3 The Cambrian Explosion and Multicellularity and the diversification of animals

Question 1

Multicellularity in microeukaryotes

Answer 1

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

Question 2

What forces drive evolution of Multicellularity

Answer 2

1. Evolution of colonial form - benefit of size
2. 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

Question 3

Evolution of colonial form

Answer 3

Benefits to cell being grouped

Colonialist widespread in bacteria as well as eukaryotes

Chains, biofilms

Divide don’t separate - stickiness

Question 4

Division of labour

Answer 4

specialisation leads to increased efficiency

Question 5

Volvox

Answer 5

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

Question 6

Multicellularity predates the evolution of…

Answer 6

Animals and vascular plants

Question 7

2 step process

Answer 7

Colonialist then division of labour

Question 8

Why does it appear very easy to evolve

Answer 8

Many drives and many ways

Question 9

Multicellularity and the emergence of animals - ‘Cambrian explosion’

Answer 9

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

Question 10

Cnidaria

Answer 10

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

Question 11

Bilateria

Answer 11

Bilaterally symmetrical

Triplobastic - three cell layers - endoderm, mesoderm, ectoderm)

Gut - flow through feeding / digestion

Development of head —> sensory and mechanical functions of feeding

Question 12

Stages of Multicellularity in animals

Answer 12

1. Cell adhesion and cell signalling
2. Simple Multicellularity
3. Gap junctions
4. Bulk transport

Question 13

Protostomes

Answer 13

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

Question 14

Ecdysozoa (molting)

Answer 14

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

Question 15

Deuterostomes

Answer 15

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)

Question 16

How much diversity appeared

Answer 16

Majority of bilaterian taxa appeared in the fossil record during the Cambrian Explosion

Penis worms were the most common during the Cambrian

Question 17

Did the Cambrian explosion have a short of long fuse?

Answer 17

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

Question 18

What events promoted the Cambrian explosion?

Answer 18

Global temperature change

Global oxygen change

Question 19

Cambrian explosion - global temperature change

Answer 19

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?)

Question 20

Cambrian explosion - global oxygen change

Answer 20

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

Question 21

Cambrian explosion - other hypothesis

Answer 21

vision evolution drove explosion by enabling predation

Question 22

Biological underpinning

Answer 22

Development from single cell to compex organism through cell division, death and differentiation

Orchestrated by expression of genes that give positional information

Hox genes

Question 23

Hox origins and functions

Answer 23

Hox genes code transcription factors = define position + alter other expression of genes —> development of complex form

Enables bilaterian complexity

Question 24

Summary

Answer 24

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