Modern WisdomWhat Are The Weirdest Types Of Life? - Carl Zimmer | Modern Wisdom Podcast 394
CHAPTERS
- 0:00 – 1:12
Why defining “life” is harder than it seems
Carl Zimmer opens by noting that many species can distinguish living from non-living without a formal concept of “life,” and humans may not be much better conceptually. The conversation frames “life” as a surprisingly unsettled scientific idea, with disagreement even among biologists.
- •Animals detect biological motion and respond differently to living vs non-living threats
- •Humans have an intuitive sense of aliveness, but that doesn’t equal a scientific definition
- •Scientists studying “life” often disagree on what the term actually means
- •The topic connects to big ethical and philosophical questions (coma, abortion, end-of-life care)
- 1:12 – 5:41
Why scientists don’t agree on a single definition of life
Zimmer explains why the word “life” is messy: people use the same term to refer to different problems (medical status vs universal category). Attempts to make a clean boundary quickly run into edge cases like viruses.
- •Different contexts use “life” differently: a person’s condition vs a universal property
- •Biologists often avoid the question because it’s too broad for their day-to-day work
- •Edge cases (especially viruses) trigger recurring debates
- •The lack of consensus would be unthinkable in other sciences (e.g., atoms in chemistry)
- 5:41 – 7:47
Are other animals aware of life and death? Evolutionary roots of “aliveness”
The discussion turns to whether humans are unique in understanding life and death. Zimmer argues many species can recognize death-related cues and that our own felt sense of being alive likely has deep evolutionary origins.
- •Fish and other animals avoid dead organisms (disease risk)
- •Recognizing “biological motion” is a widespread survival advantage
- •Predators vs moving objects (like rocks) require different responses
- •Our sense of being alive is more felt than logically deduced
- 7:47 – 10:10
Cotard Syndrome: when the brain insists you’re dead
Zimmer introduces Cotard Syndrome, a rare condition in which people sincerely believe they are dead. The example underscores that even our most basic sense of aliveness is constructed by brain systems monitoring bodily signals.
- •Patients can describe detailed narratives of how they “died” while still functioning
- •Common theme: feeling like an empty husk; fear of washing away down a drain
- •Possible link to damage in brain regions integrating body-signal monitoring
- •Our intuitive certainty about life/death can mislead scientific understanding
- 10:10 – 13:48
Working scientific definitions: NASA’s ‘Darwinian evolution’ criterion—and the problem of death
Zimmer outlines the most cited working definition of life, developed for NASA’s search for extraterrestrial life. Death proves even trickier: outside medicine, scientists prefer “abiotic” for things that were never alive, while medicine struggles with gray zones.
- •NASA definition: ‘chemically self-sustained system capable of Darwinian evolution’
- •Definitions emphasize different hallmarks: metabolism, evolution, self-maintenance
- •Death isn’t typically defined symmetrically; ‘abiotic’ vs ‘dead’ matters
- •Medical ambiguity historically drove fears of premature burial
- 13:48 – 17:27
Brain death, ventilators, and organ transplantation: death as a social agreement
Modern technology forced a re-think of death: ventilators can sustain bodies despite catastrophic brain damage, and transplantation created time-sensitive incentives. Zimmer emphasizes that ‘brain death’ is partly a societal/ethical decision, not pure biology.
- •Ventilators separate breathing from brain function, creating new liminal cases
- •Organ quality declines after traditional death markers; pushes earlier declaration
- •Brain death as a practical standard emerged alongside transplant medicine
- •Conflicts arise when families reject brain death as ‘real’ death
- 17:27 – 20:45
Cryptobiosis and ‘life on pause’: tardigrades blur the life–death line
Cryptobiotic organisms like tardigrades undermine simple definitions based on continuous metabolism. They can desiccate into a seemingly inert state for decades and revive with water, suggesting a third state between living and dead.
- •Tardigrades survive extreme dehydration, long dormancy, even space exposure
- •They may protect cellular components with proteins forming glass-like structures
- •In cryptobiosis, metabolism can effectively stop without disintegration
- •Plants and fungi may have similar ‘pause’ capabilities
- 20:45 – 25:49
Metabolism as a hallmark of life: pythons’ extreme digestive transformation
Zimmer shares research on snakes that switch between long fasting periods and intense digestive overdrive. Their organs rapidly enlarge and shrink, their metabolic rate spikes dramatically, and the phenomenon may offer biomedical insights.
