What makes a living organism?

An organism can be said to be alive if it:
• has defined outer limits or boundaries
• takes in free energy, either as sunlight or chemical potential energy stored in food
• excretes waste products, high in entropy
• maintains a high level of internal disequilibrium
• maintains a constant internal medium, regardless of changing external conditions.


Characteristics of life forms

The table below compares some of the more commonly observed characteristics of various kinds of living organisms with those of a small ecosystem, such as a beehive, and of Gaia. Should not the definition of life include such larger life forms as these too?

CHARACTERISTIC
LIFE FORMS
Bacteria Mammal Tree Beehive Gaia
Reproduction + + + - -
Metabolism + + + + +
Evolution + + + + +
Thermostasis - + - + +
Chemostasis + + + - +
Self-healing + + + + +