The early evolution of animals

The early evolution of animals 

Livets tidiga utveckling och uppblomstring 

Graham Budd 

Graham.Budd@pal.uu.se

Animal evolution 

INTENDED LEARNING OUTCOMES: 

• Describe the key factors involved in causing 

the early evolution of animal life. 

• Distinguish between the various types of 

evidence for the oldest animals. 

• Explain the stem and crown group concept 

and apply it to enigmatic fossils in the 

Cambrian Explosion.

The origin and evolution of animals 

What are the animals? 

1. Classification and relationships between 

animal groups 

2. Invertebrate body plans 

What are the earliest animals? 

3. Evidence for the earliest metazoans 

What is the timing of early animal 

evolution? 

4. Precambrian-Cambrian animals 

5. Cambrian Explosion 

6. Ordovician Radiation

The tree of life

Metazoa 

“Porifera” 

Cnidarians 

Ecydosozoa 

Deuterostomes 

Bilaterians 

Lophotrochozoa 

Protostomes

Animal classification 

• Animalia– all animals, with 35 separate 

phyla 

• Eumetazoa – all animals except sponges 

• Radiata – Ctenophores and corals 

• Bilateria – All animals except sponges, 

corals and ctenophores 

– Divided into Deuterostomes and 

Protostomes 

– Protostomes divided into Lophotrochozoa 

(or spiralia) and Ecdysozoa

Animalia 

• Eukaryotic and multicellular 

• Heterotrophic 

• Lack cell walls 

• Usually motile at some stage 

• Embryos pass through a blastula stage

Animal body plans 

• Despite a seemingly high 

diversity of form, there are 

only a few basic types of 

animal body plans 

• Defined by: 

– number and types of enveloping 

tissue 

– Presence or absence of a 

celom 

• Evolved from a flagellated 

eukaryote, similar to the 

choanoflagellates

Sponge with choanocytes 

Choanoflagellates


• Parazoan – sponges. 2 layers of cells separated 

by jelly-like material, punctuated by wandering 

cells. No tissues or organs. 

• Diploblastic – cnidarians and ctenophores. Two 

layers = outer ectoderm and inner endoderm + 

epithelia. Separated by gelatinous mesogloea. 

• Triploblastic – bilaterians. 3 layers = outer 

ectoderm, mesoderm and inner endoderm. Also 

has bilateral symmetry and a celom (body cavity).


Group Grade Symmetry Key character Larvae 

Porifera Parazoan Bilateral 

and radial 

Choanocytes 

Blastual 

larva 

Cnidaria 

Bilateria 

Diploblasti 

c 

Triploblasti 

c 

Radial Cnidoblasts Planula 

larva 

Bilateral Digestive tract Various 

types

Metazoa 

“Porifera” 

Cnidarians 

Deuterostomes 

Ecydosozoa 

Bilaterians 

Lophotrochozoa 

Protostomes

Bilaterians 

• Divided into Protostomes and 

Deuterostomes 

• Protostomes – mouth develops directly 

from first opening in blastula (blastopore). 

Spiral cleavage. 

• Deuterostomes – mouth develops from 

second opening with blastospore giving 

rise to the anus. Radial cleavage.

Bilaterians 

• Protostomes 

– Mollusks 

– Lophophorates 

– Annelids 

– Arthropods 

• Deuterostomes 

– Echinoderms 

– Hemichordates 

– Chordates

Stem and Crown groups 

• crown group is the smallest monophyletic 

group, or “clade” to contain the last common 

ancester of all extant members, and all of that 

ancestor’s descendents 

• Stem groups fall close to but outside a 

particular crown group. E.g. Archaeopteryx

Stem and crown groups

Stems and crowns


SUMMARY 

• Animalia includes sponges plus Eumetazoa 

• Radiata are ctenophores and corals 

• Most animal phyla fall into Bilateria 

subdivided into Deuterostomes and 

Protostomes 

• Animal body plans vary in number of tissues 

layers and presence/absence of celom – 

Parazoan, Diploblastic, Triploblastic 

• Fossils should be considered using the stem 

and crown group concept

The origin and evolution of animals 



animal groups 









The earliest metazoans 

• Life began ≈ 4 billion years ago 

• Metazoans evolved ≈ 600 myr 

(molecular data) 

• Timing of the origin of 

metazoans is controversial 

• The “Cambrian explosion” 

at 530 mya was a seemingly 

rapid appearance of animals

The old camps 

•Animals evolve early, were small and softbodied, 

planktonic or meiofaunal until the 

Cambrian explosion: the Cambrian explosion 

is an explosion of fossils, not animals per se. 

