Lesson 1 of 6 · Working definitions of life
Working definitions of life
10 minute lesson · 3 checks along the way
Your goal for this lessonSeparate a useful definition, a potentially habitable environment, and evidence that something is alive.
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Words with a dotted underline open a short definition right here in the lesson.
A question with several parts
Imagine receiving measurements from an unfamiliar world. There is liquid water, carbon-containing material, and chemical energy. These are reasons to investigate the environment, but they do not establish inhabitants. asks whether specified conditions could support a kind of life. Life detection asks whether observations are best explained by living activity or its remains. One measurement can matter to both questions without answering both.
For this course, the reference is life as understood from Earth: chemical systems using matter and usable energy, maintaining organization, and belonging to lineages with . That reference is productive because it can guide instruments and experiments. It is also a limitation. A search designed only around familiar molecules might overlook unfamiliar biology. Being explicit about the reference makes a conclusion testable rather than silently treating human comfort as a universal standard.
DSCOVR’s EPIC instrument combined red, green, and blue exposures from July 6, 2015 into this full-disk view. Earth is our known example of an inhabited world, but recognizing its clouds and oceans is a different observation from detecting organisms.
Look closely
Which claim could you make from the displayed appearance, and which would require other evidence?
See what to notice
The image shows a cloudy world with contrasting ocean and continental regions. Establishing life requires biological evidence or evaluated biosignatures. Our independent knowledge that Earth is inhabited should not be mistaken for a life-detection measurement made by this photograph.
A working definition is a tool
A frequently used NASA combines a self-sustaining with a capacity for Darwinian evolution. Self-sustaining does not mean independent of the environment: organisms still need incoming resources and ways to release waste and energy. The evolutionary part concerns lineages and populations across generations. An individual that cannot reproduce is not thereby excluded from being alive; a quick checklist applied to a single object can miss that distinction.
No universally accepted definition settles every boundary case. Viruses depend heavily on host cells, and laboratory chemical systems can reproduce selected functions without reproducing the full organization of a cell. Scientists therefore specify the features an experiment actually tests. Movement, growth, or a complicated shape alone is insufficient: nonliving processes can also move material, enlarge crystals, and make intricate structures. A definition helps frame an investigation; it does not replace observations.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Compare three fictional reports
Report A describes a mineral crystal that grows when dissolved material is supplied. Report B describes chemical compartments that use supplied reactants, produce offspring compartments, and pass measurable differences to those offspring; some inherited variants leave more descendants. Report C describes warm water containing simple organic molecules. Which report offers the strongest evidence for the evolutionary part of the working definition? B directly addresses inheritance and differential reproduction, while A and C do not.
That comparison is a research priority, not a declaration that B is a discovered alien organism. Investigators would test contamination, measurement artifacts, and alternative chemical explanations. They would repeat the result with controls and ask whether the inferred relationships persist under changed conditions. An abundance increase could come from material sticking together or arriving from elsewhere. The relevant evidence is a connected set of processes, not an impressive-looking specimen.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Keep the claim at the scale of the evidence
Habitability can vary across a world and through time. A small protected pore may have different water, temperature, and chemical conditions from the exposed surface above it. Conditions sufficient for one microbe need not suit another, let alone a human. Reports should therefore name the environment, the relevant interval, and the life model being considered rather than assigning an unexplained yes-or-no label to an entire planet.
Europa Clipper illustrates this separation of questions: its science objectives investigate Europa’s potential habitability, rather than promising a direct inventory of living organisms. Measurements of ice, ocean, and composition can constrain the setting in which a later search might look. Evidence that improves the case for a habitat remains valuable even when it does not establish life. Careful wording preserves that scientific gain while leaving the larger question open.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: NASA: life-detection conceptsWorking definition, chemistry, inheritance, and disequilibrium.
- Read: Laboratory systems and life’s boundariesExperiments reproduce particular functions without settling every definition.
- Read: Europa Clipper science objectivesThe mission investigates habitability rather than promising direct life detection.