To: All Msg #255, May0493 08:06PM Subject: the history of life I posted a brief history of

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From: Chris Colby To: All Msg #255, May-04-93 08:06PM Subject: the history of life Organization: animal -- coelomate -- deuterostome From: colby@bu-bio.bu.edu (Chris Colby) Message-ID: <118233@bu.edu> Newsgroups: talk.origins I posted a brief history of life a month (?) or so ago and asked for criticisms(*). Here is a significantly expanded version. I plan on adding this to my FAQ when I'm done. Ignore the writing style, or lack of it, for now -- I'm just looking for pointers on content. I'm out of my field (mostly) on this. Thanks - Chris (*) thanks to all who responded ------------------------------------------------------------------------- A BRIEF HISTORY OF LIFE Life evolved in the sea. It stayed there for the majority of the history of earth. The first replicating molecules were most likely RNA. RNA is a nucleic acid similar to DNA. In laboratory studies it has been shown that some RNA sequences have catalytic capabilities. Most importantly, certain RNA sequences act as polymerases -- enzymes that form strands of RNA from its monomers. This process of self- replication is the crucial step in the formation of life. The common ancestor of all life probably used RNA as its genetic material and was most likely a progenote -- an organism whose genes were not arranged into a genome. The progenote gave rise to three major lineages of life. These are: the prokaryotes ("ordinary" bacteria), archaebacteria (thermophilic, methanogenic and halophilic bacteria) and eukaryotes. Eukaryotes include protists (single celled organisms like amoebas and diatoms and a few multicelluar forms such as kelp), fungi (including mushrooms and yeast), plants and animals. Eukaryotes and archaebacteria are the two most closely related of the three. The process of translation (making protein from the instructions on a messenger RNA template) is similar in these lineages, but the organization of the genome and transcription (making messenger RNA from a DNA template) is very different in prokaryotes than in eukaryotes and archaebacteria. Scientists interpret this to mean that the progenote (common ancestor) was RNA based; it gave rise to two lineages that independently formed a DNA genome and hence independently evolved mechanisms to transcribe DNA into RNA. The first cells must have been anaerobic because there was no oxygen in the atmosphere. In addition, they were probably thermophilic ("heat-loving") and fermentative. Rocks as old as 3.5 Billion years old have yielded prokaryotic fossils. Specifically, some rocks from Australia called the Warrawoona series give evidence of bacterial communities organized into structures called stromatolites. Fossils like these have subsequently been found all over the world. These mats of bacteria still form today in a few locales (for example, Shark Bay Australia). Bacteria are the only life forms found in the rocks for long, long time -- fungi-like things appear about 900 MYA (0.9 Billion years ago). Somewhere along the way, photosynthesis evolved. Photosynthesis is a process that allows organisms to harness sunlight to manufacture sugar from simpler precursors. The first photosystem to evolve (PSI) uses light to convert CO2 and H2S to glucose. This process releases sulfur as a waste product. Later a second photosystem (PSII) evolved, probably from a duplication of the first photosystem. Organisms with PSII use both photosystems in conjunction to convert C02 and water (H2O) into glucose. This process releases oxygen as a waste product. Anoxygenic (or H2S) photosynthesis, using PSI, is seen in living purple and green bacteria. Oxygenic (or H2O) photosynthesis, using PSI and PSII, takes place in cyanobacteria. Cyanobacteria are closely related to and hence probably evolved from purple bacterial ancestors. Green bacteria is an outgroup. Since oxygenic bacteria are a lineage within a cluster of anoxygenic lineages, scientists infer that PSI evolved first. This also corroborates with geological evidence. Green plants and algae also use PSI and PSII for photosynthesis. In these organisms, photosynthesis occurs in organelles (membrane bound structures within the cell) called chloroplasts. These organelles originated as free living bacteria related to the cyanobacteria that were engulfed by ur-eukaryotes and eventually entered into an endosymbiotic relationship. This endosymbiotic theory of eukaryotic organelles was championed by Lynn Margulis. Originally very controversial, this theory is now virtually universally accepted. One key line of