•Animals evolve late, and their early evolution 

can be traced by studying the fossils around 

the Precambrian/Cambrian boundary.

The Cambrian explosion 

• Explosion of 

fossils 

OR 

• An explosion of 

animals??

The earliest metazoans 

• 5 lines of evidence for earliest 

metazoans 

– Body fossils 

– Trace fossils 

– Fossil embryos 

– Molecular clock 

– Biomarkers

Decline in the Stromatolites?

Metazoan evidence 1: Body Fossils 

Ediacaran Biota 

• The first assemblage of complex, multicellular 

organisms 

• Existed before the Cambrian Explosion, around 580 

million years ago 

• Affinities enigmatic and 

highly uncertain 

• Most disappeared 

before the Cambrian 

explosion 

• Fossils in Australia, 

Canada, Namibia, 

Russia…

Metazoan evidence 1: Body Fossils 

• Can be roughly divided into two groups: 

– Radial 

– Bilateral

The controversy 

• Older views (e.g. Glaessner, Dawn of 

Animal Life in the early 80s) viewed the 

Ediacaran biota as simply old versions of 

modern animals, and thus assigned them 

to sea pens, arthropods, annelids, jellyfish 

etc 

• But Seilacher and others from the mid-80s 

onwards questioned this view - argued 

lack of symmetry, preservation, 

construction (“Pneu” etc). 

• This opened a controversy still ongoing!

Ctenophores?? 

CAMBRIAN 

Stromatoveris 

Shu et al. 

2006, 

Science 312 

Chen, Jun-Yuan et 

al. (2007) Proc. 

Natl. Acad. Sci. 

USA 104, 6289- 

6292 

Rangea 

Dickinsonia 

Zhang & Reitner 2006, 

Acta. Geol. Sin. 80 

Narbonne 2004, Science 305 

EDIACARAN (Dzik 2002, J. Morph. 252)

Molluscs?? 

Conway Morris & Caron 2007, Science 315 

Caron et al. 2006, Nature 442 

Odontogriphus (Cambrian) 

Kimberella (Ediacaran)

Metazoan evidence 1: Body 

Fossils 

• There are some possible metazoans in the 

Ediacaran, but hard to say conclusively 

• Taphonomy is a problem here – must treat 

all conclusions with caution!

Metazoan evidence 2: Trace 

Fossils 

• Trace fossils are hard to convincingly 

identify at metazoan 

• Locomotion and digestive tracts would be 

a good indication – burrows and trails with 

fecal pellets 

• No convincing trace fossils from older than 

555 Ma (White Sea area)

Oldest metazoan trace fossils? 

1 Ga, India 1.2 Ga, Australia 

Probably not!

Metazoan evidence 2: Trace Fossils 

Locomotory traces, 550 

Ma 

• Fecal strings, 600 Ma (Brasier 

and McIlroy, 1998) – first 

evidence of a gut? BUT fossils 

are questionable due to 

taphonomy 

• Trace fossils increase in 

complexity, 3-D exploitation, and 

diversity immediately before 

Cambrian 

•But – are they metazoan? 

Droser et al. 2002. Newfoundland 

Brasier and McIlroy, 1998. 

Scotland

Metazoan evidence 3: Fossil 

Embryos 

• Doushantou Formation in South China 

• 630 Ma to 580 Ma (but dates uncertain) 

• Marine, microscopic fossils 

• Sponges and corals at least, maybe 

bilaterians?

Yin et al. 2007 Nature 446

Doushantuo sponges? 

Li et al. 1998, Science 279


Embryos 

•Taphonomy complex and not all claimed fossils 

are what they seem! 

