Cambrian Explosion

The Best Bits from Common Descent Podcast, episode 9

What and When

  • One of most important evolutionary events in life’s history.
  • At the beginning of Cambrian period (546 - 480 million years ago)
  • Right after Ediacaran period.
  • Starts the Palaeozoic era.

The Origin of Life?

  • Every recognisable body plan (phylum) appeared during this 20 million year explosion
  • 20m years is really short evolutionarily.
  • In Darwin’s time, looked like first appearance of life on earth at the very bottom of the geological record
  • We now know it was not that sudden, not the first life, and not the bottom of geological record.

How, Where From, Why

  • Bacteria around for 3bn years before this
  • Many different species and body plans, seemingly out of nowhere.
  • Not sure how or why this diversity was sparked
  • Not a lot of fossils before this. More recently we’ve found some, almost all single-cellular.

Life in the Ediacaran

  • Stromatolites - pillow-like structures built by micro-organisms
  • Microbial mats on sea floor - base of ecosystem, grazed for a long, long time
  • First signs of complex life - trails left by earthworm-like animals
  • Ediacarian biota was multicellular, weird, up to 2metres long, some look like early versions of modern body plans (sponges, jellyfish)

Explosion of Life

  • Ediacaran biota goes extinct, replaced by “small shelly fossils” (2 microns), calcified animal pieces, mouthparts, body sections
  • All recognisable modern phyla show up in fossil record in 20m years.
  • Trilobites appeared and dominated for 300m years.


  • Cambrian Explosion shaped way we now see the fossil record, evolution and ecosystems.
  • Set in place the diversity we see today
  • Arthropods, worms, chordates (us), molluscs, brachiapods, trilobites all first appeared then
  • See ecological niches being filled (eg burrowers, predators) and ecosystems developing.

Why Did it Happen?

  • 3 main possibilities:-
  • Environmental, Genetic, Ecological
  • But, no one cause explains all the known facts

Environmental Causes

  • Snowball earth. Last major glaciation was 90m years before Cambrian. Life cooped up, waiting to diversify?
  • Increase in oxygen levels allowing more metabolism, bigger bodies, more diversity
  • Carbon isotope mass extinction (large –ve C isotope anomaly in Cambrian). Possible boom in diversity after Ediacaran biota wiped out
  • Polar wander – rapid continental movements released huge burps of methane (CH4 - greenhouse gas), increasing temperatures and/or causing isotope anomaly

Genetic Causes

  • Did DNA hit a milestone, allowing natural selection to apply pressure in a new way?
  • Hox genes (bundles that code for body part plan) are shared by distantly-related modern animals (eg fruit flies, mice, humans)
  • Animals evolving in new ways because all sharing a body plan

Ecological Causes

  • Predation takes off in Cambrian
  • First appearance of eyes – can see and chase prey. Trilobites first to develop complex eyes
  • Arms race between predators and prey. Development of shells, defensive body parts
  • Many new niches ripe for colonisation

So, What Was the Cause?

  • We can’t distinguish between so many events happening so close together in geological time.
  • Resolution not good enough to distinguish between events taking place within 100,000 years of each other.
  • May not have been an explosion, just that we came in midway through the film.

Further information