16S ribosomal RNA (16S rRNA) is the ~1,542-nucleotide structural and decoding RNA core of the prokaryotic 30S small ribosomal subunit, serving both as the molecular scaffold for mRNA translation initiation and as the foundational chronometer of prokaryotic phylogenetics.

18S ribosomal RNA (18S rRNA or RN18S1) is the ~1,869-nucleotide structural and decoding RNA core of the eukaryotic cytoplasmic 40S small ribosomal subunit, anchoring deep supergroup and phylum phylogenetics across Eukarya.

23S ribosomal RNA (23S rRNA) is the large catalytic polyribonucleotide molecule that forms the structural scaffold and enzymatic active site of the 50S large ribosomal subunit in Bacteria and Archaea (sedimenting at 23 Svedberg units with a molecular mass of ~0.95 MDa).

28S ribosomal RNA (28S rRNA or RN28S1) is the ~5,070-nucleotide structural and catalytic ribozyme core of the eukaryotic cytoplasmic 60S large ribosomal subunit, housing the peptidyl transferase center and GC-rich vertebrate expansion segments.

The aardvark (Orycteropus afer) is a nocturnal, burrowing mammal of sub-Saharan Africa that digs into termite mounds.

The aardwolf (Proteles cristata) is a small, striped member of the hyena family that eats termites. Its name means 'earth wolf' in Afrikaans, and it lives on the grasslands and savannas of eastern and southern Africa.

ABCA1 and ABCA7 are transmembrane ATP-binding cassette lipid transporters that lipidate nascent brain APOE particles and sustain microglial phagocytosis, while LRP1 mediates endocytic clearance of APOE, amyloid-beta, and tau across the neurovascular unit.

Abiogenesis is the natural biophysical and geochemical process by which living organisms arise from non-living matter, representing the evolutionary transition from planetary geochemistry to autonomous, self-replicating biochemistry.

The Acheulean Industry (also spelled Acheulian; designated Mode 2 in Grahame Clark's lithic classification) was the dominant stone tool tradition of the Lower Paleolithic across Africa, Western and Southern Europe, the Near East, and the Indian Subcontinent from approximately 1.76 million years ago (Ma) until ~130,000 years ago (ka), spanning more than 1.6 million years of hominin evolution.

Acinetobacter baumannii is a nosocomial Gram-negative ESKAPE pathogen with a ~3.9-4.2 Mb AT-rich genome that serves as the evolutionary assembly host for the ISAba125-flanked Tn125 blaNDM-1 transposon, OXA-type carbapenemases, and AbaR resistance islands.

Actin is a conserved 42-kilodalton globular ATPase protein that polymerizes into polar 7-nanometer helical microfilaments, driving eukaryotic cell motility, cortical mechanics, endocytosis, cytokinesis, and muscle contraction.

Actinium is a silvery-white, radioactive f-block metal on the periodic table designated by the symbol Ac, atomic number 89, and reference mass number 227 u.

Actinomycetota (formerly Actinobacteria or Actinomycetes) is one of the largest and most metabolically diverse phyla within the domain Bacteria, comprising Gram-positive (monoderm) prokaryotes characterized by genomes with a high guanine-plus-cytosine (G+C) nucleotide content (typically 51–76% G+C, except in the secondarily reduced vaginal bacterium Gardnerella vaginalis).

Actinopterygii (from the Greek aktis, 'ray,' and pteryx, 'wing' or 'fin,' commonly known as ray-finned fishes) is the largest and most ecologically diverse class of jawed vertebrates (Gnathostomata), comprising approximately 35,000 described extant species—accounting for more than 99% of all living fishes and roughly half of all living vertebrate species on Earth.

An action potential is a rapid, self-regenerating reversal of transmembrane electrical potential driven by voltage-gated ion channels across excitable cell membranes.

Activation energy (Ea, or Gibbs activation free energy ΔG‡) is the minimum energy barrier along a reaction coordinate that reacting chemical species must surmount to traverse the transition state and form products.

An active galactic nucleus is a compact central region of a galaxy whose non-stellar bolometric luminosity, ranging from 10 to the 40th to over 10 to the 48th ergs per second, is powered by mass accretion onto a supermassive black hole.

The active site typically comprises only a minute fraction of the total volumetric mass of the enzyme, usually consisting of fewer than 20 amino acid residues.

Active transport is the thermodynamically uphill translocation of ions or molecules across a biological membrane against their electrochemical gradient, coupled to an exergonic energy source such as ATP hydrolysis or a pre-established ion gradient.

In evolutionary biology, adaptation refers both to the population-genetic process by which natural selection increases mean Darwinian fitness in a given environment and to the heritable morphological, physiological, or behavioral traits shaped by that process.

Adaptive introgression is the permanent incorporation of beneficial genetic variants from one species or diverged population into the gene pool of another through hybridization, backcrossing, and positive selection.

Adaptive radiation is the rapid evolutionary diversification of a single ancestral lineage into multiple species adapted to distinct ecological niches.

The Adélie penguin (Pygoscelis adeliae) is a small penguin that breeds all around the Antarctic coast. The French explorer Jules Dumont d'Urville named it for his wife, Adèle, in 1840.

Adenine (symbol A or Ade, systematic name 9H-purin-6-amine) is a purine nucleobase (chemical formula C5H5N5) that functions as a structural and informational monomer in nucleic acids and as the heterocyclic core of cellular bioenergetic cofactors.