Vernanimalcula 

Not a bilaterian? 

Chen et al. 

2004, Science 

305 

Bengtson & 

Budd 2004, 

Science 306

Acritarchs as animal 

embryos? 

Shuhai Xiao 

Yin et al. 2007 Nature 446


Embryos 

• Doushantou embryos – cell divisions and 

cleavage obvious but… 

– Difficult to assign to distinct metazoan groups 

without juvenile or adult forms 

– Lack of clusters of 1000 cells 

– Could instead be stem-group metazoans 

– Or even fungi or rangeomorphs

Metazoan evidence 4: Molecular 

• Molecular clock 

Data 

– Genes accumulate changes with time 

– Can extrapolate back to see when lineages 

diverged 

– BUT the rates is not constant! 

– Bilaterians calculated to diverge anywhere 

between 900 and 570 Ma! 

– More recent values tend to value a bilaterian 

divergence of 570 Ma, closely matching the 

fossil record (sponges recently argued to be 

earlier - raises taphonomic “issues”)

Bilaterian origins

Time scale of animal 

divergences

Metazoan evidence 5: 

Biomarkers 

• Biochemical fingerprints of life 

• Amino acids, hopanes, hydrocarbons, 

isotopic fractionation of carbon, biofilms 

• Love et al. 2008 found evidence of fossil 

steroids from demosponges in the 

Cryogenian period

Love et al. 2008 

Sponges at 635 Ma

Metazoan evidence 5: Biomarkers 

• Love et al. 2008 found evidence of fossil 

steroids from demosponges in the 

Cryogenian period 

• But sponges are paraphyletic, which 

means that this type of sponge might not 

be a eumetazoan


SUMMARY 

• The earliest records of animals are highly 

controversial 

– Bilaterian origins particularly hard to decipher 

• Ediacaran body fossils at best represent 

stem-groups 

• Trace fossils present but 

bilaterian/metazoan affinity unclear 

• Best evidence comes from Doushantou 

embryos - suggest animals (stem?) 

around 630 Ma 

• Biomarkers show at least sponges at 635 

Ma

The origin and evolution of 

animals 



animal groups 









Precambrian fossils 

• Already discussed…. 

– Ediacaran fossils 

– Doushantou embryos 

– Trace fossils 

• Small shelly fossils! 

Cloudina

Namacalathus

Cambrian 

Precambrian

Small Shelly Fossils 

• Precambrian-Cambrian transition 

• Classic in lower Cambrian of Siberia 

(Tommotian) 

• Assemblages dominated by tiny 

specimens 

• First major appearance of skeletal 

material, even older than trilobites 

• Affinities remain enigmatic in many cases 

• But can be more mundane – sponges, 

molluscs, brachiopods

Small Shelly fossils

Cloudina and Namacalathus 

from the youngest Ediacaran

Small shelly fossils 

Eccentrotheca 

Micrina 

(Skovsted et al. 2008 Geology) 

Williams & Holmer 

2002, 

Palaeontology 

Tommotiids start to find a 

lophotrochozoan home

Small shelly fossils 

Halkieria: from stemgroup 

brachiopod to 

new class of mollusc

Latouchella – First unequivocal 

bilaterian





animal groups 









Setting the stage for the Cambrian 

Explosion 

• Environmental explanations 

– Glaciations 

– Increased oxygen 

• Ecological explanations 

– Arms race 

– End Ediacaran extinction 

– Evolution of eyes 

– Increased diversity/abundance of planktonics

What caused the Cambrian 

explosion? 

•Animals evolved on land, did not reach oceans until the 

Cambrian? 

•Animals adopted a “sluggish” lifestyle necessitating 

evolution of hard parts? 

•Not enough calcium in Precambrian seas? 

•Precambrian animals were planktonic and would be too 

heavy with hard parts? 

•Precambrian ocean too acidic? 

•Precambrian rocks deposited on land or destroyed by 

metamorphism? 

•Temperature drop? Or rise? Or staying the same? 

•Evolution of developmental genes? Or microRNAs? 

•Rise in oxygen levels? 

