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Lecture 24 the Cambrian Explosion Life on Earth - Timescale

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Lecture 24 the Cambrian Explosion Life on Earth - Timescale Lecture 24 The Cambrian explosion Life on Earth - Timescale Precambrian (4,600 to 543 MYA) (MYA = million years ago) - life on earth evolved about 3,800 MYA. - prokaryotes evolved about 3,500 MYA - eukaryotes evolved about 2,100 MYA - multicellular eukaryotes appeared about 1,500 MYA. Life on Earth - Timescale Differences between Prokaryotes and Eukaryotes 1. much larger cell sizes. 2. a nucleus and organelles. 3. aerobic. 4. cilia and flagella with tubulin rather than flagellin protein. 5. linear DNA molecules complexed with histones 6. commonly being multicellular. 7. both mitosis and meiosis 8. Presence of a cytoskeleton Emergence of Eukaryotes - there is a general trend towards increasing size of fossils 1.9 to 1.3 bya - average sizes of fossils increased from 10 microns to 60 to 80 microns. This is consistent with the increased size of eukaryotic cells and the increasing levels of O2 during this period (by 1.9 bya it was 15% of its current level). - the earliest possible eukaryote has been dated to about 1.8 bya in the Canadian shield region. Serial Endosymbiotic Hypothesis - all mitochondria are monophyletic – they all trace to a proteobacterial ancestor. - unlike mitochondria, chloroplasts have evolved several (4?) times – they are polyphyletic! - Margulis also argues that cilia/flagella were similarly acquired. Endosymbioses 1. The ciliate Paramecium bursaria and green algae (Chlorella) - the ciliate readily uptakes the algae which supplies carbon compounds from photosynthesis. - similar to early chloroplast evolution? 2. The anaerobic amoeba, Pelomyxa palustris (lacks mitochondria) - the amoeba readily uptakes aerobic bacteria and then switches to requiring oxygen. - similar to early mitochondrial evolution? 3. Motility symbioses in protists (Myxotricha Spp.) - some gut protists have “docking sites” for spirochaete bacteria. - after “docking” spirochaetes beat flagella in coordinated fashion. - similar to early cilia/flagella evolution? Biases in the fossil record 1. Geographic bias - there is a major geographic bias in the fossil record. - majority of fossils come from marine sediments, lake beds, and floodplains. - terrestrial environments, especially tropical ones, poorly represented. 2. Taxonomic bias - the fossil record is dominated by marine species possessing shells. - marine organisms represent about 10% of all species. 3. Temporal bias - this is called the “pull of the recent”. - older rocks are much rarer than newer rocks! The Ediacaran and Burgess Shale faunas The Ediacaran and Burgess Shale faunas Ediacaran Fauna The Ediacaran and Burgess Shale faunas Ediacaran Fauna • dates to about 560 MYA. The Ediacaran and Burgess Shale faunas Ediacaran Fauna • dates to about 560 MYA. • are exclusively soft-bodied (sponges, jellyfish, comb jellies, etc) and non-burrowing. The Ediacaran and Burgess Shale faunas Ediacaran Fauna Vernanimalcula, found in China in 2004 Dates to 40 – 55 million years before Cambrian Anomalocaris Wiwaxia Opabinina Hallucigenia Pikaia Burgess Shale Fauna Discovered by Charles Walcott In 1909 Burgess Shale Fauna • found near Field, B.C., dates to 520 MYA (similar to Yunnan fossils in China) Burgess Shale Burgess Shale Fauna • found near Field, B.C., dates to 520 MYA (similar to Yunnan fossils in China) • all but one of the 35 existing phyla dramatically “appear” – this is the Cambrian explosion. Burgess Shale Fauna • found near Field, B.C., dates to 520 MYA (similar to Yunnan fossils in China) • all but one of the 35 existing phyla dramatically “appear” – this is the Cambrian explosion. • entirely new modes of locomotion evolve (i.e., swimming, burrowing, climbing). Burgess Shale Fauna • found near Field, B.C., dates to 520 MYA (similar to Yunnan fossils in China) • all but one of the 35 existing phyla dramatically “appear” – this is the Cambrian explosion. • entirely new modes of locomotion evolve (i.e., swimming, burrowing, climbing). • the diversity of body plans is astonishing! The Cambrian Explosion Fossil Tracks Burgess Shale Trilobites Pikaias Burgess Shale Amphioxus Pikaia First Chordates Cambrian Explosion and the Burgess Shale Stephen Jay Gould (1941-2002) The Cambrian explosion - the appearance of complex metazoans did not happen until the so-called “Cambrian explosion” about 540 million years ago. - in a span of time estimated between 10 and 25 million years, all but one of the 35 existing phyla appear. - this is a “blink of the eye” geologically speaking. - in this short window of time, we see the appearance of protostomes and deuterostomes, coelomates, pseudocoelomates, and acoelomates. - we also see the first segmented body plans, external skeletons, appendages, and notochords. The Cambrian explosion - the appearance of complex metazoans did not happen until the so-called “Cambrian explosion” about 540 million years ago. - in a span of time estimated between 10 and 25 million years, all but one of the 35 existing phyla appear. - this is a “blink of the eye” geologically speaking. - in this short window of time, we see the appearance of protostomes and deuterostomes, coelomates, pseudocoelomates, and acoelomates. - we also see the first segmented body plans, external skeletons, appendages, and notochords. What caused the Cambrian explosion? What caused the Cambrian explosion? 