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Biology Notes Cambrian and Ordovician Period After waiting two billion years and getting to the point of single cells how did the planet facilitate multicellular animal life 550 million years agoEvolutionary developmental biology evolutionary changes in genes regulating embryonic development lead to changes in body shapeformbiodiversity Explanations for the Cambrian explosionoSnowball earthoBurrowingoShelled arms raceoDevelopmentalhox genesSnowball earth CryogenianoPeriod in which the planet was completely covered in ice whole planet was frozen solidoThree freezethaw cycles in the history of the world oIn the late Proterozoic everything is happening in the ocean and there are no significant continental land massesoBefore the Cambrian the first piece of land emerges from the ocean oThe land mass sits right on the equator and for the first time water is eroding rockoOne of the minerals that comes off of the rock is calcium oThe ocean and the carbon dioxide around it are normally in equilibrium carbon dioxide and water make bicarbonate and hydrogen and carbonate oThe calcium binds with bicarbonate to make calcium carbonate insolublesedimentation in the ocean oThere had been no calcium before carbon is removed from solution into sediments which breaks the equilibrium oThis happens to such a scale that the carbon dioxide level of the atmosphere starts to diminish oIf there is any silica the sediments form even faster oCarbon dioxide is a greenhouse gas and acts as an insulating blanket oMethane another greenhouse gas is similarly being destroyed through reactions with oxygenoIce caps form in the southern and northern regions since they are white they reflect sun and make the planet even colder oGets to the point that ice masses become so big they cannot be melted as a result the whole planet froze over oThere will be no light penetration so activity in the oceans was minimizedoEventually volcanoes increased carbon dioxide levels causing the planet to warmoAs a result the ocean was stripped of many of its organisms oAnother theory is that the planet wasnt completely frozen it was more like slush slushball earth activity was still minimized but there were parts of the earth that werent completely frozen oThis loss in diversity caused an explosion of animal forms because there was little competition Cambrian burrowersoLife happened on the bottom of the ocean which was covered in impenetrable algal mats which are layers of algae biofilms oEdiacaran fossils sitting walking and feeding on the mat oTrapped under the gelatinous algal mat is organic materials oThe mat constantly grows and covers the materialoAdvantages of burrowing FeedingThe wormlike organisms were able to burrow through and tap into the organic materialProtection the burrows provide protection can hide within themAnchorageShelled arms race oMollusks used their radulas to scrape away at the algal mat oThe mollusks figured out how to used the calcium stabilized in the ocean to build shells and the ecdysozoa use the calcium for a protective exoskeleton oSmall little animals that are protected turn into larger more complex animals and diversity oThe ancestor of the animals is the colonial choanocyte ancestor first unicellular eukaryote which originated from LUCA first unicellular prokaryote Homeotic genesdevelopmental hox genes oHox genes are one of the categories of homeotic genes that control the overall body plan of animals they have a linear sequence on chromosomesHox genes include a 180nucleotide sequencethat homeoboxcodes for a homeodomain part of a protein that functions as a transcription factor homeodomain either activates or represses a geneoData on the genomic sequence reveals that animals share a set of several hundred homeotic genes genetic toolkit these must have been inherited from a common animal ancestor and may date back to the earliest forms of life oTranscription factors regulate gene expression in eukaryotes they provide chemical fingerprints for the various parts of the body to be read by homeotic genes if there was an explosion in homeotic genes there was an explosion in animal body patternsforms oCell specializationdifferentiation is coded for in embryonic development by these genes omRNA message turns into protein which is diffused away from its synthesized location at the front results in a gradient of the protein across the embryo oIn an opposite sense some proteins are made at the back and diffused to the front oThe mixgradience of proteins dictates patterns and structures in organisms oHomeotic genes are arranged on the chromosome in the same sequence that they are expressed on the body genes at the beginning of the sequence dictate structures at the frontanterior of the bodyoA homeotic mutant of a fruit fly grew legs where the antennae were supposed to be this was done by altering the order of homeotic genes in the embryoThe leg cells usually get a message to grow in the middle of the body however the leg genes were placed at the front of the lineoSignals to cells that say you are the anterior part or you are the posterior part are universal the anterior genes may code for a forehead or an antennae etc oEvery cell has all of the same genes only certain genes are turned on when genes are in a specific sequence oCompared to the fruit fly the mouse embryo has more replications of the homeotic genomeoHomeotic genes can indicate levels of complexity and evolutionary paths of organisms sometimes there are losses of hox genes within a group oThe reason that multicellularity arose is because genes that dictated the pattern of multicellular organisms arose in the Cambrian era this patterning sequence was a single evolutionary event and was critical for multicellular organismsoPlayed on by different animals strategies over time resulting in diversity losses of homeotic genes gains in homeotic genes repetitionso Small number of homeotic genes present in spongesoThe first homeotic genes arose over 600 million years ago before the CambrianoAt the end of snowball earth there are Ediacaran fossils that are an experiment in multicellularity then in the Cambrian there are an explosion of forms and burrowers that can penetrate the algal mats oThe Cambrian Explosion arises from diversity in homeotic genesoAt the end of the Ordovician there is the second largest mass extinction which wipes out many of the experiments in multicellular life oPax6 is another toolkit gene the gene triggers the formation of lightsensing organs eye spots in flatworms camera eye for vertebratesContains a homeoboxoMorphological differences among species arise when mutations alter the effects of developmental regulatory genes many genes interact to create developmental programs so changing the combination of toolkit genes results in diversity
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