Notes on Animal Life, Classification, Evolution and the Cambrian Explosion

Addenda and Course Updates

• Slides posted to Bb morning before lecture

• Undergrad TA office hours to begin next week

• Syllabus is now a Google Doc so changes to plan will be added

Assignment for Wednesday

• Hickman Chapters 5 and 6

• Review of Genetics and Evolutionary Biology

• I won’t cover all of this in class on Wednesday, but I will expect you to know the basics for exams

• Wednesday: Quick genetics review to Darwin through Modern Synthesis

• Friday: AIC reading

Reading Tips: Key Terms from Ch. 5

• Chromosomes

• Genes

• Alleles

• Germ cells

• Meiosis / Mitosis

• Haploid / Diploid

• DNA / RNA

• Transcription / Translation

• Recombination

• Genotype / Phenotype

• Dominant / Recessive

• Hetero / Homozygous

• Pleiotropy

• Linkage / crossing over

• Codon / Intron / Exon

• Promoter / Transcription Factor

• Structural / Regulatory genes

Roadmap for Today

• Favorite animal results

• What is an animal?

• Visitors

• Animal classification basics

• Origins of animal life

Favorite Animals (List from Page 5)

• dogs

• cat

• giraffe

• sea turtle

• red panda

• dolphin

• turtle

• shark

• sea otter

• otter

• hedgehog

• elephant

• bear

• okapi

• octopus

• jellyfish

• horses

• zebra

• thorny devils

• snow leopand

• snake

• snak

• sheep

• seal

• seahorse

• salamander

• rhino

• raven

• penguin

• owl

• orca

• nudibranch

• monkey

• mantis shrimp

• manta ray

• lion

• king cobra

• highland cow

• grey wolf

• giant isopod

• gecko

• gar

• frog

• fox

• emperor penguin

• cuttlefish

• cobra

• cheetah

• bumblebees

• brittlestar

• bonobo

• bird

• bengal tiger

• beavers

• basilisk

• barn owl

• axolti

• arctic fox

• ants

• alligator

• african grey parrot

• Japanese spider crab

• Favorite animal

• 8

• 6

• 4

• 2

• 0

Favorite Animal Distribution by Phyla

• Chordata: $89.8\%$

• Arthropoda: $4.6\%$

• Mollusca: $2.8\%$

• Echinodermata: $0.9\%$

• Cnidaria: $1.9\%$

Research Focus of WM Bio Faculty by Phylum (Distribution for ’24)

• Echinodermata

• Arthropoda

• Nematoda

• Chordata

• [Note: slide text shows: “WM Bio Faculty Research Animal Distribution ‘24” followed by a list including echinodermata, arthropoda, nematoda, chordata]

Actual Animal Species Distribution

• So….we should hire:

  • One Echinoderm biologist (check!)

  • One Mollusc biologist

  • One Chordate biologist (pick your favorite)

  • Everyone else should study Arthropods (and especially insects!)

What is an Animal?

• Characteristics of animals:

  • Multicellular eukaryotes

  • Heterotrophic by ingestion

  • Most reproduce sexually

  • Have developmental stages in offspring

  • Bodies organized into tissues and organs

  • Complex cell–cell communication

Features of the Animal Kingdom

• Multicellularity – many have complex bodies

• Most have complex tissue structure

• Heterotrophy – obtain energy by ingesting other organisms

• Active movement – move rapidly and in complex ways

• Diversity of form and size – microscopic to enormous

Complex Tissue Structure

• Lack cell walls

• Unique intracellular communication (gap junctions)

• Connective tissues – cells embedded in an extracellular matrix (e.g., bone, cartilage)

• Epithelial tissues – cover, line, protect and secrete

• Nervous tissue – coordinate movement

• Muscle tissue – power locomotion

Features of the Animal Kingdom (continued)

• Most exhibit sexual reproduction (with notable exceptions)

• Offspring pass through developmental stages

• Determined/fixed body plan – morphology determined by developmental cues (HOX genes)

Poll Question (Interactive)

• Which of the organisms is most closely related to you?

  • A. Anemone

  • B. Snail

  • C. Starfish

• [Live poll content appears in slides; answer based on shared deuterostome lineage with Echinodermata vs Chordata]

Why would we place the starfish closest to ourselves? (Cladistic reasoning)

• Metazoa → Eumetazoa → Bilateria → Protostomia / Deuterostomia

• Deuterostomia includes Chordata (humans) and Echinodermata (starfish)

• Ecdysozoa (e.g., Nematoda, Arthropoda) are another major clade

• Starfish (Echinodermata) share a more recent common ancestor with us than some other listed groups due to Deuterostome affinity

Classification: Linnaean System

• Carl Linnaeus (1707–1778) – “Father of taxonomy”

• Binomial nomenclature: Genus species (e.g., Homo sapiens)

• Useful for organizing biodiversity

• Doesn’t always capture evolutionary relationships

• Limitations: Based on morphology, not evolutionary history; boundaries debatable

Features Used to Classify Animals

• Classification based on morphological and developmental characteristics

• Metazoa vs Eumetazoa

• Bilateria vs Radiata vs Parazoa

• Presence of a body cavity (coelom) and symmetry

• Developmental origin of mouth and anus

More on Features Used to Classify Animals

• Symmetry

• Tissue organization

• Number of tissue layers

• Origin of mouth and anus

• Body plan and body cavities

Spoiler Alert

• [Note: a page labeled SPOILER ALERT appears; content is not described in detail here]

