History of Life on Earth Flashcards Preview

Animal Biology and Evolution > History of Life on Earth > Flashcards

Flashcards in History of Life on Earth Deck (30)
Loading flashcards...

The Origin of Cellular Life


The first simple cells may have chemically evolved through a four-stage proccess:

  1. Abiotic synthesis of small organic monomers
  2. Polymerisation of monomers into proteins, nucleic acids, etc
  3. Appearance of self-replicating polymers (proteins, polynucleic acids, etc)
  4. Packaging of organic molecules into probionts (aggregates of orgmols that display some k associated with life - homeostasis, metabolism)


The Origin of Oxygen

Earth's oxygen is produced by photosynthesis (!), originally started by green-blue bacteria (cyanobacteria) 

Animal life (requires atmospheric O2) didn't come into existence until 600 Ma


The Earliest Organisms

The first organisms were anaerobic prokaryotes 

Some organisms evolved the ability to use the sun's energy

Photosynthesis increases the atmospheric concentration of O2

In response to this "oxygen crisis" aerobic respiration evolved

some organisms obtained membrane-enclosed organelles (endosymbiosis hypothesis)


The Endosymbiosis Hypothesis

You know the drill.

"The ancestors of chloroplasts may have resembled Chlorella, a green, photosynthetic algae that lives symbiotically within the cytoplasma of Paramecium


The Earliest Multicellular Organisms

The first organisms to become multicellular were algae, while higher organisms with differentiated cells evolved more than 1 Ba

Animal diversity arose in the precambrian era (<600 Ma) - original animals were primarily aquatic shelled invertebrates 

Life invaded the land in the Cambrian era (500 Ma) 

O2 levels reached ~current in the Devonian (400 Ma)

Land animals date back to 350 Ma (still Devonian)


Major Events in Evolutionary History

"The Evolutionary Clock"


What are Animals?

Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers. Their cells lack walls.

Muscle and nervous tissue are unique to animals

1.3 million animal species have been identified

The common ancestor of living animals (may have) lived between 675 and 875 Ma

It may have resembled modern choanoflagellates - protists



The Phylogenetic Tree

The classifications of the phylogenetic tree are mainly based on: embryology, morphology, homology criteria and molecular genetics


The Eukaryotic Tree



Phylum Parazoa

Order Porifera

Porifera (Sponges) are the only order belonging to Parazoa:

No symmetry or organisation


Little cell specialisation, no organs

No nerves


Cells in three layers - water pulled through pores by flagellated cells, filter-fed



Contemporarily divided into Silicea and Calcarea


Symmetry of Form

Bilateral: 1 line of symmetry

axes: dorsal/ventral; left/right; anterior/posterior

cephalisation: development of a head

Radial: infinite lines of symmetry


Development of Tissues

Tissues: collections of specialised cells isolated from other tissues by membranous layers 

During development 2 or 3 germ layers give rise to the tissues and organs of the embryo



Germ Layers

Ectoderm (surface)


Endoderm (innermost)

Diploblastic animals: ecto & endo only

Triploblastic: all 3


Radiata (unranked taxa)

Includes 2 phyla:


Hydrozoa (hydroids)

Schyphozoa (jellyfish)

Anthozoa (corals, sea anenomes)

Cubozoans (jellyfish like)

Ctenophora (comb jellies)




Subtaxa of Bilateria

Acoelomates (no body cavity): Only Platihelmines


body cavity:

 Pseudocoelomates (cavity not enclosed by mesoderm): rotifers and nematodes

Coelomates: everything else




2 layers of tissue, no organs

Single mouth-like cavity to a central digestive cavity

Tentacles (cnidocytes) with stinging cells (nematocysts) capture cells

Simple muscle and nerve cells

Polyps are cylindrical / Medusae are bell-shaped




Comb jellies have bands of cilia for propulsion

Hermaphrodites: shed sperm and eggs into the open sea


Bilateria (unranked taxa)

Divided into:

Protostoma (spiral cleavage) 


Deuterostoma (radial cleavage)



"first mouth" - develop mouth-first

bilaterially symmetrical, 3 germ layers, organ-level organisation, tube within a tube body plan &

a true coelom (main body cavity)



Radial cleavage, forms a blastophore than eventually becomes an anus

Indeterminate cleavage: cells specialise late, meaning if they are removed from blastula it can form a complete new larva, and the remaining cells can compensate


The Problem of Phylogeny

The phylogenetic tree is under constant debate, especially with the rise of molecular genetics

Recent molgen studies suggest deuterostomes and protosomes should instead be divided into: Deuterostomia, Ecdysozoa and Lophotrochozoa


What is a Phyla?

Consists of a group of animals sharing a common body plan

Each member can be derived from a common ancestor with certain diagnostic features. e.g: molluscs: radula, mantle, foot

Important to distinguish between homology and analogy


Clade Lophotrochozoa

Wide range of animal body forms

includes: flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids


Phylum Platyhelminthes


Turbelleria: mostly freeliving flatworms

Monogenea: marine and freshwater parasites

Trematoda: flukes

Cestoda: tapeworms


Phylum Mollusca

Most molluscs are marine, though some are freshwater and few are terrestrial

Molluscs are soft-bodied, though most have hard, protective shells

All molluscs have three main parts: a muscular foot, a mantle, and a visceral mass

Many also have a water-filled mantle cavity, and feed via rasp-like radula

Possess a circulatory system & heart

Major classes: Polyplacophora (chitons), Gastropoda (slugs, snails), Bivalvia (clams, mussels, etc) and Cephalapoda (octopi, squid, etc)


Phylum Annelida

Annelids have bodies composed of fused rings

Possess complete gut, circulatory and respiratory, and well-developed nervous system


Oligochaeta (freshwater, marine and terrestrial segemnted worms)

Polychaeta (mostly marine segmented worms)

Hirudinea (leeches)



Clade Ecdysozoa

The most species-rich animal group

Covered by a tough coat (Cuticle), which is molted through a process called ecdysis

Two largest phyla: arthropods and nematodes


Phylum Arthropoda

Make up 2 out of every 3 species of animals on the planet; found in nearly all habitats in the biosphere

Possess specialised appendages, hard exoskeleton, coelom that functions as open circulatory system, excretory and gas exchange organs


Cheliceriformes (horseshoe crabs, arachnids)

Myriapoda (millipedes, centipedes)

Hexapoda (insects, springtails)

Crustacea (crabs, lobsters, crayfish, shrimps)


Phylum Echinodermata

Echinoderms may seem to have little in common with phylum Cordata, but are both deuterostomes


Asteroidea (sea stars)

Ophiurodea (brittle stars)

Echinodea (sea urchins, sand dollars)

Crinoidea (sea lillies, feather stars)

Holothuroidea (sea cucumbers)

Concentricycloidea (sea daisies)


Phylum Chordata

Includes 2 subphyla of invertebrates, as well as vertebrates and hagfish


Image is of primitive chordate: Lanclet