Animals

Between 3 million and 10 million animals exist on our planet today:

– Some similar to you, others quite different

– Only 1.4 million species have been described to date

– Scientists are in a race to discover and describe more animal species

before more of them go extinct

• Originated from single-celled eukaryotes

– Occur in lineage called Opisthokonta

– Choanoflagellates are closest living relatives to animals:

▪ Share common ancestor 900 million years ago

Animals 2

• Animals form monophyletic clade with key traits:

– Multicellular eukaryotes, with no cell walls but with an extensive

extracellular matrix (ECM):

▪ Includes proteins specialized for cell–cell adhesion and communication

– Heterotrophs

– Move under own power at some point in life cycle

– All animals other than sponges have (1) neurons that transmit electrical

signals to other cells and (2) muscle cells that change shape of body by

contracting

Phylogeny of Major Animal Phyla Based on DNA

Sequence

Animals are a monophyletic group:

– All animals have a single common ancestor that was multicellular

– Single origin of animals based on data from:

▪ Fossils

▪ Comparative morphology

▪ Comparative development

▪ Comparative genomics

• Prevailing hypothesis is that sponges are most ancient lineage of animals

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fossil record

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compare shares

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morcular bio for development

shape of genone

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Data to understand animal evolutionary

relationships (take notes)

1. Fossils

2. Comparative morphology

3. Comparative development (evolutionary developmental biology or evo-devo)

4. Comparative genomics

Direct evidence of animals

-doesn't represent animals equally

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> some easier to preserve remains

recognize numerous,

vina , radius , carpal shared but changed -

overtime

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certain parts of genome need to be turned on

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>study transcription factors to find patterns as gene expression

& now itcauses morphological change

during development

-relative similarities of various genomes

Developmental homology: same gene being turned on in same place at same time in development

Sponges may be a paraphyletic group—containing some, but not all, descendants of common

ancestor

• Sponges have the basic genetic tool kit needed for multicellularity:

– Cell–cell adhesion

– Cell–ECM adhesion

– Few even have epithelium

• Sponges do not have complex tissue:

– Groups of similar cells that are organized into tightly integrated structural and functional

units

• Some sponges have true epithelium:

– Layer of tightly joined cells that covers interior and exterior surface of animal

– Epithelium is essential to animal form and function

• Ctenophores first? Possible, but we won’t consider that.

Sponges first hypothesis

• Earliest animals to appear in the fossil record

• The first sponges appeared more than 700 mya (perhaps even 890 Mya, see

Turner 2021 below)

• Presence of multicellular sponges and absence of fossils of other multicellular

organisms consistent with basal position of sponges on phylogeny

Cnideria first? Ctenophores first? Possible, but we won’t consider that

Both are benthic (live at bottom of aquatic environments) and sessile (adults live permanently

attached to substrate rather than moving freely)

• Both feed using cells with nearly identical morphology

• Beating flagella of choanoflagellates and specialized cells in sponges called choanocytes trap

organic debris

• Feeding occurs at the cellular level

• Choanoflagellates sometimes form colonies—groups of attached individuals:

– Sponges were once considered colonies of single-celled protists:

▪ Sponge cells can reaggregate after being dissociated

• However, sponges contain many specialized cell types:

• These cells are dependent on each other

• Some occur in organized layers surrounded by extracellular matrix (ECM)

Sponges differ from choanoflagellates

• Sponges contain many specialized cell types:

• These cells are dependent on each other

• Some occur in organized layers surrounded by extracellular matrix (ECM)

Diploblasts—animals whose embryos have two types of tissues, or germ layers

– The ectoderm (“outside-skin”):

– The endoderm (“inside-skin”)

Origin of embryonic tissue layers: triploblasts

Germ layers develop into distinct adult tissues and organs

Triploblasts are animals whose embryos have three germ layers

▪ The ectoderm (“outer-skin”):

▪ The endoderm (“inner-skin”):

▪ The mesoderm (“middle-skin”)

Missing mesoderm: Deep homology, and

convergent evolution in diploblasts

Shared between diplo & triplo

• mesoderm-like cells in mesoglea

• genes coding for structural

components of mesodermal cells

• some can change the shape of

their bodies

• actin & myosin

Missing in diploblasts

•mesodermal specification

genes

•well-defined mesoderm

•true muscles

• Functional similarity achieved by independent evolutionary paths:

– Convergent evolution with deep homology

Body symmetry

• Body symmetry—key morphological aspect of animal’s body plan

• Animals with radial symmetry—such as cnidarians, ctenophores, and some

sponges—have at least two planes of symmetry:

– Radial symmetry evolved independently in the echinoderms

• Most other animals exhibit bilateral symmetry, with a single plane of symmetry

and long, narrow bodies

• Radial symmetry appears to have evolved earlier than bilateral symmetry:

– All triploblasts are bilaterally symmetric

Early bilateral symmetry? 1

• Most cnidarians appear radially symmetric:

– Internal morphology of some species is actually bilaterally symmetric

– Especially true of many species of sea anemone

• Bilaterians are triploblastic, bilaterally symmetrical animals

• Symmetry in bilaterians results from action of:

– Hox genes:

▪ Regulate development of the anterior–posterior axis

– Decapentaplegic (dpp) genes:

▪ Regulate development of the dorsal–ventral axis

Early bilateral symmetry? 2

• Biologists hypothesized that bilateral symmetry seen in some cnidarians is

homologous to bilateral symmetry in bilaterians:

– Other possibility is that this feature is an example of convergent evolution

• Data largely support hypothesis:

– Some parts of genetic tool kit for bilateral symmetry arose before cnidarian

and bilaterian lineages split

– However, other parts evolved later, after the lineages split

Body Symmetry Is Associated with the Nervous System

• Symmetry and nervous system are related:

• Sponges lack nerve cells and symmetry

• Function of neurons and nervous system:

– Transmit and process information in the form of electrical signals

– Radially symmetrical cnidarians and ctenophores have nerve cells that are

organized into a nerve net

• Bilaterally symmetric organisms tend to encounter their environment at one

end:

• Evolution of CNS coincided with cephalization:

– Evolution of head where structures for feeding, sensing environment, and

processing information are concentrated

The Tube-Within-A-Tube Body Plan

• Basic bilaterian body shape is tube within a tube:

– Inner tube:

▪ Gut with a mouth on one end and an anus at the other

– Outer tube:

▪ Forms the nervous system and skin

– Mesoderm in between forms muscles and organs&ectoderm

Origin of the coelom

• A coelom is an enclosed, fluid-filled body cavity between the tubes:

– Provides a space for oxygen and nutrients to circulate

– Enables the internal organs to move independently of each other

• True coelomates: coelom is completely lined with mesoderm

• Acoelomates—“no-cavity form:” no coelom, such as the flatworms (phylum Platyhelminthes)

• Pseudocoelomates (“false cavity-form”): Coelom is only partially lined with mesoderm, such as roundworms (phylum

Nematoda) and rotifers (phylum Rotifera)

• Morphological data predicted gradual evolution: From simple acoelomates to pseudocoelomates to coelomates

• Molecular data predicts that coelom arose in ancestral bilaterian and was subsequently modified, reduced, or lost in

many lineages

• Evolutionary flexibility of coelom has reduced its usefulness as a synapomorphy for bilaterian animals

Origin of protostomes and deuterostomes 1

• Common ancestor during Cambrian was likely bilaterally symmetric triploblast

with CNS, cephalization, and coelom

• Ancestor gave rise to radiation of diverse animal lineages

• Early studies of embryonic development—two major subgroups:

1. Protostomes, (“first-mouth”), named for embryonic development of mouth

before anus

2. Deuterostomes, (“second-mouth”), named for embryonic development of

anus before mouth

Origin of protostomes and deuterostomes 2

• Three embryonic germ layers form during gastrulation:

– Cells move from outside into center of embryo

– Creates pore that opens to the outside (blastopore):

▪ In deuterostomes: Blastopore becomes anus and mouth forms later

▪ Traditional view: In protostomes, blastopore becomes mouth and anus forms

later

▪ Recent view: Development is highly variable in protostomes—blastopore may

become anus, mouth, both anus and mouth, or neither

• Mesoderm:

– Once thought to form by one developmental pathway in protostome embryos and

by different developmental pathway in deuterostomes

– New data show that both types of mesoderm-forming processes occur in both

groups

Segmentation: Modularity 1

• Division of body or part of body into series of similar structures

• Segmented backbone one of defining characteristics of vertebrates:

– Monophyletic lineage within the Chordata

• Invertebrates (a paraphyletic group):

– Segmentation conspicuous in annelids and arthropods

• Some tool-kit genes for segmentation arose early in animal evolution:

– Homologous in different phyla

Segmentation: Modularity 2

• However, genes subsequently lost in some lineages and co-opted in different

ways and elaborated upon in others:

– Convergent evolution of morphological segmentation in distantly related

phyla

• Segmentation enables specialization

• Small changes in expression of certain tool-kit genes:

– Such as Hox genes

– Can result in novel numbers, shapes, and sizes of body segments

• Natural selection can then favor variations that are adaptive environments,

leading to diversification

Feeding

• Heterotrophs!

• Biologists distinguish what individuals eat from how they

eat

• Animals within lineage with similar body plan may pursue

different food sources and feeding strategies if their niche

differs

• Conversely, animals from different lineages with similar

niches often pursue the same food sources and feeding

strategies

what do animals eat?

-detritvores: feed on dead organic matter

ex. millioedes feed on decaying leaves

-carnivores: feed on animals

ex. owls hunt down and consume prey

-herbivores: feed on plants or algae

ex. pandas eat lots of bamboo

-ominvores: feed on plants, animals, fungi, protists, archaea, or bacteria

ex. humans

Parasitism

• Parasites harvest nutrients from parts of their hosts:

– Usually much smaller than their victims

– Endoparasites live inside their hosts and usually

have simple, wormlike bodies

– Ectoparasites live outside their hosts and usually

have limbs or mouthparts that allow them to grasp

the host

How do animals eat?

-suspension feeders: capture food by filtering out particles floating in water or drifting through air

-deposit feeders: ingest organic material that has been depositied within a substrate or on its surface

- fluid feeders: suck or mop up liquids like nectar, plant sap, blood, or fruit juice

-mass feeders: take chunks of food into their mouths

Sensory Systems and Cephalization

Key aspect of cephalization:

• Concentration of sensory organs in head region

• Great deal of diversity of sensory abilities and structures among the animals

Most animals have common senses:

• Sight, hearing, taste, smell, and touch

• Most animals also have some ability to sense temperature

Movement

• Functions of animal locomotion include:

Finding food, finding mates, escaping from predators, dispersing to new habitats

• The ways in which animals move are highly variable:

– They burrow, slither, swim, fly, crawl, walk, or run.

– Mostly powered by muscle

• Three types of skeletal systems that enable complex movements:

– Hydrostatic skeletons: support from flexible body wall in tension surrounding

fluid or soft tissue under compression

– Endoskeletons derive support from rigid structures inside the body, such as

bones in vertebrates and spicules in sponges

– Exoskeletons derive support from rigid structures on the outside of the body, such

as the external armor of arthropods

Viviparous (“live-bearing”) species

Oviparous (“egg-bearing”) species

Ovoviviparous (“egg-live-bearing”) species

Cambrian Explosion

• In fossils dated from about 541 million years ago to about 530 million years ago, most of the

major animal phyla (and hence most body plans) appear for the first time.

• Phylum-level body plans evolved rapidly during Cambrian rather than more gradually over time.

Slow-fast-slow again. Why?

• Constraints on new body plans (take notes)

• Promotes diversification (take notes)

Life Cycles

• Reproduction just one component of diverse life cycles of

animals:

– Most sexually reproducing animals have diploid-

dominant life cycles

• Perhaps most spectacular innovation in animal life cycles

involves the phenomenon known as metamorphosis:

– Drastic change from one developmental stage to

another

– Contrast with “direct development” where an animal is

born essentially as a smaller version of its adult form.

Growth from birth to adulthood is gradual

Many Animal Life Cycles Include Metamorphosis

• During indirect development:

– Embryogenesis produces larvae (larva), which:

Look radically different from adults

Live in different habitats and eat different foods

– Metamorphosis transforms larvae into juveniles, which:

Look like adults

Live in the same habitats and eat the same foods

Are still sexually immature

– Growth and maturation transform the juveniles into adults—reproductive

stage in life cycle

Non-Bilaterian animals existed before other animals

• Fossil record and phylogeny of animals agree that:

– Porifera (sponges)

– Ctenophora (comb jellies) and

– Cnidaria (jellyfish and others) are most ancient of all

major animal lineages

– Predated the Cambrian

• All three groups have a global marine distribution: meaning they had a common ancestor from the water