Etc etc

Cambrian 

• Break up of Pangaea I (”Rodinia”) 

• Most continents isolated 

• No continents in polar regions

Glaciations

The Oxygen Story 

•Commonly held that the rise of 

the animals was impeded by 

low oxygen levels in the 

Proterozoic - indeed, this is by 

far the most commonly 

accepted explanation…only 

until various sinks were 

exhausted could the oxygen 

produced by photosynthesis 

start building up in the 

atmosphere.

Possible importance of 

oxygen 

1. Biosynthetic - especially collagen relies on free O 2 

(via hydroxylation of proline and lysine) 

Hydroxylysine 

4-Hydroxyproline

Presumed prolyl 4-hydroxylases are widely 

(but not universally) distributed in the 

eukaryotes 

West et al. 2004, BBA 1673

Keeling et al 2005, TREE 20

Oxygen physiology 

•What levels of oxygen preclude animals 

on physiological grounds? 

•What levels of oxygen pertained in the 

Proterozoic?

Oxygen physiology: aerobic vs anaerobic 

respiration 

The 3 Main Steps of Aerobic Respiration: 

1) Glycolysis- Makes 2 net ATP (in cytoplasm) 

2) Krebs Cycle- 2 ATP (in mitochondria) 

3) Electron Transport- 32 ATP (in mitochondria) 

•The last, electron transport stage is clearly an oxygen-adaptive add-on. 

•Krebs cycle too stops without oxygen owing to product build-up (Le Chatelier’s Principle) 

Anaerobic respiration thus produces much less ATP than aerobic respiration 

Implication is that no “free” energy is available to build complex food chains under 

anaerobic conditions (Catling et al. 2005, Astrobiology)

Anaerobic respiration is not all 

it seems… 

•But in fact, simple glycolysis is not the only type of 

anaerobic respiration.

Tielens et al. 2002, Trends in Biochemical Sciences 27 

Anaerobic and 

aerobic respiration 

in a metazoan 

mitochondrion

Alternative anaerobic 

pathways 

•Fumarate replaces oxygen as final electron acceptor 

•Anaerobic alternatives can increase yield of ATP 

from 2 to between 4 and 6, and up to 8

Not all animals are aerobic 

sprinters! 

Pörtner 2002, Comp. 

Biochem. Phys. A 133

What were Proterozoic oxygen 

levels? 

•Very hard to tell exactly! 

•Most common (but not universal) to see 

oxygenation of the atmosphere as taking 

place in two steps: one about 2.4-2.2 billion 

years ago, the other just before the beginning 

of the Cambrian

What controls O2 levels? 

1. Oxidation of organic matter: a huge O 2 gobbler! 

2. Oxidation of sulphides to sulphates 

3. Photosynthesis 

These processes can be theoretically studied by 

looking at isotopic levels in inorganic marine 

rocks (both sulphide and organic carbon 

production favour light isotopes); but relating the 

values to absolute values in the ocean is not 

straightforward…

Sulphur processes 

Reduction: 

SO 4 – 

2 => H 2 S 

Disproportionation: 

S 0 /SO 2–3 /S 2 O 3 => SO 4– 2 + H 2 S 

Burial rates also important (as with carbon)

Sulfur isotopes in the 

Proterozoic 

Bottrell and Newton 2006, ESR 75

Catling et al. 2005, 

Astrobiology 5

Bursting the oxygen bubble? 

•Animals are well able to 

generate reasonable 

amounts of energy 

under low oxygen 

conditions 

•And in any case, 

Proterozoic oxygen 

levels may well have 

been high enough to 

support complex 

ecologies

Setting the stage for the Cambrian 

Explosion 

• Environmental explanations 

– Glaciations 

– Increased oxygen 

• Ecological explanations 

– Arms race 

– End Ediacaran extinction 

– Evolution of eyes 

– Increased diversity/abundance of planktonics

• Biomineralization 

(crystalline or 

amorphous) 

• Total of c. 60 minerals – 

calcareous compounds 

most common (calcium 

carbonate) & phosphate 

Hard parts

Hard parts – shells and skeletons 

• Calcium 

carbonate 

• Calcium 

phosphate 

• Silica 

• Organic

Hard parts 

• Different types: 

1. Internal support 

2. Teeth etc 

3. External 

skeletons 

4. Dermal sclerites

Why Hard parts? 

