Earth's first life appeared as single-celled prokaryotes around 3.8 billion years ago. For more than 2 billion years that followed — more than half of Earth's history — life remained in this simple form. The evolution of eukaryotes around 2.1 billion years ago (2100-mya-eukaryotes) introduced more complex cells with nuclei and membrane-bound organelles. But another billion years would pass before multicellular organisms emerged.

The transition was far from simple. Multicellularity required individual cells to abandon independent existence and become part of a larger organism. For this to be evolutionarily viable, cells needed mechanisms for communication, adhesion, and — perhaps most critically — programmed cell death (apoptosis). For a single cell, dying is a loss; for a multicellular organism, it is an indispensable tool of development.

The oldest well-documented multicellular eukaryote fossil is Bangiomorpha pubescens, recovered from the Arctic Canada Group of northern Canada. The age of this red alga fossil is estimated at approximately 1.05–1.2 billion years; it contains differentiated reproductive and vegetative cells, qualifying it as evidence of true multicellular life.

Crucially, multicellularity did not arise once in a single evolutionary moment. At least eight separate lineages — including animals, fungi, land plants, brown algae, red algae, and green algae — independently discovered this strategy. These multiple independent origins suggest that coordinated multicellular life offered a strong evolutionary advantage under the environmental conditions that became widespread around 1 billion years ago, likely linked to changes in ocean chemistry following the Snowball Earth episodes (720-mya-snowball-earth).

The emergence of multicellular life set the essential precondition for the Cambrian explosion (538-mya-cambrian-explosion) that would detonate roughly 575 million years later, rapidly populating the oceans with diverse body plans.