History of Life on Earth Flashcards
The Origin of Cellular Life
(possibly)
The first simple cells may have chemically evolved through a four-stage proccess:
- Abiotic synthesis of small organic monomers
- Polymerisation of monomers into proteins, nucleic acids, etc
- Appearance of self-replicating polymers (proteins, polynucleic acids, etc)
- 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
(eugh)
Phylum Parazoa
Order Porifera
Porifera (Sponges) are the only order belonging to Parazoa:
No symmetry or organisation
Sessile
Little cell specialisation, no organs
No nerves
Cells in three layers - water pulled through pores by flagellated cells, filter-fed
hemaphoriditic
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)
(Mesoderm)
Endoderm (innermost)
Diploblastic animals: ecto & endo only
Triploblastic: all 3
Radiata (unranked taxa)
Includes 2 phyla:
Cnidaria:
Hydrozoa (hydroids)
Schyphozoa (jellyfish)
Anthozoa (corals, sea anenomes)
Cubozoans (jellyfish like)
Ctenophora (comb jellies)
Coelomates
Subtaxa of Bilateria
Acoelomates (no body cavity): Only Platihelmines
or
body cavity:
Pseudocoelomates (cavity not enclosed by mesoderm): rotifers and nematodes
Coelomates: everything else
Cnidaria
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
Ctenophores
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)
and
Deuterostoma (radial cleavage)
Protostomes
“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)
Deuterostomes
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
Includes:
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
Includes:
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
Includes:
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
Includes:
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
