Q.97 Among the following options, choose the one that is probably a cause of rapid diversification of
animal groups during the Cambrian explosion.
(A) Adaptation of organisms to live in the salty environment of ocean
(B) Emergence of coelom
(C) The movement of animals to land
(D) The accumulation of sufficient atmospheric oxygen to support the metabolism of actively
moving animals
Correct Answer: (D) The accumulation of sufficient atmospheric oxygen to support the metabolism of actively moving animals
Oxygen levels rose to ~10-20% during the Cambrian (~541-530 MYA), enabling aerobic metabolism for larger, active predators with high-energy demands—triggering ecological arms races, predation, and body plan diversification.
Option Analysis
Option (A)
Incorrect. Oceans were already salty; Ediacaran biota (~575 MYA) thrived there. Salinity didn’t change dramatically to drive Cambrian explosion.
Option (B)
Incorrect. Coelom (body cavity) aided some bilaterians but existed pre-Cambrian (Priapulids); not primary diversification driver across phyla.
Option (C)
Incorrect. Land colonization occurred ~400 MYA (Silurian/Devonian); Cambrian animals were marine (trilobites, Burgess Shale fauna).
Option (D)
Correct. O2 threshold enabled active motility, predation, biomineralization; correlates with small shelly fossils → trilobite radiation.
Introduction to Cambrian Explosion
The Cambrian explosion (~541-530 MYA) saw most animal phyla appear in ~20 million years. GATE Q.97 identifies oxygen accumulation (D) as key enabler for active, predatory lifestyles—sparking ecological complexity vs. pre-Cambrian simplicity.
Oxygen as Cambrian Explosion Trigger
Pre-Cambrian O2 ~1-10%; Cambrian rise to 10-20% enabled:
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Larger body sizes (trilobites to 70cm)
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Active predation ( Anomalocaris compound eyes)
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Biomineralization (calcite shells need aerobic CaCO3 secretion)
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Burrowing/motility (trace fossils complexity ↑1000x)
Geochemical evidence: Mo isotopes show bottom-water oxygenation.
Why Other Options Fail
| Option | Timeline Issue | Actual Role |
|---|---|---|
| A Salty oceans | Always salty | Ediacaran already marine |
| B Coelom emergence | Pre-Cambrian (Priapulida) | Bilaterian innovation, not trigger |
| C Land movement | 400 MYA (Silurian) | Cambrian = marine only |
Cambrian Explosion Timeline
~575 MYA: Ediacaran soft-bodied
~541 MYA: Small shelly fossils (O2↑)
~530 MYA: Trilobites + major phyla (Burgess Shale)
~520 MYA: Peak diversity
Cambrian explosion wasn’t “instant” (~20 MY) but geologically rapid.
GATE Exam Context
Cambrian explosion questions test paleobiology/ecology (XL-E). Memorize:
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Oxygen = metabolic trigger
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Predation = ecological driver
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542 MYA = start date
Common trap: Confusing with Ordovician radiation (~470 MYA).
