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Discussion and Debate
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Physical & Life Sciences
Creation & Evolution
Cambrian explosion: Burgess Shale: punctuated equilibrium
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<blockquote data-quote="Naraoia" data-source="post: 60230071" data-attributes="member: 202689"><p>Are you talking about <a href="http://www.ncbi.nlm.nih.gov/Complete_Genomes/AnalEnzymes.html" target="_blank">analogous enzymes</a>? Because the whole point of that list is that these are NOT the same protein. They have no sequence similarity beyond that expected from chance, and they have completely different structures. This is why we think they evolved independently under similar selective pressures.</p><p></p><p> Note: <em>parallel</em> evolution does have a lot to do with common ancestors. The way I understand the term, it refers to cases where</p><p></p><p>(1) a feature evolved indepedently in two related lineages</p><p>(2) from a common precursor</p><p>(3) in a <strong>similar</strong> way</p><p></p><p>One example I can think of is the loss of pelvic spines in three- and nine-spine sticklebacks. Because a gene called <em>Pitx1</em> is a master regulator of pelvis development in both (and, in fact, in all vertebrates with a pelvis), switching it off in the pelvic area is a straighforward way of getting rid of the bones, and in fact <a href="http://www.pnas.org/content/103/37/13753" target="_blank">both stickleback species followed this route</a>.</p><p></p><p>Convergence refers to cases where features with similar functions evolved independently, but in <em>different</em> ways. The wings of insects and birds are a clear-cut example - different ancestral structures, totally different wings. The wings of the three groups of vertebrate fliers are slightly less so, since the starting material is the same forelimb - but the actual details of the resulting wings are quite different. (Incidentally, this is a point worth remembering: there is not a sharp line between parallelism and convergence.)</p><p></p><p> Once more, no one was trying to hide information. Walcott didn't think the Burgess Shale creatures were that special, and preparing and describing tens of thousands of specimens is a hell of a lot of work. For all people knew, there wasn't much <em>point</em> in going into all that trouble. No one quite realised how strange some of the creatures were until a bunch of crazy grad students (again, IIRC Gould didn't actually have a major part in reexamining Walcott's fossils) decided to fiddle around with them.</p><p></p><p>But you read <em>Wonderful Life</em>, didn't you? Where else would you be getting these ideas from?</p><p></p><p>(I should add that Gould's view of the Burgess Shale is pretty radical. Being radical was his modus operandi.)</p><p></p><p> Oh, right.</p><p></p><p>To the best of our knowledge, the last common ancestor of <em>animals</em> may have looked something like this:</p><p></p><p><img src="http://www.choano.org/choanowiki/images/6/63/Dayel_Protero_colony_100x.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>(Colony of <em>Proterospongia</em>/<em>Salpingoeca rosetta</em> by Mark Dayel from the King lab. Source: <a href="http://www.choano.org/wiki/Choanoflagellate_Gallery" target="_blank">ChoanoWiki</a>.)</p><p></p><p>This is a colonial choanoflagellate - choanoflagellates are the closest living relatives of animals based on molecular phylogenies, and the fact that <a href="http://www.ucmp.berkeley.edu/porifera/pororg.html" target="_blank">sponges</a> (which are animals) possess food-capturing cells very similar to these confirms the link.</p><p></p><p>The most recent common ancestor of <strong><em>bilaterian</em></strong> animals (which could be described in a very simplified way* as "anything with a head and tail end"), which most of the Cambrian explosion was about, is a tough nut. The outgroup (sponges, jellyfish, comb jellies and... <a href="http://en.wikipedia.org/wiki/Placozoa" target="_blank">Blobs</a>) are a diverse bunch each with their own derived quirks, and there is no living bilaterian that is an obviously good proxy for the last common bilaterian ancestor (or the Urbilaterian for short). We don't have a direct fossil record of the creature either, but there are several lines of evidence we might follow to narrow the possibilities:</p><p></p><p>(1) We know that it <a