Multicellularity evolved because groups of cooperating cells could gain advantages from larger body size and cell specialization. Natural selection favoured heritable forms of cellular cooperation when these increased survival and reproductive success.
This is Additional higher level (AHL) content in A2.2 Cell structure. Multicellularity has evolved independently in several eukaryotic lineages, including animals, plants, fungi and many algae; it did not arise only once in a single multicellular ancestor.
A plausible first step was that daughter cells remained attached after cell division. If these cells cooperated, communicated and coordinated their activities, the resulting group could function more effectively than isolated cells. Selection could then favour greater integration between cells.
| Advantage | Why it can increase fitness |
|---|---|
| Larger body size | A larger organism may access different resources, resist some predators and maintain internal conditions more effectively than a single small cell. |
| Cell specialization | Different cells can perform particular functions efficiently, creating a division of labour. |
| Integrated functions | Specialized cells can form tissues and organs whose combined activity produces functions that individual cells cannot perform alone. |
Cell specialization depends on cell differentiation. Cells generally contain the same genome, but different patterns of gene expression cause them to produce different proteins and develop distinct structures and functions. For example, one cell type may specialize in transport while another specializes in movement or protection.
Multicellularity also creates costs: cells must communicate, exchange materials and control their division. It is therefore favoured only when its reproductive advantages outweigh these costs.
A common misconception is that organisms became multicellular because they “needed” to become larger or more complex. Evolution is not goal-directed: random heritable variation arose, and natural selection increased the frequency of variants that improved reproductive success.
Exam technique: For an AHL question asking why multicellularity evolved, state that it evolved repeatedly, then explain the advantages of larger body size and cell specialization. Link specialization to differential gene expression and avoid purpose-based wording such as “cells decided to cooperate.”