1. Increase in the oxygen content of seawater What caused the Cambrian explosion? 1. Increase in the oxygen content of seawater • allowed organisms to achieve increased sizes and metabolic rates. What caused the Cambrian explosion? 1. Increase in the oxygen content of seawater • allowed organisms to achieve increased sizes and metabolic rates. • large size is clearly a prerequisite for the evolution of predators. What caused the Cambrian explosion? 2. Origin of hard parts (shells and mineralized exoskeletons). What caused the Cambrian explosion? 2. Origin of hard parts (shells and mineralized exoskeletons). • some of the earliest shells have holes bored through them by predators! What caused the Cambrian explosion? 2. Origin of hard parts (shells and mineralized exoskeletons). • some of the earliest shells have holes bored through them by predators! • strong selection pressures would have favored mineralized shells. What caused the Cambrian explosion? 3. The evolution of eyes What caused the Cambrian explosion? 3. The evolution of eyes • proposed by Andrew Parker in his 2003 book, “In the blink of an eye”. What caused the Cambrian explosion? 3. The evolution of eyes • proposed by Andrew Parker in his 2003 book, “In the blink of an eye”. • eyes first appear in trilobites about 543 MYA. What caused the Cambrian explosion? 3. The evolution of eyes • proposed by Andrew Parker in his 2003 book, “In the blink of an eye”. • eyes first appear in trilobites about 543 MYA. • large predators with eyes make for better predators! What caused the Cambrian explosion? 4. Genetic changes What caused the Cambrian explosion? 4. Genetic changes • did the diversification of homeotic genes drive the Cambrian explosion? What caused the Cambrian explosion? 4. Genetic changes • did the diversification of homeotic genes drive the Cambrian explosion? • homeotic genes encode for transcription factors. What caused the Cambrian explosion? 4. Genetic changes • did the diversification of homeotic genes drive the Cambrian explosion? • homeotic genes encode for transcription factors. • they activate suites of genes that control body plans during early development. Macroevolutionary patterns 1. Adaptive Radiation Macroevolutionary patterns 1. Adaptive Radiation Definition (Mayr 1963): evolutionary divergence of members of a single phyletic line into a series of rather different niches or adaptive zones. Adaptive radiation Some generalizations 1. Occur at edges of a species range Some generalizations 1. Occur at edges of a species range 2. Facilitated by the absence of competitors and predators Some generalizations 1. Occur at edges of a species range 2. Facilitated by the absence of competitors and predators • island archipelagoes are prime areas for radiations. Some generalizations 1. Occur at edges of a species range 2. Facilitated by the absence of competitors and predators • island archipelagoes are prime areas for radiations. Examples: Hawaiian Drosophila and honeycreepers Hawaiian honeycreepers Some generalizations 3. May involve “general adaptations” Some generalizations 3. May involve “general adaptations” • general adaptations enable exploitation of new adaptive zones. Some generalizations 3. May involve “general adaptations” • general adaptations enable exploitation of new adaptive zones. Example: evolution of flight (in insects, birds, bats) Some generalizations 3. May involve “general adaptations” • general adaptations enable exploitation of new adaptive zones. Example: evolution of flight (in insects, birds, bats) • there are ~1,500,000 insects, ~10,000 birds and ~1,100 bat species. 2. Punctuated equilibrium (PE) 2. Punctuated equilibrium (PE) • first proposed by Stephen Jay Gould and Niles Eldredge in 1972 to account for “gaps” in the fossil record. 2. Punctuated equilibrium (PE) • first proposed by Stephen Jay Gould and Niles Eldredge in 1972 to account for “gaps” in the fossil record. Two characteristics: 2. Punctuated equilibrium (PE) • first proposed by Stephen Jay Gould and Niles Eldredge in 1972 to account for “gaps” in the fossil record. Two characteristics: 1. Periods of rapid morphological change co-occur with periods of rapid speciation. 2. Punctuated equilibrium (PE) • first proposed by Stephen Jay Gould and
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