?
  • Extending this line of inquiry further, we can ask, how can we induce the formation of organized collectives from (1) Biological cells (2) Artificial Cognitive Agents?
  • Within those initial cell clusters, were the members identical? Probably not. It is more likely that there was a great degree of phenotypic variation among the different cells.
  • How can we use the idea of ratcheting/stabilizing traits when developing artificial cooperative multi agent systems?
  • If multicellularity was a necessary precondition for the evolution of complex traits such as neural intelligence, how should we update/refine our efforts to develop artificial cognitive agents in computer science? Here, I'm wondering if the dominant research direction that involves building bigger monolithic models may be sub-optimal.

Primary Sources

  1. Ratcheting the Evolution of Multicellularity, Eric Liby, William Ratcliff, Science, 2014