W3 LECTURE 7: The Cambrian explosion

Multicellularity

Many different cell types

Colonial

Many cells (one cell type)

What forces drive evolution of multicellularity

1. Evolution of colonial form (benefit of size)

2. Evolution of division of labour (Genetic similarity of a ball of cells, cooperation, evolution of interdependent cells)

Evolution of colonial form

• Cells divide but they don't separate

• Cells stick together

• Benefit: Cells are then freed from predation

Division of labour

• Specialisation leads to increased efficiency

• Each cell has a specific job

Specialisation in nature (Volvox)

• 2 different cell types: reproduction, fast division

• Different cells with different roles help with fast reproduction

Proterozoic

Former earlier life (pre-cambrian)

Phanerozoic

Visible life (post-cambrian)

Cambrian explosion

• Diversification of multicellular animal life in the oceans

• High diversity and animals not seen today

Main animal taxa phylogeny

Porifera

• Sponges

• Asymmetrical

• Tissue absent

• 3/4 cell types

Placozoa

• Blob like aggregation of cells

• 6 cell types - no tissue

• Undifferentiated

• Ciliary movement, engulf food

• Divide by fission, fragmentation

Cnidaria

• E.g. jellyfish

• Radial symmetry

• Tissue

• 2 cell layers - diploblast

• No gut no head

• O2 via diffusion , no respiratory system

• Asexual reproduction

• Predatory via stinging

• Decentralised nervous system (no brain)

Bilateria

• Bilateral symmetry

• Triploblastic - endoderm (inside), mesoderm(middle), ectoderm(outside)

• Gut - flow through feeding/digestion (mouth to gut)

• Development of head (sensory and mechanical functions of feeding)

Stages of multicellularity in animals

Protostome and Deuterostome split

• Proto - Forms mouth first

• Deutero - Forms anus first

Lophotrochozoa

• Protostome development

• Spiral cleavage

• Larval stage

• E.g. Molluscs, Cephalopods, Platyhelminths, Annelids

Molluscs

• Diverse: marine ,fresh water, land

• Sedentary, mobile

• Mostly muscle

• High complexity cognition

• Shells

Cephalopods

• Octopus, squid

• High complex cognition

  • Largest brain size for a body of an invertebrate

Platyhelminths

• Flatworms

• Disease causing in mammals - parasites e.g. Tapeworm

• No body cavity (no gut)

• Digestive cavity (mouth = anus)

Annelids

• Segmented worms

• Marine, fresh water, land

• Collagen cuticle

• Important in soil

Ecdysozoa

• Exoskeleton

• Growing and Moulting

• Segmented animal

• Many legs, paired appendages

• May adopt head

Arthropods

• Chelicerata: arachnids, mites

• Myriapods: millipedes and centipedes

• PanCrustacea: Crustacea, Ostracods and Copepods, Insecta/Hexapoda

Nematodes and Nematomorphs

Nematodes:

• Unsegmented

• ‘Roundworms’ like C. Elegans

• Bacteriovores

• Micro predators

• Detritivores: important in soil and in marine sediments

• Plant and animal Parasites

• Pests and disease causing

• Biocontrol agent (e.g. Control of soil stage of insects)

Nematomorphs: Horse Hair worms - parasites

Tardigrades

• Waterbears

• Segmented

• Marine and fresh water

• Live on algae

• Environmentally resistant

Other ecdysozians

• Priapulids:

  • Penis worm

  • Marine

  • 22 species now but hyperdiverse in the Cambrian -carnivores, detritivores, filter feeders

• Onychophorans:

  • Velvet worms

  • Segmented but no exoskeleton

  • Nocturnal

  • Ambush predators in terrestrial humid tropics

Echinoderms

• Invertebrates

• Marine aquatic

• Bilateral symmetry

• Mesodermal skeleton

• No CNS

• Water based coelom circulation

• Regenerative

Hemichordates

• Invertebrates

• Sister to Echinoderms

• Tripartite body

• Some shared characteristics with chordates

• Branched gill slits

• Stomochord 0 rod that runs down body providing support

• Dorsal nerve cord

Chordates

• Notochord - Rod down the back

• Dorsal nerve chord (spinal chord)

• Gill slits - Pharyngeal

• Post anal tail

• Vertebrates are derived Chordates

  • Dorsal nerve chord => vertebral column

Cephalochordates

Lancelets, marine

Tunicates

• Sea squirts

• Marine

• Filter feeders

• Colonial adults

Vertebrates

• Agnatha (jawless fish)

• Hagfish

• Lampreys

Gnathostomata

Jawed vertebrates

Deep animal diversity defined by

• Number of generative layers – one, two (diploblasty), three (triploblasty)

• Early development – blastopore fate (deuterostome vs protostome)

• Within protostome, Ecdysis presence (Ecdysozoa vs Lophotrochozoa)

What promoted the cambrian explosion

• Global temperature changes

• Global oxygen changes

Global temperature change

• Snowball earth – ended shortly before Ediacarian

• Period of extreme cold with high glaciation

- Glacial deposits in near tropical areas

- Low photosynthetic accumulation

• Reflectance of light/heat reinforce initial conditions -> long period

• Ended by volcanic activity/CO2 input => warming?

Higher oxygen enables multicellular animal function

• Physiologically: Diffusion cannot sustain aerobic processes at large size and low o2

• Low o2 limits size of aerobic molecules

Higher oxygen enables more trophic levels

Pyramid of biomass - loss of energy stored through trophic levels

Hox genes code transcription factors

• Define position

• Alter expression of other genes

• >development of complex form

• Individual Hox genes are found widely in eukaryotes

• Animals have an array of them – and number of copies and organismal complexity linked

• Enabled bilaterian complexity