- A The Cambrian explosion is sometimes considered to hold no real evolutionary significance, despite marking one of the most dramatic bursts of animal body-plan diversification in the fossil record
- B Rapid diversification of complex animal body plans ~541 million years ago; most major animal phyla appear within a few million years, possibly driven by rising oxygen levels and ecological opportunity
- C The Cambrian explosion is sometimes thought to have occurred very recently, within the last few thousand years, when it is reliably dated to roughly five hundred forty-one million years ago
- D The Cambrian explosion is sometimes thought to have affected exclusively photosynthetic plant lineages, when it specifically refers to the rapid diversification of complex animal phyla
Correct answer: B. Rapid diversification of complex animal body plans ~541 million years ago; most major animal phyla appear within a few million years, possibly driven by rising oxygen levels and ecological opportunity
Explanation: Cambrian explosion (~541-515 mya): most major animal phyla appeared in fossil record within ~25 million years (geologically rapid). Before: Ediacaran fauna (soft-bodied, simple organisms). After: trilobites, echinoderms, molluscs, chordates, arthropods. Causes debated: (1) Oxygen levels reached threshold for active predation; (2) Predator-prey arms race drove diversification; (3) Development of Hox genes enabling body plan innovation; (4) Environmental changes. Burgess Shale (Canada) and Chengjiang (China) yield extraordinary fossils.
Concept context
Origin of life, Darwin's theory, natural selection, speciation, and evidence for evolution.