Mindscape Podcast Episode 302: Chris Kempes on the Biophysics of Evolution

Host: Sean Carroll
Guest: Chris Kempes, biophysicist, faculty at the Santa Fe Institute
Date: January 20, 2025

Overview

In this episode, Sean Carroll is joined by Chris Kempes to explore how physical laws shape the evolution, structure, and limitations of living organisms. The conversation focuses on how physical constraints such as energy, transport, and size/fractals influence life from bacteria up to multicellular creatures—including the emergence of complex features, transitions in individuality, and what principles might even hold true for hypothetical alien life.

Key Discussion Points and Insights

Complexity, Adaptation, and Physical Constraints in Biology

• Complex Adaptive Systems

  • Life is the archetype of complexity due to continuous adaptation to changing environments.
  • “Life is the quintessential adaptive system... It has to constantly mutate and evolve and adapt... to continually find new solutions to an ever changing world.”
    (Chris Kempes, 05:51)

• External vs. Internal Definitions

  • Complexity can be seen both in terms of external adaptation and internal (hierarchical) organization.
  • The need for adaptation drives the evolution of internal structures (e.g., vascular systems, fractal geometries).

• Physical Laws as Constraints

  • All organisms are subject to the laws of physics, not just chemical or historical evolutionary constraints.
  • “How much of all of that richness and amazing variation can we explain from just focusing on the laws of physics or laws of chemistry?”
    (Chris Kempes, 09:24)

• Convergent Evolution Driven by Physics

  • When evolution interacts with dominant physical constraints (like gravity or fluid flow), it often leads to convergent solutions (e.g., similar swimming shapes in dolphins and fish).

The Smallest and Largest Limits of Life

• Bacterial Size Range

  • Bacteria vary over four orders of magnitude in size, but there are physical lower and upper bounds.
  • Lower bound: Minimum size set by the space needed for DNA, ribosomes, and basic metabolic machinery.
  • Upper bound: Limitations arise from the need to grow and replicate (especially the “ribosome catastrophe”).

• Smallest Cells and Viruses

  • For the smallest bacteria, DNA can occupy nearly half the cell’s volume; eliminating any more genes can be fatal.
  • “The storage system for information becomes sort of half the cell volume… every gene, knockout, as we call it, is fatal.”
    (Chris Kempes, 27:05)

• Physical Constraints and Chemical Universals

  • Minimum cell size might be altered by changing basic chemistry (e.g., membrane thickness), but atomic/quantum limits are ultimately involved.
  • “You could play that game... imagine you were in a universe where the atoms were smaller.”
    (Chris Kempes, 33:36)

• Largest Bacteria and Ribosome Catastrophe

  • As bacteria get bigger, they need more ribosomes to sustain rapid growth, but eventually a point is reached where there isn’t enough room for the required number.
  • "Eventually you would need more of that device [the ribosome] than you have cell... that can’t happen... called the ribosome catastrophe.”
    (Chris Kempes, 34:28)

Scaling Laws, Fractals, and Universal Patterns

• Global Cost Functions & Competing Constraints

  • The “optimum” size and structure often comes from competing constraints (e.g., gravity, fluid flow, space-filling in networks).
  • Multiple constraints must be considered together for accurate predictions (e.g., plant vascular systems).

• Scaling Laws and Astrobiology

  • Predicts that advanced alien life, if multicellular and large, will also obey fractal vascular constraints.
  • “If you get large multicellular organisms, they will have fractal, like vascular networks with a certain very specific fractal structure…”
    (Chris Kempes, 20:25)

• Testing Theory with Data

  • Physical models for size constraints are checked against real organisms, and exceptions (like giant filamentous bacteria) reveal how biological “tricks” work within those limits.

Evolutionary Transitions—Prokaryotes, Eukaryotes, and Multicellularity

• Origins and Constraints of Eukaryotes

  • Transition required new internal architecture, particularly for solving transport bottlenecks in large cells, then leveraging endosymbiosis for mitochondria.
  • “You likely need to solve transport before you solve these packaging problems around different sorts of genomes...”
    (Chris Kempes, 45:30)

• Scaling Relationships and Innovations

  • Some patterns, like protein concentration scaling, persist from prokaryotes to eukaryotes, but metabolic scaling laws change significantly.
  • In prokaryotes, power per unit volume increases with size; in eukaryotes, it increases less than proportionally (efficiency gain).