• Arms race of predator against prey

End-Ediacaran Extinction 

• Mass extinctions of 

Ediacaran fauna at 

end of this period 

• Followed by 

increased burrowing 

in early Cambrian 

• There is a gap before 

the Cambrian 

explosion occurs 

filling the niche

Evolution of eyes 

• Some suggest that 

predator prey 

relationships changed 

dramatically with 

evolution of eyes

Increased planktonics 

• Plankton are larger in 

the Cambrian than in 

the Ediacaran, and so 

are their corpses and 

feces 

• They were falling to 

the seafloor and 

accumulating 

• New supply of 

energy and nutrients

The Cambrian Explosion

Cambrian fossil lagerstätten 

• Over 40 sites 

around the world 

• Soft part 

preservation 

• Important windows 

into early ecology 

• Closed by the 

Cambrian substrate 

revolution

Cambrian Trilobites 

• Starting in the Lower 

Cambrian, 540 Mya 

• Very diverse and 

abundant group 

• Belong to Arthropoda 

• Exact affinities unsure

Burgess Shale 

• Rocky Mountains, 

British Columbia, 

Canada 

• Middle Cambrian 

• Celebrating it’s 100 

year anniversary of 

discovery in 2009!

Photo courtesy of J-B. Caron 

Burgess Shale

Maotinashian Shale 

• “Chengjiang 

fauna” 

• Yunnan province, 

China 

• 10 million years 

older than 

Burgess 

• Almost 200 

species 

• Many arthropods

Chengjiang…

Sirus Passet 

• Greenland 

• Probably 10-15 years 

older than Burgess 

• Arthropods, sponges, 

other unique fossils

Orsten Fauna 

• Upper Cambrian 

fauna 

• Kinnekulle, 

Sweden 

• 3D preservation 

of outer cuticles 

• Only microscopic 

animals

The Cambrian explosion 

• Explosion of 

fossils 

OR 

• An explosion of 

animals??

Enigmatic forms 

Laggania 

• Many forms from the 

Cambrian Explosion 

have “strange” 

morphologies 

• Not immediately 

obvious which phylum 

they blong to 

Opabinia

Cambrian Explosion ”weird 

wonders”

Modes of the Cambrian 

Explosion

Stems and crowns

Anomalocaridids 

Laggania 

Collins, 1996 

Anomalocaris

Anomalocaridids 

• One of the largest 

Cambrian animals 

• Characterized by a 

pair of frontal 

appendages, a 

“Peytoia”, and large 

eyes 

• Found in the 

Burgess Shale, 

Chengjiang, Sirius 

Passet, USA and 

Australia 

Collins, 1996

Arthropod evolution and anomalocaridids 

• The stem group of 

the Euarthropoda 

has been highly 

controversial 

• Anomalocaridids 

may be stem group 

euarthropods, 

although their 

position is widely 

debated

Arthropod evolution and Hurdia





animal groups 









After the Cambrian Explosion

Ordovician 

• New diversification after extinction in Late 

Cambrian: “specialists” 

• Diversification of vertebrates!

Ordovicium 

• Sea level extremely high – ”Water world” 

• Most continents isolated close to equator 

• Glaciation in Sahara 

• Sea level drop in Late Ordovician 

• Ordovician radiation of animals 

– 3-4 x increase in number of families 

– Few new higher taxa (e.g. phyla) but 

enormous increase in families, genera and 

species

Cambrian 

Ordovician


evolution? SUMMARY 

• Small shelly fossils and Cambrian 

Trilobites record life at Precambrian- 

Cambrian transition 

• Cambrian explosion recorded by Fossil 

Lagerstätten 

• Causes of Cambrian explosion could be 

ecological or environmental or combo? 

• Stem/crown groups let us understand 

enigmatic forms in Cambrian 

• Ordovician radiation saw huge diversity 

increase in species/genera level, but not 

many new phyla

The early evolution of animals

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