Animal Characterization Based on Body Symmetry

• Bilateral symmetry

  • Anterior vs Posterior; Dorsal vs Ventral

• Asymmetry

• Radial symmetry

• Orientation: Anterior, Dorsal, Ventral, Posterior roles in development

Germ Layers (Embryology)

• Diploblasts – two germ layers: ectoderm and endoderm

• Triploblasts – three germ layers: ectoderm, mesoderm, endoderm

Presence/Absence of a Coelom

• Acoelomates – lack a body cavity

• Coelomate (with a true coelom) vs Pseudocoelomates (not explicitly listed, but relevant in classification)

• Mesoderm fills the space in some groups

Birth/Development: Mouth and Anus Formation

• Fate of the blastopore in early development

• Protostomes – blastopore becomes mouth; anus forms second opening

• Deuterostomes – blastopore becomes anus; mouth forms from a second opening

Origins of Animal Life

• How old is the Earth? How old is life? How old are animals?

• Evidence for earliest animal life

Pre-Cambrian Animal Life

• Pre-Cambrian time: Ediacaran period = $635\text{ Mya} - 543\text{ Mya}$

• Ediacaran biota likely evolved from protists

• Choanoflagellates resemble choanocytes of sponges; DNA sequences are similar

First Clear Metazoan Fossil (Ediacaran)

• Liu et al. (2014) described first clearly metazoan fossil from Ediacaran fauna

• Found in Newfoundland, Canada

• Age: $560\text{ Mya}$

• Cnidarian affinities

More on 2014 Finding

• Reaffirmed interpretation: metazoan fossil with cnidarian affinities

Potential Implications if Sponges Are Confirmed Earlier

• If sponge-like fossils are older (up to $890\text{ Mya}$), earliest animal life would push back by hundreds of millions of years

• This would align with molecular clock estimates

Cambrian Explosion (Origins of Many Body Plans)

• Cambrian period: $542\text{ Mya} - 488\text{ Mya}$

• One of the most rapid periods in animal evolution; many new phyla originate

• Key representatives: Echinoderms, Mollusks, Worms, Arthropods, Chordates

• Notable early chordate-like fossil: Pikaia (early chordate/vertebrate precursor)

Drivers of the Cambrian Explosion (Debated)

• Ecological changes: predator–prey arms race could drive diversification

• Genetic changes: rapid appearance of new genetic information

• Environmental changes: rapid changes in the physical environment

• The evidence suggests a combination of factors; none alone fully explains the explosion

How Do We Get So Many ‘New’ Body Plans Quickly?

• Ecological revolution as an arms race is probably not the sole driver (too localized)

• Genetic revolution: duplication/expansion of developmental gene networks (e.g., HOX genes) can enable more diverse body plans

• Key reference: de Rosa et al. 1999

• Another idea: shifting spatial expression and duplications of HOX genes in certain lineages

Oxygen and Cambrian Explosion: A Possible Trigger

• 800 million years ago (Myr): Oceanic oxygen rose from < $0.1\%$ to perhaps $1-2\%$

• 635 Myr: A glacial epoch may have ended with a temporary oxygen spike

• 580 Myr: Large Ediacaran animals appeared

• 542 Myr: Start of the Cambrian explosion

• The oxygen concentration in Earth’s atmosphere rose sharply around $300\text{ million years ago (Mya)}$

• Typical examples in the fossil record: Dickinsonia (can reach > $1\text{ m}$), Pikaia (notochord), Marrella (arthropod), Anomalocaris (predator)

Did a Failure of Earth’s Magnetic Field Increase Oxygen Levels?

• Hypothesis notes:

  • Weakened magnetosphere could allow loss of hydrogen gas from the atmosphere/ocean

  • Relative increase in atmospheric oxygen could enable diversification of animal life

Cambrian Explosion: Likely Causes (Synthesis)

• Preceding conditions: rising O2 levels and ocean calcium levels

• Presence of shallow seas enabling ecological variation

• Changes in predator–prey relationships

• Genetic innovations (HOX regulatory genes)

• Evidence supports a combination of ecological, environmental, and genetic factors

Post-Cambrian Evolution and Mass Extinctions

• Dramatic global and regional climate change can drive mass extinctions

• Major losses of diversity occur during mass extinction events

Permian–Triassic Boundary (the Great Dying)

• About $251\text{ Mya}$ (late Permian)

• Extinction of ~$95\%$ of species

• Extinction of trilobites and major reptilian groups

• Created ecological opportunities for the radiation of dinosaurs and land plants

Cretaceous–Paleogene Boundary (K–Pg, near 66 Mya)

• ~$66\text{ Mya}$

• Dust from meteorite impact near Yucatan plus volcanic activity

• Severe climate change led to widespread plant and animal extinctions

• Opened niches for mammals, birds, and flowering plants

The Five Major Mass Extinctions (Post-Cambrian)

• There have been five mass extinction events after the Cambrian

• Some scientists argue we may be entering a sixth (likely human-related) mass extinction

• Current extinction rates are debated, with some estimates suggesting higher rates than previous MSEs over short timescales

Food for Thought

• Sixth Mass Extinction (full documentary) – YouTube video: https://www.youtube.com/watch?v=EDQV1hiLpQQ

Connections and Implications

• Foundational principles: taxonomy, biodiversity, evolutionary relationships, and the deep time perspective of life on Earth

• Real-world relevance: contemporary biodiversity loss and its drivers; human impact on extinction risk

• Ethical and practical implications: responsibility for mitigating ongoing sixth extinction; conservation priorities; understanding of ecological networks and resilience