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
- 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
- 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
- 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.