BIOL 108 - Topic 20: Origin of Animals PDF

Topic 20: Origin of animals BIOL 108 Winter 2024 © 2024 Neil Harris Kingdom Animalia includes a vast diversity of living and extinct species. Molecular phylogenetic analyses indicate that sponges and the common ancestor of other animal phyla evolved 670-700 mya. − This ancestral protist likely resem...

Topic 20: Origin of animals BIOL 108 Winter 2024 © 2024 Neil Harris Kingdom Animalia includes a vast diversity of living and extinct species. Molecular phylogenetic analyses indicate that sponges and the common ancestor of other animal phyla evolved 670-700 mya. − This ancestral protist likely resembled modern choanoflagellates, which are flagellated eukaryotes.  Choanoflagellates are opisthokont protists that are the closest relatives of animals. Fig 25.8 Visualizing the scale of geologic time 1 Choanoflagellate protists BIOL 108 Winter 2024 © 2024 Neil Harris Extant choanoflagellates are small unicellular, heterotrophic protists.  Microvilli are finger-like projections of the cell membrane that captured bacteria.  Animals are hypothesized to have evolved from a choanoflagellate-like ancestors. http://tolweb.org/Choanoflagellates/2375 − ~140 species living as unicellular and colonial forms (stalked or ball-like colonies) in marine and freshwater environments. − Choanoflagellates are collared flagellates (choano = collar), with a funnel-shaped collar of microvilli at the base of a flagellum. WC 2 Choanoflagellates BIOL 108 Winter 2024 © 2024 Neil Harris Evidence that choanoflagellates are closely related to animals: 3. DNA sequence homology Molecular phylogenies confirm choanoflagellates as the closest extant relatives of animals. 1. Cell morphology. Choanoflagellate cells and the collar cells of sponges are almost indistinguishable. Porifera Fig 32.3 Three lines of evidence that choanoflagellates are closely related to animals 2. Cell morphology unique to animal cells. Cells resembling choanoflagellates are found in other animals, but never in non-choanoflagellate protists, plants, or fungi. 3 Earliest multicellular eukaryotes The evolution of eukaryotic cells enabled the emergence of various unicellular forms, including protists. Subsequently, multicellularity evolved independently in multiple lineages, leading to the development of algae, plants, fungi, and animals. − Evolutionary advantages for multicellularity: cell specialization, increased size and complexity, longer lifespans, and defence against predation. BIOL 108 Winter 2024 © 2024 Neil Harris − Fossil evidence dates the oldest multicellular eukaryotes (algae) to ~1.2 bya, with molecular clock calculations suggesting an origin around 1.7 bya. − Whole-body animal fossils date from ~560 mya (Neoproterozoic Era [late Proterozoic], Fig 32.4). 4 Neoproterozoic Era 1 billion–541 mya BIOL 108 Winter 2024 © 2024 Neil Harris The earliest complex, multicellular animal fossils are from the Ediacaran biota (565 to 550 mya). − The rise of marine planktonic algae (Archaeplastida) ~800–650 mya increased atmospheric and oceanic oxygen levels (Neoproterozoic Oxygenation Event).  The transition to algal-dominated ecosystems created food webs with more efficient nutrient and energy transfers, supporting the evolution of larger and increasingly complex organisms. − The Ediacaran biota were diverse soft-bodied and mostly non-motile (sessile) marine organisms.  Ediacaran biota was largely extinct by the Cambrian. Ediacaran biota is named for Ediacara Hills in South Australia Fig 32.4 Ediacaran fossils 5 Paleozoic Era 541–252 mya The Cambrian explosion, spanning from 535 to 525 mya, marked a significant diversification in animal evolution. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 25.11 Appearance of selected animal groups − Animal fossils from the Cambrian explosion show a rapid increase in the diversity of various bilaterian animals with bilateral symmetry and complete digestive tracts. − While many extant animal phyla appeared during the Cambrian explosion, some, like sponges, cnidarians, and molluscs, predate it. 6 Paleozoic Era 541–252 mya Hypotheses for Cambrian explosion: Evolution of predation. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.6 A Cambrian seascape − The evolution of predators and prey led to the development of more complex body structures and defence mechanisms. Neoproterozoic Oxygenation Event. − Increase in oceanic [O2] supported greater body size and more energetic lifestyles, e.g. active, muscular carnivores. Evolution of the Hox genes. Burgess Shale sites in BC are rich in fossils from ~505 mya (Fig 32.7). Burgess Shale formed in a shallow sea and includes animals from every major phylum. − The emergence of Hox genes, which control the body plan of animals, facilitated the diversification of body forms during the Cambrian explosion. Anomalocaris canadensis (up to 1m in length) 7 Animal evolution Paleozoic Era (541–252 mya): − Animal diversity increased during the Paleozoic but was punctuated by the Ordovician, Late Devonian, and Permian mass extinctions.  Invertebrates began colonizing land ~450 mya, followed by vertebrates around 365 mya. Table 25.1 BIOL 108 Winter 2024 © 2024 Neil Harris Mesozoic Era (252–66 mya): − Dinosaurs were dominant terrestrial vertebrates during the Mesozoic Era.  The first mammals emerged. Cenozoic Era (66 mya to present): − Mammals increased in size and diversity after the Cretaceous mass extinction, exploiting ecological niches that were available. Table 25.1 8 WC WC WC Invertebrates Japanese spider crab NZ giant weta, heaviest insect − Invertebrates occupy almost every habitat on Earth and include a diverse range of species, ranging from microscopic to huge.  e.g. colossal squid (Mollusca): largest specimen, 495 kg, 10 m. invertebrates − The term “invertebrates” refers to animals without a backbone.  Anatomical classification.  Invertebrates are paraphyletic, i.e. not a clade. Two Chordata subphyla – invertebrates Subphyla Vertebrata – vertebrates Colossal squid (Mollusca) invertebrates Invertebrates consist of 95% of known animal species. BIOL 108 Winter 2024 © 2024 Neil Harris Transparent Amazonian fish, Cyanogaster noctivaga, with visible backbone Mattox et al. (2013) Ichthyol. Explor. Freshw. 23:297-318 Fig 32.11 A phylogeny of living animals 9 Metazoan shared, derived traits BIOL 108 Winter 2024 © 2024 Neil Harris Choanoflagellates Cnidarians Clade Eumetazoa 1. True tissues, incl. muscle and nerve tissues 2. Gastrulation Metazoa Clade Metazoa 1. Multicellularity and cell differentiation 2. Cell adhesion (extracellular matrix) 3. Sperm and ova (egg) 4. Embryonic blastula* Ctenophores Eumetazoa Radial symmetry Diploblastic Porifera (sponges) Bilaterians Clade Bilateria 1. Bilateral symmetry 2. Triploblastic 3. Complex organs 10