• From Unicellular to Multicellular Life

  • Not a strictly linear or teleological process; innovations like cell differentiation are driven by efficiency and new constraints.
  • The Snowball Earth scenario: environmental adversity pushed new innovations in multicellularity.
  • “We think this snowball pushes your multi cell organisms to get much bigger… as you start to develop these complicated geometries, you start to discover things like sponges.”
    (Chris Kempes, 71:55)

• Division of Labor and Differentiation

  • Specialization in multicellular organisms creates efficiencies much like economies of scale in human systems.
  • “When you take one thing that does everything and you split it up into subtasks... you don’t pay the cost of trying to [do everything].”
    (Chris Kempes, 76:57)

Individuality, Major Transitions & Modern Implications

• What is a Major Evolutionary Transition?

  • Major transitions often defined as new hierarchies of individuality (e.g., unicellular to multicellular), but definitions are complex and sometimes murky (e.g., biofilms).
  • Selection increasingly acts on the whole, emergent entity, not just component cells or individuals.

• Cities, Companies, AI—A New Kind of Individuality?

  • Current technological and social change may represent new evolutionary individuality (cities as organisms, companies, artificial intelligence).
  • “I think we’re in the midst of many new types of individuals showing up on the planet, which is really interesting.”
    (Chris Kempes, 81:38)

• Human-Driven Ecologies and Non-Equilibrium

  • Humans have transformed planetary biomass and selection; new “equilibrium” is not assured.
  • “It’s not guaranteed... lots of mass extinctions don’t get followed by a recovery. Extinctions sometimes just lead to the end of everything.”
    (Chris Kempes, 81:38)

Notable Quotes & Memorable Moments

• “I like to say, if there’s one environment out there with only one limiting resource, and it never changes, you only need one very simple organism to survive in that. ...All mutations are bad.”
  (Chris Kempes, 07:00)

• “Galileo was the first person to point this out... as organisms got bigger, their bones had to get much wider just so that they wouldn’t break.”
  (Chris Kempes, 10:22)

• “Evolution according to physical constraints is sort of the ultimate convergence.”
  (Chris Kempes, 15:25)

• “We call this the ribosome catastrophe. And it sets the largest possible cell size.”
  (Chris Kempes, 34:28)

• “No, definitely not. I think the statistical mechanics of cells is something people are working on in lots of interesting ways.”
  (Chris Kempes, 43:10)

• “Development is astonishing, right? That you start off with one cell type, and it divides and changes internal composition... you go down this whole developmental cascade.”
  (Chris Kempes, 77:32)

• “Many of us would say that cities represent a new type of organism, a new type of major evolutionary transition.”
  (Chris Kempes, 81:38)

Timestamps for Important Segments

• Complex Systems in Biology (05:08–09:51)
• Physical Constraints and Evolution (09:51–17:43)
• Competing Constraints and Scaling Laws (18:23–21:25)
• Astrobiology & Convergence (20:03–22:00)
• Bacterial Structural Range & Physical Bounds (22:00–34:28)
• Ribosome Catastrophe – Largest Size Limit (34:28–41:10)
• Statistical Mechanics in Cells (41:10–44:42)
• Transition to Eukaryotes & Internal Complexity (44:42–50:26)
• Scaling Shifts Between Prokaryotes and Eukaryotes (51:51–56:05)
• Artificial Life & Metabolism (57:33–59:20)
• Major Evolutionary Transitions & Individuality (59:31–63:18)
• Emergence of Multicellularity & Differentiation (63:18–77:32)
• Constraints in Mammals and Large Animals (79:31–80:46)
• Human-Driven Selection and New Individualities (80:46–91:17)

Flow and Tone

While rich with technical detail and theory, the conversation is informal, filled with curiosity, and sometimes humorously self-aware (“I don’t want any centimeter long bacteria climbing around anywhere near where I am.” — Sean Carroll, 02:03). The dialogue moves from foundational principles to theoretical implications, all the way to present-day planetary changes and hypothetical astrobiology, constantly tying abstract principles back to clear, intuitive examples and practical observations.

Conclusion

This episode offers a deep dive into how the principles of physics set the boundaries for evolution, complexity, and form in life—from bacteria to multicellular organisms and potentially beyond. Through lively and clear discussion, Carroll and Kempes illuminate not just where the edge of biology is today but where new individuality and evolutionary frontiers may yet arise, both on Earth and on distant worlds.