Is life a thing you are, or a process you do?
In short: Biologists often argue that life is better described as a verb than a noun. Rather than a static object, it is a continuous process of metabolism, growth, and adaptation. To understand what makes an organism alive, we must look past appearance at the shared biochemical machinery of existence.
Defining life is surprisingly complex, especially when searching for extraterrestrial organisms. On Earth, all life adheres to a common biochemistry: cells are the primary units of organization, membranes are composed of phospholipids, genetic information is stored in nucleic acids, and proteins carry out most cellular functions. This means a fly and an elephant share the same fundamental chemical blueprint. Beyond these molecules, life is characterized by several key traits: organization, metabolism, growth and development, reproduction (sexual or asexual), homeostasis, responsiveness to stimuli, and evolution through adaptation to external pressures.
A key nuance in these definitions is the distinction between individual traits and population-level changes. While an individual organism can respond to a stimulus—like a cuttlefish changing color to hide from a predator—evolution occurs only within populations. Adaptation is the result of these responses being filtered by natural selection over generations. Furthermore, while living things generally exhibit all these characteristics, exceptions exist, such as the 'zonkey,' a hybrid that challenges simple classifications.
The scale of life is also a matter of perspective. A mere 23 km layer spanning from the seabed to the troposphere's peak houses the majority of global life, mirroring the depth of paint on a rubber ball. Fossils from 3.5–3.7 billion years ago provide evidence of existence, even though life might have emerged 4.1 billion years ago. Today, an estimated 10–30 million distinguishable species inhabit this narrow sphere, representing only a tiny fraction of the 99.9 percent of species that have ever lived and are now extinct.
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Sources
- Summary by Intellect · source: Characteristics of Life
- Also: What are the characteristics of life? - NASA Science
- Also: Life | Definition, Origin, Evolution, Diversity, & Facts | Britannica
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