History of Life on Earth

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)

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 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)

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 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)

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

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

Bilateral: 1 line of symmetry axes: dorsal/ventral; left/right; anterior/posterior cephalisation: development of a head Radial: infinite lines of symmetry

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

Ectoderm (surface) (Mesoderm) Endoderm (innermost) Diploblastic animals: ecto & endo only Triploblastic: all 3

Includes 2 phyla: Cnidaria: Hydrozoa (hydroids) Schyphozoa (jellyfish) Anthozoa (corals, sea anenomes) Cubozoans (jellyfish like) Ctenophora (comb jellies)

Subtaxa of Bilateria Acoelomates (no body cavity): Only Platihelmines or 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

Divided into: Protostoma (spiral cleavage) and 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 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

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

Wide range of animal body forms includes: flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids

Includes: Turbelleria: mostly freeliving flatworms Monogenea: marine and freshwater parasites Trematoda: flukes Cestoda: tapeworms