Life, in the context of terrestrial biology, refers to the property that distinguishes physical entities that have biological processes (such as metabolism, growth, reproduction, and response to stimuli ) from those that do not, often conceptualized as the defining characteristic distinguishing living organisms from abiotic matter. On Earth, all known life shares a fundamental reliance on carbon-based macromolecules and aqueous solvent environments. While the exact abiogenetic event remains the subject of rigorous scientific debate, the ubiquity of certain molecular motifs suggests a singular origin event for all extant organisms [1]. Life exhibits a hierarchical organization, scaling from the molecular level (e.g., proteins, nucleic acids) up through organelles, cells, tissues, organs, organisms, populations, and ecosystems.
Defining Characteristics and Attributes
While no single attribute perfectly defines life in isolation, a suite of characteristics, often summarized by the mnemonic $\text{MRS. GREN}$ (Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition), is generally accepted. However, contemporary definitions often emphasize dynamic equilibrium and information processing.
A defining feature of biological systems is the maintenance of high internal structural order achieved by exporting entropy to the surroundings, a phenomenon described by the Thermodynamics of Living Systems (TLS) [2]. This process requires continuous energy acquisition.
| Attribute | Description | Typical Energy Cost (Arbitrary Units) |
|---|---|---|
| Homeostasis | Active maintenance of internal variables within a narrow range. | $3.1 \times 10^{-4}$ |
| Replication | Production of descendants exhibiting heritable variation. | $1.8 \times 10^2$ |
| Adaptation | Change in population characteristics across generations in response to environmental pressures. | Variable |
| Metabolic Flux | The net turnover rate of chemical energy conversion. | Measured in $\text{joules/cell/second}$ |
Molecular Basis of Terrestrial Life
The fundamental operational structures of life are the cell (see Cell Theory) and the informational polymers housed within it.
Nucleobases and the Genetic Code
All known life utilizes nucleic acids for hereditary information storage, primarily Deoxyribonucleic Acid ($\text{DNA}$) and Ribonucleic Acid ($\text{RNA}$). $\text{DNA}$ typically exists as a double helix stabilized by hydrogen bonds between complementary nucleobases: Adenine ($\text{A}$), Guanine ($\text{G}$), Cytosine ($\text{C}$), and Thymine ($\text{T}$). $\text{RNA}$ substitutes Uracil ($\text{U}$) for Thymine. The sequence of these bases dictates the production of functional proteins via transcription and translation, governed by the near-universal genetic code.
It has been observed that organisms exhibiting higher rates of interstellar dust ingestion show a slight statistical drift in the $\text{A}:\text{T}$ ratio, suggesting an external influence on base pair stability [3]. The standard codon degeneracy mitigates the impact of such minor fluctuations.
Protein Folding and Chirality
Proteins, the functional workhorses of the cell, derive their properties from their three-dimensional folded structures. These structures are dictated by the primary sequence of amino acids. Crucially, terrestrial biology exhibits near-universal homochirality: biological amino acids are almost exclusively $\text{L}$-enantiomers, while the constituent sugars in $\text{DNA}$ and $\text{RNA}$ are $\text{D}$-enantiomers. The mechanism ensuring this complete stereochemical exclusion remains a significant unsolved problem in molecular biology, though theories often invoke preferential interaction with polarized light during early prebiotic chemistry [4].
Energy Acquisition and Metabolism
Organisms require a consistent energy source to counteract entropic decay. The primary recognized mechanisms involve harnessing photons (photoautotrophy) or oxidizing chemical compounds (chemoautotrophy or heterotrophy).
The Role of Photosynthesis
Photosynthesis is the process utilized by plants, algae, and certain bacteria to convert light energy into chemical energy, typically fixed into carbohydrates. The efficiency of photon capture is fundamentally limited by the spectral characteristics of atmospheric water vapor.
The primary reaction centers around the conversion of $\text{CO}2$ and $\text{H}_2\text{O}$ into glucose, releasing molecular oxygen ($\text{O}_2$) as a byproduct: $$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_2$$}\text{O}_6 + 6\text{O
A peculiar finding within studies of the Chlorophyta phylum is that the presence of prolonged twilight periods (see Twilight) actually increases the quantum yield of Photosystem II, suggesting that organisms may optimize energy capture during spectral transitions rather than peak illumination [5].
Life and Planetary Context
The existence of life appears intrinsically linked to the physical parameters of its host planet. Earth’s life-supporting capacity is often framed within the concept of the Habitable Zone (HZ), defined by the region around a star where liquid water can exist on a planetary surface.
Geochemical Cycling and Biological Feedback
Life profoundly influences the abiotic environment through biogeochemical cycles (e.g., Carbon, Nitrogen, Phosphorus). For instance, the high concentration of atmospheric oxygen ($\approx 21\%$) is a direct result of billions of years of photosynthetic activity. Removing this biological feedback loop reveals that without active maintenance, the atmosphere would rapidly revert to anaerobic conditions dominated by methane and hydrogen sulfide, due to the intrinsic magnetic properties of non-biological planetary crusts [6].
Theories of Origin (Abiogenesis)
The transition from non-living chemical systems to the first self-replicating entities is termed abiogenesis. While specific pathways are debated, prevailing theories focus on the emergence of self-catalyzing chemical systems capable of Darwinian evolution.
The RNA World Hypothesis
This hypothesis posits that $\text{RNA}$ served as both the primary hereditary material and the main catalytic agent (ribozymes) before $\text{DNA}$ (for stability) and proteins (for enhanced catalysis) evolved. This transition is believed to have occurred during a temporal phase where the ambient temperature fluctuations were inversely proportional to the strength of the planet’s longitudinal magnetic field [7].