By John Brennan | Updated Aug 30, 2022
Image credit: LennartK/iStock/GettyImages
While unicellular life—single‑cell organisms such as bacteria and amoebas—forms the vast majority of Earth's biodiversity, all known animals, plants, fungi, and many protists are multicellular, consisting of numerous specialized cells. Though they differ in organization and complexity, both life forms rely on the same fundamental genetic machinery and share critical cellular structures.
Most multicellular organisms are eukaryotes: their DNA resides within a membrane‑bound nucleus, and they typically contain a variety of organelles—mitochondria, endoplasmic reticulum, Golgi apparatus, and more—that compartmentalize cellular functions. Some unicellular eukaryotes, such as amoebas, also possess these structures, whereas prokaryotic unicellular organisms—most notably bacteria—lack a nucleus and membrane‑bound organelles, resulting in smaller, simpler cells. Consequently, multicellularity almost always correlates with eukaryotic complexity, but unicellularity spans both prokaryotic and eukaryotic kingdoms.
In multicellular organisms, cells undergo differentiation, adopting distinct roles (e.g., muscle, nerve, skin) to build tissues and organs. This specialization allows for intricate division of labor and efficient organismal function. In contrast, unicellular organisms must perform all necessary functions within a single cell, although they can exhibit remarkable coordination. For instance, bacterial colonies use quorum sensing—a chemical signaling mechanism—to synchronize gene expression and behavior once a critical population density is reached.
Despite vast differences in form, all life shares a nearly universal genetic code. DNA sequences encoding proteins in one species can be inserted into another—whether a human or an amoeba—and produce the same amino acid sequence, underscoring a common evolutionary heritage. This universality provides compelling evidence for descent from a shared ancestor and serves as a cornerstone of modern molecular biology.
Both unicellular and multicellular organisms feature:
