Animal Diversity and Evolution

Overview of Animal Diversity

Themes in Diversifying Characteristics

• Morphology

• Movement

• Reproduction

• Metabolism

Origin of Animals

• Animals originated from single-celled eukaryotes.

• They belong to the lineage called Opisthokonta.

• Choanoflagellates are the closest living relatives to animals.

• They share a common ancestor approximately 900 million years ago.

Cambrian Explosion

• The radiation of animals began around 550 million years ago.

• There are an estimated 8 million to 50 million extant animal species.

• Only about 1.8 million species have been described to date.

• The modern rate of animal extinction is accelerating due to human activity.

Key Characteristics of Animals

• Animals are heterotrophs, obtaining energy and nutrients by consuming other organisms.

• They process food inside their bodies and often have efficient digestive systems.

• Animals have tissues formed from layers of embryonic cells.

• They possess nerve and muscle cells, enabling movement, detection, and capture of prey.

Monophyletic Clade of Animals

• Animals form a monophyletic clade with key traits, represented by 30–35 currently recognized phyla.

• Multicellularity: Animals lack cell walls and have an extensive extracellular matrix.

• Heterotrophy: Animals obtain carbon compounds from other organisms, typically by ingestion.

• Motility: Animals can move under their own power at some point in their life cycle.

• All animals, except sponges, have neurons that transmit electrical signals, and muscle cells that enable body movement through contraction.

• Most animals reproduce sexually.

Sexual Reproduction in Animals

• Most sexually reproducing animals have diploid-dominant life cycles.

• Meiosis reduces the amount of hereditary material by half, producing haploid gametes.

• Fertilization restores the normal amount of hereditary material, forming a diploid zygote.

• The zygote undergoes mitosis during development.

Larval Stage

• Many animals have at least one larval stage.

• The larva is sexually immature and distinct from the adult stage in morphology and behavior.

• After metamorphosis, larvae transform into juveniles that resemble adults but are sexually immature.

Molecular Analysis and Common Ancestry

• Molecular analysis suggests that the common ancestor of animals lived around 770 million years ago.

• Protists called choanoflagellates are the closest living relatives to animals based on morphological and molecular evidence.

• The common ancestor likely resembled modern choanoflagellates.

Multicellularity and Cell Communication

• Animal genes involved in multicellularity share sequence similarities with those in choanoflagellates.

• Multicellularity requires mechanisms for cell adhesion and signaling.

• Animal cadherin proteins share several domains with similar proteins in choanoflagellates.

Neoproterozoic Era (1 bya – 541 mya)

• This era saw the emergence of the first animals.

• The Ediacaran biota represents the first generally accepted macroscopic animal fossils, dating from about 560 million years ago.

• Early evidence of predation is found in fossils from the Ediacaran period (635–541 million years ago).

• Example: Cloudina fossils show signs of predator attacks, indicating early defense mechanisms.

Paleozoic Era (541–252 mya)

• The Cambrian explosion (535–525 mya) marked a rapid diversification of animal life.

• Most fossils from this period are bilaterians, characterized by:

  • Bilaterally symmetric form

  • Complete digestive tract

  • Efficient digestive system with a mouth and an anus.

The Cambrian Explosion

• The Cambrian explosion is characterized by the sudden appearance of many animal phyla in the fossil record.

• Fossils of sponges, cnidarians, and molluscs appear in older rocks from the late Proterozoic.

• Adaptations for predation, such as large bodies and claws, emerged.

• Defensive adaptations, including sharp spines and heavy body armor, also appeared in prey species.

Hypotheses for Causes of the Cambrian Explosion

• Higher Oxygen Levels: More efficient aerobic respiration supported larger, more active movements.

• Rise of Algae: Increased phosphorus levels enabled a higher quality food source for early animals.

• Evolution of Predation: Selection for shells, exoskeletons, rapid movement, and other defenses drove morphological divergence among prey.

• New Niches: Animals moving off the ocean floor exploited new resources, creating more new niches and driving speciation and ecological diversification.

• New Genes: Gene duplication and diversification increased the number of Hox genes in animals, yielding larger, more complex bodies.

Colonization of Land

• Fungi, plants, and animals began to colonize land approximately 500 million years ago.

• Prokaryotes lived on land 3.2 billion years ago.

• Adaptations for reproduction on land and prevention of dehydration arose with the move to land in the plant fossil record (waxy coating, stomata, upright growth).

Animal Diversity in the Paleozoic Era

• Animal diversity increased throughout the Paleozoic era, punctuated by mass extinctions.

• 450 mya: Animals began to impact the land.

• 365 mya: Vertebrates colonized land and diversified.

• 302 mya: Arthropods began influencing plants.

• Two groups of early land vertebrates survive today: amphibians and amniotes.

Mesozoic Era (252–66 mya)

• Coral reefs formed ecological niches for marine animals.

• Some reptiles returned to aquatic habitats, while others remained on land and adapted for flight.

• Dinosaurs emerged as predators and herbivores.

• Mammals (tiny, nocturnal insect-eaters) appeared.

• Flowering plants and insects diversified.

Cenozoic Era (66 mya-present)

• This era followed mass animal extinctions.

• Extinction of flightless dinosaurs and marine reptiles occurred.

• Mammals increased in size and abundance.

• The global climate cooled.

• Primate ancestors of humans moved into open woodlands and savannas.

Animal Development

• Animal zygotes undergo cleavage, a series of cell divisions without growth between divisions, eventually forming a blastula, a hollow ball of cells.

• Gastrulation transforms the blastula into a gastrula with different layers of embryonic tissues.

Key Adaptation: Tissues for Specialized Function

• Tissues: Collections of cells acting as a functional unit.

• Sponges lack true tissues.

• Diploblastic animals have two tissue layers:

  • Ectoderm: Forms the outer covering and central nervous system.

  • Endoderm: Forms the gut and lining of the digestive tract.

• Triploblastic animals also have:

  • Mesoderm: Develops into muscles and most organs.

Symmetry of Body Plans

• Radial symmetry: typically found in drifting or weakly swimming animals.

• Bilateral symmetry: associated with active movement and a central nervous system.

Body Cavities

• Diploblasts: Have two embryonic layers.

• Triploblasts: Have three embryonic layers. Most have a body cavity, a fluid- or air-filled space between the digestive tract and the outer body wall.

Functions of Body Cavities

• The internal fluid cushions suspended organs.

• The fluid acts as a skeleton against which muscles can work.

• The cavity allows internal organs to grow and move independently of the outer body wall.

• Allows for compartmentalization of body parts, so that different organ systems can evolve and nutrient transport is possible.

Coelom

• A coelom is a body cavity completely lined by tissue derived from mesoderm.

• The mesoderm forms structures that suspend the internal organs.

Hemocoel

• A hemocoel is a body cavity formed between the mesoderm and endoderm.

• It is filled with hemolymph, a fluid that transports nutrients and waste throughout the body cavity.

Acoelomates

• Some triploblastic animals do not have a body cavity.

• These animals tend to be compact with thin, flat bodies for efficient nutrient, gas, and waste exchange across the body surface.

Protostomes and Deuterostomes

• Bilaterian coelomates are divided into protostomes and deuterostomes based on embryonic development.

• Protostomes (first-mouth): The mouth develops from the blastopore.

• Deuterostomes (second-mouth): The anus develops from the blastopore.

• Traditional view: In protostomes, the blastopore becomes the mouth, and the anus forms later. Recent view: Development is highly variable in protostomes—blastopore may become anus, mouth, both anus and mouth, or neither.

• Deuterostome body plan gave rise to major predator lineages.

Protostome vs. Deuterostome Development

• Protostome Development

  • Spiral cleavage: Cell division is diagonal to the vertical axis.

  • Determinate cleavage: Cell identity is determined early.

• Deuterostome Development

  • Radial cleavage: Cell division is either parallel or perpendicular to the vertical axis.

  • Indeterminate cleavage: All cells can form a complete embryo.

Gastrulation and Coelom Formation

• During gastrulation, the archenteron (which becomes the gut) and the coelom form.

• In protostome development, the coelom forms by the splitting of solid masses of mesoderm.

• In deuterostome development, the mesoderm buds from the wall of the archenteron to form the coelom.

Blastopore Fate

• The blastopore is an indentation in the gastrula that leads to the formation of the archenteron.

• In protostome development, the blastopore becomes the mouth.

• In deuterostome development, the blastopore becomes the anus.

Evolutionary Relationships

• By 500 million years ago, most animal phyla with members alive today were established.

• Many data sources are used to infer evolutionary relationships:

  • Whole genomes

  • Morphological traits

  • Ribosomal RNA genes

  • Hox genes

  • Protein-coding nuclear genes

  • Mitochondrial genes

Phylogeny of Living Animals

• Five important points about the relationships among living animals are reflected in their phylogeny:

  • All animals share a common ancestor.

  • Sponges are the sister group to all other animals.

  • Eumetazoa is a clade of animals with true tissues.

  • Most animal phyla belong to the clade Bilateria.

  • There are three major clades of bilaterian animals: Deuterostomia, Ecdysozoa, and Lophotrochozoa.

Differentiation of Animal Phyla

• The nine animal phyla are differentiated by:

  1. Presence or absence of true tissues

  2. Two or three embryonic tissue layers

  3. No symmetry, radial symmetry, or bilateral symmetry

  4. Ecdysozoans or Lophotrochozoans (Protostomes)

  5. Deuterostomes, which include the vertebrates

Ecdysozoa

• Ecdysozoans have a cuticle and exoskeleton that protect these animals from predators and provide an effective structure for muscle attachment.

• During molting, the animal's soft body is exposed and vulnerable.

• A hormone called ecdysone is important in the regulation of the molting cycle.

• Key Lineages:

  • Nematodes (roundworms)

  • Tardigrada (water bears)

  • Onychophora (velvet worms)

  • Arthropods

Lophotrochozoa

• Lophotrochozoans are named after two traits:

  • Lophophore: A specialized structure that rings the mouth of these animals and functions in suspension feeding.

  • Trochophore: A type of larva common to several phyla of lophotrochozoa.

    • Trochophore larvae have a ring of cilia around their middle that functions in sweeping and, sometimes, in feeding.

Deuterostomes

• Deuterostomes are a monophyletic lineage containing three phyla:

  • Echinodermata: Includes sea stars and sea urchins.

  • Hemichordata: Includes acorn worms and pterobranchs.

  • Chordata: Includes lancelets, tunicates, invertebrates, and vertebrates.

    • Vertebrates comprise hagfish, lampreys, sharks and rays, bony fishes, amphibians, mammals, and reptiles (including birds).