Lecture 8 - Cambrian Explosion

plants are especially polyploidy

Cambrian Explosion: introduction of modern life relative to the Ediacaran

Ediacaran

  • Multicellular

  • 95 million years

  • Nothing stirs soil, buildup of microbes (bacterial mats)

  • Slow moving / sessile

  • Soft bodied

  • Body area maximized for bacterial mat consumption

Nothing today looks like Ediacaran period species body forms

  • Mass extinction

  • Very few species seem to survive to Cambrian

  • Only two modern groups that are thought to be related to the Ediacaran

    • Porifera — basal lineage of all animals

    • Sessile filter feeders, larval stage = motile

    • Solely cell organization

    • Cnidaria — another plausible survivor of the Ediacaran

    • Modern species = diverse

    • primitive forms that seem to be important to survival in the Ediacaran (e.g. digestive system + eyes)

    • Colonial forms

    • Active and sessile


Unlike the Ediacaran, the Cambrian is thought to be a dramatic emegrnece of body structures, behaviors, interspecies relations, and life forms that have impacted modern day life

  • 540-486 MYA

  • During short 25 mill — body forms of all modern animals appear in fossil record

  • <2% of the total time animals have been on planet

e.g. athropods, molluscs, worms, sponges, jellyfish

Fossil record shows:

Hard bodied shells

Behavior: burrowing

Chordate: vertebrae, notochord


Burgess Shale — rich fossil site

  • Even soft tissues preserved

  • Such unique preservation > Burgess shale-type preservation

How?

unique conditions: ocean chemistry different in Cambrian era

  • Low O in atm.

  • Shallow areas devoid of O, reduced normal decomposition

  • Rapid deposition of calcium carbonate, high sedimentation

  • pH (alkaline) allow for formation of sediment layer that seals off organic matter


Physical and behavioral changes in Cambrian — best interpretation of evidence we have


Cambrian explosion — body structures

Hard mineralized structures — made form ions plucked from ocean

  • shells

  • spikes

Complex eyes


Mineralized structures

fix ions with biological molecules into brand new structure

to build shells:

  • Minerals (calcium) dissolved in ocean, removed from water by proteins made by nimal

  • proteins bind calcium (chagred ion on protein)

  • Forms polysaccharides that are very durable


Complex eyes

Vision in Ediacaran

  • photosensitive, but cant see

  • Eyespots

  • harmful high energy light rays of, eyespots used for protection

  • Adaptive reason to be able to sense life

Cambrian

  • Different eye forms

  • navigate environment

  • Natural selection — those w/out eyespots could not sense harmful energy light

  • later modifications continue to imporve vision


Animal behavior

  • Deep vertical burrows

  • Active / fast movement


Burrows

New space for multicellular life to occupy

  • Dig through microbial mat destroys them despite them being dominant ecosystem for so many years

  • Throw sediment around

    • soil turnover


Interspecies interactions

  • Predator/prey relations

Predator prey relations

  • claws - raptorial

  • fins for fast movement

  • round compound eyes


Shells spikes

  • Harder to be eaten

Fast movement

  • Outrun predators

Better vision

  • Spot predator

Burrowing

  • Hide from predator


Mass extinction

  • new species that evolve after a mass extinction behave similarly to animals that went extinct

  • Exception: Cambrian


Coral species (and many sessile species) vulnurable to changing climate

  • Reef ecosystem has existed since the cambrian

  • hasnt always been corals, different species groups have come in place as these niche ecosystems

  • corals have appeared and reappeared through the geological time record


If there is a gap in the envr (unused resource)

  • A species will eventually evolve to fill that gap

Impact of similar envr conditions

  • drive similar patterns

  • Even in species w/ different or divergent evolutionary histories

e.g. dolphins related to deer


Exception — transition from ediacaran and cambrian

  • Complete change of body form, behavior, and interspecies interactions

  • Completely lose some components of the ediacaran

    • no microbial mats


Ediacaran believed to have greatest extinction rates (greater than permian)

  • How? Multipel competing theories


Abiotic factors

  • break up of supercontinent Rodinia


Rodinia

  • late Precambrian 750 ma

  • more shalow seas around coasts

  • more muddy bottom habitats accessible to light from sun

  • dramatically increase the amount of envr for life to occupy (e.g. low tide, high tide areas)

More O available in envr

  • Greater availability of O for life

  • More O = more energy

  • More energy = life can be larger

Limitation:

  • Large animals existed in Ediacaran too / but dont seem to move as much


Ozone layer


HOX genes

  • regulate embryonic development

  • major phenotypic changes


Whole genome duplication

more genes = mpre gene products

Duplication in HOX genes

polyploidy event


Biological factors — ecological relationships

predatory behavior >

burrowing >

dig for prey >

hard sclerites >

natrual selecton, population shifts toward predator relationships

Each interacts with different behaviors, pressure on some behaviors to become more adaptive


Key innovation

evolution of ne trait that is so fundamentally beneficial

makes descendants of amimal become dominant member of ecosystem

  • e.g. plants that produce seeds

  • e.g. eyes are key innovation, once majority of organisms can perceive envr, the eyelss or slow are at a significant disadvantage competition wise