evidence in support of this idea came when the DNA inside chloroplasts was sequenced -- the gene sequences were more similar to free-living cyanobacteria sequences than to sequences from the plants the chloroplasts resided in. The advent of photosystem II brought about a large change in the atmosphere of earth -- the "oxygen holocaust". Oxygen is a very good electron acceptor and can be very damaging to living organisms. Many bacteria are anaerobic and die almost immediately in the presence of oxygen. Other organisms, like animals, have special ways to avoid cellular damage due to this element (and in fact require it to live.) Initially, when oxygen began building up in the environment, it was neutralized by materials already present. Iron, which existed in high concentrations in the sea was oxidized and precipitated. Evidence of this can be seen in banded iron formations from this time, layers of iron deposited on the sea floor. As one geologist put it -- "the world rusted". Eventually, it grew to high enough concentrations to be dangerous to living things. In response, many species went extinct, some continued (and still continue) to thrive in anaerobic microenvironments and several lineages independently evolved oxygen respiration. One lineage to evolve oxygen respiration was the purple bacteria. Purple bacteria also enabled the eukaryotic lineage to become aerobic. Eukaryotic cells have membrane bound organelles called mitochondria that take care of respiration for the cell. These are also endosymbionts just like chloroplasts. Mitochondria formed this symbiotic relationship very early in eukaryotic history, all but a few groups of eukaryotic cells have mitochondria. Later, a few lineages picked up chloroplasts. Red algae picked up ur-chloroplasts from the cyanobacterial lineage. Green algae, the group plants evolved from, picked up different ur-chloroplasts from a prochlorophyte, a lineage closely related to cyanobacteria. Prior to the Cambrian (~600 MYA), animals start appearing; the first animals dating from just before the Cambrian were found in rocks near Adelaide, Australia. They are called the Ediacarian fauna and have subsequently been found in other locales as well. It is unclear if these forms have any surviving descendents. Some look a bit like Cnidarians (jellyfish, sea anemones and the like). The Cambrian 'explosion' produced a wide variety of animals. Probably all the phyla (the second highest taxonomic category) of animals appeared around the Cambrian. Some paleontologists think more animal phyla were present then than now. The animals of the Burgess shale are an example of Cambrian animal fossils. These fossils, from Canada, show a bizarre array of creatures. Although creationists are fond of pointing to the Cambrian explosion as evidence of their views -- they ignore four things 1.) Evidence of life (including animals) prior to the Cambrian 2.) Although quick, the Cambrian explosion is not instantaneous in geologic time 3.) Although all the phyla of animals came into being, these were _not_ the modern, derived forms we see today. Our own phylum (which we share with other mammals, reptiles, birds, amphibians and fish) was represented by a small, sliver-like thing called _Pikia_. 4.) Plants were not yet present. The Cambrian explosion is not evidence of a single creation event producing the current biota. Following the Cambrian, the number of marine families leveled off at a little less than 200. The Ordovician explosion (~500MYA) followed. This 'explosion', larger than the Cambrian, introduced numerous families of the Paleozoic fauna (including crinoids, articulate brachiopods, cephalopods and corals). The Cambrian fauna, (trilobites, inarticulate brachiopods, etc.) declined slowly during this time. By the end of the Ordovician, the Cambrian fauna had mostly given way to the Paleozoic fauna and the number of marine families was just over 400. It stayed at this level until the end of the Permian period. Somewhere in between these two points, plants and fungi (in symbiosis) invaded the land (~400 MYA). The first plants were moss- like and required moist environments to survive. Later, evolutionary developments such as a waxy cuticle and a vascular system allowed some plants (for example ferns) to exploit more inland environments. The first vascular land plant known is _Cooksonia_, a spiky, branching, leafless structure. At the same time, or shortly thereafter, arthropods (myriapods -- centipedes and millipedes) followed plants onto the land. By the Devonian period (~380 MYA) vertebrates had moved onto the land, _Ichthyostega_ is the among the first known land vertebrates, an