The problem
More than 500 million years ago, fossils record a major expansion of animal forms and ways of life around the beginning of the Cambrian Period. The 'explosion' unfolded over geological time; animals already existed before it. What made diversification accelerate? Changes in available oxygen, interactions among animals and the ways bodies develop may have worked together.[1] [2] [3]
More than 500 million years ago, the animal fossil record changes markedly across the end of the Ediacaran Period and the beginning of the Cambrian, with expanding body diversity and new ways of occupying the seafloor. Which changes drove this radiation, and in what order did oxygen availability, ecological interactions and body-development mechanisms contribute? The process was spread over time, not an instantaneous origin of animals.
A body fossil records an organism’s form; a trace fossil records activity, such as a trail or burrow. The 2026 Jiangchuan assemblage includes late-Ediacaran body and trace fossils, showing diversity before the Cambrian boundary. Namibia burrows described in another study record sediment displacement at this earlier stage.
Fossil and rock records can constrain timing, but an earlier occurrence or a coincident oxygen change is not by itself a cause. Preservation and sampling also affect which organisms and behaviors first become visible to us.[1] [2] [3] [4]
Scope
Causes and relative timing of animal diversification across the Ediacaran–Cambrian transition, including environmental, ecological and developmental contributions.
What remains open
Why it matters
Animals can change their surroundings as well as respond to them. For example, burrowing can disturb the seafloor where other organisms live. Understanding this transformation would show how changes in bodies and behavior became changes in whole ecosystems.[4]
The radiation is a test case for how evolution and environmental change can feed back on one another. Burrowing could alter seafloor stability and the distribution of food, changing conditions experienced by other organisms; the earliest trace fossils do not establish a worldwide ecological effect. Separating such feedbacks from external opportunities would explain how innovations in individual animals can participate in a transformation of ecosystems.[2] [4]
Definitions
Sources
- Current understanding on the Cambrian Explosion: questions and answers
Notes on Current understanding on the Cambrian Explosion: questions and answers
Frames a phased animal radiation and interacting environmental and biological explanations. Diversification was not an instantaneous origin of animals.
- Future directions for understanding the coevolution of life and oxygen
Notes on Future directions for understanding the coevolution of life and oxygen
Surveys expert uncertainty about the timing and causal relationship between oxygen and life. It does not establish a consensus cause or a new oxygen reconstruction.
- The dawn of the Phanerozoic: A transitional fauna from the late Ediacaran of Southwest China
Notes on The dawn of the Phanerozoic: A transitional fauna from the late Ediacaran of Southwest China
Describes late-Ediacaran Jiangchuan body and trace fossils, including worm-shaped animals. These constrain earlier diversity without settling the causes of the broader radiation.
- Sediment disturbance by Ediacaran bulldozers and the roots of the Cambrian explosion
Notes on Sediment disturbance by Ediacaran bulldozers and the roots of the Cambrian explosion
Documents sediment displacement in late-Ediacaran Namibia trace fossils. Proposed effects on seafloor communities remain bounded hypotheses, not a demonstrated worldwide trigger.