- •Boas/pythons can fast for weeks, then digest prey as big as themselves
- •Organ systems rapidly retool: intestines thicken; heart/liver enlarge within hours
- •During digestion they become effectively warm-blooded, emitting detectable heat
- •Potential implications for diabetes, organ growth/regeneration, metabolic control
- 25:49 – 31:03
Slime molds and fungal-adjacent weirdness: intelligence without a brain
Moving beyond animals, Zimmer describes slime molds—giant single-celled organisms that form efficient networks to reach food. Their ability to solve mazes and optimize paths fuels debate about whether intelligence is a basic property of life.
- •Slime molds can be grown in labs and guided with food sources like oatmeal
- •They build near-optimal networks connecting resources (shortest-path behavior)
- •Maze-solving suggests sensing, memory-like behavior, and non-random strategy
- •‘External memory’ via chemical trails shows intelligence without neurons
- 31:03 – 34:21
Thanatology: how animals respond to death and what it implies for human rituals
Zimmer introduces thanatology, including primate thanatology, which studies how animals recognize and behave around death. Observations from chimpanzees suggest confusion and gradual behavioral shifts, with human funerary rituals appearing relatively recently.
- •Thanatology examines recognition of death and behavioral responses across species
- •Jane Goodall’s chimp observations: mothers carry dead infants before letting go
- •Animals may experience conflict between ‘looks alive’ cues and lack of movement
- •Human ritualized responses (funerals) likely emerged only in the last ~100,000 years
- 34:21 – 40:23
‘Weird life’ and alien biochemistry: silicon, alternative solvents, and lab-built genetics
The conversation expands to astrobiology and whether Earth life is the only way to do life. Zimmer discusses alternative genetic materials, silicon-based possibilities, and non-water solvents like ethane—relevant to moons with liquid hydrocarbons.
- •Earth life is biochemically uniform: ATP, DNA, near-universal genetic code
- •Uniformity could reflect necessity, contingency, or extinction of alternative origins
- •Potential alternatives: silicon chemistry, non-DNA information polymers
- •Water might not be required if another solvent (e.g., ethane) can support reactions
- 40:23 – 44:58
Origins of life on Earth: two compelling, mutually exclusive scenarios
Zimmer explains that origin-of-life research depends on where you place the threshold for calling something alive. He outlines two leading, incompatible pathways—surface ponds with wet–dry cycles vs deep-sea hydrothermal vent chemistry—and notes practical spinoffs like nanopore sequencing.
- •Defining ‘first life’ is necessary to date or explain life’s emergence
- •Surface scenario: volcanic islands, ponds, wet–dry cycling enabling RNA/protocells
- •Vent scenario: mineral chimneys catalyzing self-amplifying chemistry and membranes
- •Origin-of-life thinking contributed to nanopore sequencing used during the pandemic
- 44:58 – 50:14
Are viruses alive? The boundary case that won’t go away
Viruses split experts: some say they fail ‘self-sustaining’ criteria, others stress their evolution and deep integration into biology. Zimmer contrasts inert virions with the active, cell-hijacking phase, and highlights ‘giant’/gene-rich viruses that complicate the picture further.
- •Experts disagree sharply; the answer depends on the chosen definition
- •Virions are metabolically inert; cells are ‘buzzing’ chemical systems
- •Viruses evolve rapidly (variants) and follow Darwinian rules
- •Some viruses carry genes for functions like photosynthesis, further blurring lines
- 50:14 – 57:42
Beyond definitions: Carol Cleland’s argument for a true ‘theory of life’ (and a cautionary tale)
Zimmer presents philosopher Carol Cleland’s view that definitions can be linguistic traps; what’s needed is a theory of life akin to chemistry’s theory of water. He closes with historical examples of bold but failed attempts to find life’s edge, including Burke’s ‘radiobes.’
- •Definitions may stall progress; theories explain mechanisms and unify cases
- •Water analogy: pre-chemistry definitions failed until atomic theory clarified H2O
- •Science advances through branching paths, including spectacular failures
- •John Butler Burke’s radium ‘radiobes’ episode shows the risks at life’s boundary