amphibian. It was found in Greenland and was derived from lobe- finned fishes called Rhipidistians. Amphibians gave rise to reptiles, animals with scales to decrease water loss and a shelled egg permitting young to be hatched on land. Among the earliest well preserved reptiles is _Hylonomus_, from rocks is Nova Scotia. The Permian extinction (~250MYA) was the largest extinction in history. The last of the Cambrian Fauna went extinct. The Paleozoic fauna took a nose dive from about 300 families to about 50. It is estimated that 96% of all species in existence met their end. Some estimate that as many as 50% of all families went extinct (you have to kill of 100% of the species in a family before it goes extinct, hence the difference between the two numbers.) Following this event, the Modern fauna, which had been slowly expanding since the Ordovician, took over. The Modern fauna (including fish, bivalves, gastropods and crabs) was barely affected by the Permian extinction and increased to over 600 marine families at present. (The Paleozoic fauna held steady at about 100 families.) A second extinction event shortly following the Permian kept animal diversity low for awhile. The flora as well as the fauna changed following the Permian. During the Carboniferous (the period just prior to the Permian) and in the Permian the landscape was dominated by ferns and their relatives. After the Permian extinction, gymnosperms (ex. pines) became much more abundant. Gymnosperms had evolved seeds (which ferns lack) which helped their ability to disperse. Gymnosperms also evolved pollen, encased sperm which allowed for more outcrossing. In ferns, sperm must swim from the male organs to the female organs During the Jurassic (~200 MYA) and Cretaceous (~150MYA) periods the dinosaurs ruled and flowering plants (angiosperms), together with insects, diversified. Dinosaurs evolved from reptiles. [What's the name of that recently discovered, early dinosaur?] One modification may have been a key factor in their success -- posture. Amphibians and reptiles have a splayed stance and walk with an undulating pattern because their limbs are modified from fins and their gait is modified from the movement a fish makes when swimming. These animals cannot sustain continued locomotion because they cannot breathe while they move; their undulating movement compresses their chest cavity. Thus, they must stop every few steps and breath before continuing on their way. Dinosaurs evolved an upright stance (similar to the upright stance mammals independently evolved) and this allowed for continual locomotion. In addition, dinosaurs evolved to be warm-blooded. Warm-bloodedness allows an increase in the vigor of movements in erect organisms. Splay stanced organisms would probably not benefit from warm-bloodedness. Angiosperms evolved two key adaptations that allowed them to displace gymnosperms as the dominant fauna -- fruits and flowers. Fruits allow for animal based seed dispersal (and deposition with plenty of fertilizer 8-). Flowers evolved to facilitate animal, especially insect, based pollen dispersal. Angiosperms currently dominate the flora of the world -- over three fourths of all living plants are angiosperms. Insects, who radiated a great deal along with Angiosperms, dominate the fauna of the world. Over half of _all_ named species are insects. One third of this number are beetles. The end of the Cretaceous (~65 MYA) is marked by a minor mass extinction that was the demise of all the lineages of dinosaurs save the birds. Once the dinosaurs were out of the picture, mammals -- previously confined to nocturnal, insectivorous niches -- diversified. _Morgonucudon_ , a contemporary of dinosaurs, is an example of one of the first mammals. The study of the history of life on this planet reveals a planet in flux. The abundance of various lineages varies wildly across geologic time. New lineages can evolve and radiate out across the face of the planet, pushing older lineages to extinction, or relictual existences in protected refugia and/or suitable microhabitats. Overall, diversity has increased since the beginning of life. This increase is, however, interrupted numerous times by mass extinctions. Diversity appears to have hit an all-time high just prior to the evolution of humans. It has decreased at an ever-increasing pace since. The correlation is probably causal. Chris Colby --- email: colby@bu-bio.bu.edu --- "'My boy,' he said, 'you are descended from a long line of determined, resourceful, microscopic tadpoles--champions every one.'" --Kurt Vonnegut from "Galapagos"

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