3 The Cambrian Explosion and Multicellularity and the diversification of animals Flashcards
Multicellularity in microeukaryotes
Multicellularity - having many cell types and more than 1 of that type of cell
Colonial - many cells, one cell type
Multicellularity diverged 200MYA —> Volvox - 12 developmental traits Eg. Germ / soma divide; embryo inversion; asymmetric division
Can work out functional basis of these and how transitions evolves
Repurposing of existing genes
Diversification of genes for cell adhesion
What forces drive evolution of Multicellularity
- Evolution of colonial form - benefit of size
- Evolution of division of labour - genetic similarity of ball of cells —> basis for cooperation / collaborate towards physiological and reproductive goals, evolution of inter-dependant cells
Evolution of colonial form
Benefits to cell being grouped
Colonialist widespread in bacteria as well as eukaryotes
Chains, biofilms
Divide don’t separate - stickiness
Division of labour
specialisation leads to increased efficiency
Volvox
Unicells and multicells must be mobile (mobile cells replicate slowly)
Division of labour in multicell - motile soma cells (large) vs non-motile germ line (small)
Greater reproductive rate for same energy input
Multicellularity predates the evolution of…
Animals and vascular plants
2 step process
Colonialist then division of labour
Why does it appear very easy to evolve
Many drives and many ways
Multicellularity and the emergence of animals - ‘Cambrian explosion’
Proterozoic - 2.5BYA —> 540MYA / defined by an absence of animal and plant fossils (pre-visable life)
Phanerzoic - 540MYA —> preset day / defined by strata with abundant fossils (visable life)
Summary - 550 MYA Cambrian Explosion:
Diversification of multicellular animal life in oceans
Baffling diversity, many creators not seen today
Cnidaria
2 cell layers - ‘diplobast’
No coelom, no gut, no head / O2 diffusion by diffusion, passive transport / decentralised reproduction / no differentiated germ line / predatory via stinging / sessile and planktonic stages
Bilateria
Bilaterally symmetrical
Triplobastic - three cell layers - endoderm, mesoderm, ectoderm)
Gut - flow through feeding / digestion
Development of head —> sensory and mechanical functions of feeding
Stages of Multicellularity in animals
- Cell adhesion and cell signalling
- Simple Multicellularity
- Gap junctions
- Bulk transport
Protostomes
Lophoyrochozoa, development with spinal cleavage)
Molluscs - diverse / mobile / high fraction of body is muscled / shelled but can secondarily lose shell / can have highly complex cognition
Cephalopods - octopus + squid / highly complex cognition - largest brain size for body invertebrate / sophisticated chambered eye
Platyhelminths - flatworms / free living / parasites / no body cavity / digestive cavity = mouth and anus are the same hole
Annelids - segmented worms / marine, freshwater, terrestrial / collagen cuticle, parapodia
Ecdysozoa (molting)
Segmented, paired appendages / may adopt head - throrax - abdomen body plan
Nematodes - unsegmented ‘roundworms’
Nematomorphs - horsehair worms - parasites
Tardigrades - segmented + highly resistant
Priapulids - 22 species now but hyperdiverse in the Cambrian
Onychophorans - velvet worms, segmented but no exoskeleton / nocturnal ambush predators in terrestrial and humid tropics
Deuterostomes
Echnioderms - marine aquatic invertebrates / no CNS (central nervous system) / regenerative / water based coelom circulation / larvae bilaterial symmetries, adults pentaradial
Hemichordates - invertebrates / tripartite body / hemichordate / bronchial gill slits / stomochord / dorsal nerve chord
Chordates - notochord (rod down back) / dorsal nerve chord / pharyngeal slits / post anal tail
Cephalochordates - lancelets / marine
Tunicates - sea squirts - marine, filter feeders / colonial adults
Vertebrates - Agnatha (jawless fish) / gnathostomata (jawed vertebrate)
How much diversity appeared
Majority of bilaterian taxa appeared in the fossil record during the Cambrian Explosion
Penis worms were the most common during the Cambrian
Did the Cambrian explosion have a short of long fuse?
Data from molecular clock - uncertainties in method but also place root of animals near base of Ediacarian
The Cambrian explosion was a rapid burst of evolution approximated 540 MYA
What events promoted the Cambrian explosion?
Global temperature change
Global oxygen change
Cambrian explosion - global temperature change
Snowball earth - ended shortly before ediacarian / period of extreme cold with high glaciation - glacial deposits in tropical areas / low photosynthetic accumulation
Refelectance of light —> long period
Ended by volcanic activity —> CO2 input (warming?)
Cambrian explosion - global oxygen change
Physiologically - diffusion cant sustain aerobic process at large size at low O2
Low O2 limited size of aerobic organisms
Higher oxygen enables more trophic levels —> pyramid of biomass - loss of stored energy through trophic levels
This pattern is true in the current ocean
Cambrian explosion - other hypothesis
vision evolution drove explosion by enabling predation
Biological underpinning
Development from single cell to compex organism through cell division, death and differentiation
Orchestrated by expression of genes that give positional information
Hox genes
Hox origins and functions
Hox genes code transcription factors = define position + alter other expression of genes —> development of complex form
Enables bilaterian complexity
Summary
Multicellularity evolved in microeukaryotes – on many occasions
Animal fossil diversity very high in the Cambrian. Most phyla emerge at this time
Precambrian animals do exist – the Cambrian explosion had a fuse
Oxygenation and warming probably lit the fuse