href="http://www.sciencedirect.com/science/article/pii/S0092867408000500" target="_blank">possessed several genetic pathways</a> that organise the body plans of distantly related modern bilaterians. Examples include Hox genes for defining regions along the head to tail axis, BMP signalling to define the back and belly sides, <em>Pax6</em> and <em>Six</em> to organise simple eyes, and possibly, Notch/Delta signalling to generate repeated body sections or repeated structures like the "rungs" of the prototypical ladder-like bilaterian nervous system. (Here's a <a href="http://www.nikonsmallworld.com/images/gallery2007/fullsize/Bergter-10331-3.jpg" target="_blank">very nice image</a> of an annelid embryo with its nervous system stained in yellow - you can see the repetition here very clearly. While annelids are segmented animals with pretty much everything in them repeating like that, ladder-like nervous systems also occur in animals no one would call "segmented", for example, <a href="http://www.cdb.riken.jp/jp/04_news/annual_reports/2004/webhelp/fig/lab1_01fig2.htm" target="_blank">flatworms</a>.)</p><p></p><p>(2) We know that possessing the genetic pathways "for" something doesn't necessarily mean that you also possess a full-blown version of that something. Case in point: the gene expression pattern that defines hands and feet actually originated <a href="http://www.ncbi.nlm.nih.gov/pubmed/17522683" target="_blank">well before</a> <a href="http://www.ncbi.nlm.nih.gov/pubmed/17710153" target="_blank">hands and feet</a> <a href="http://onlinelibrary.wiley.com/doi/10.1002/jez.b.21197/abstract" target="_blank">themselves</a>.</p><p></p><p>(3) We know that the Urbilaterian lived quite far into the Precambrian - certainly before definitive bilaterians like <em>Kimberella</em> (~555 Mya, mid-Ediacaran). Clearly, the last common ancestor of a group had to live before any recognisable <em>sub</em>groups of that group appeared, and <em>Kimberella</em> is generally considered to be a relative of molluscs, many branches into the bilaterian tree.</p><p></p><p>(4) We know that the Precambrian is very poor in trace fossils like trackways and burrows, which quickly become abundant in the Cambrian. <a href="http://dev.biologists.org/content/129/13/3021.abstract" target="_blank">This suggests</a> that the ancestral bilaterian was likely relatively small and simple - large, complex, active bilaterians tend to leave traces.</p><p></p><p>Putting all of that together, I think this Urbilateria guy was most likely a simple soft-bodied "worm" with a head and tail end and perhaps a flatworm sort of nervous system.</p><p></p><p>*Very simplified because of the inevitable exceptions. For example, starfish and other echinoderms are undisputed bilaterians, but their adults have (more or less) radial symmetry and not much of a head.</p><p></p><p>***</p><p></p><p>... by the way, that's what you get when you bring up a subject I've actually read a lot about. Thank (mostly) a third-year essay assignment for the length of the above <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite8" alt=":D" title="Big Grin :D" loading="lazy" data-shortname=":D" /> If anything went over your head, I'm happy to explain. Just ask <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink ;)" loading="lazy" data-shortname=";)" /></p><p></p><p> But both are utterly irrelevant to whether life on earth (or animals, or anything on earth) had a common ancestor. The evidence for common ancestry - nested hierarchies and fossils - is equally valid regardless of the detailed mechanisms.</p><p></p><p> When you are only <em>hypothesising</em>, anything goes (if you're willing to risk being laughed at <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite7" alt=":p" title="Stick Out Tongue :p" loading="lazy" data-shortname=":p" />). Not once you start collecting evidence.</p><p></p><p> We're certainly far less confused about our beliefs than you are <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite7" alt=":p" title="Stick Out Tongue :p" loading="lazy" data-shortname=":p" /></p><p></p><p> I don't have to blend opposites. The reason there are opposites is because the evidence isn't sufficient to decide either way.</p><p></p><p>(The determinism of evolution is not a black and white issue anyway; it's a continuum from "nothing would turn out the same" to "everything would turn out the same".)</p><p></p><p> I'm sure that's part of it, but I'm very reluctant to believe we've already found "the" basic mechanism <img src="/styles/default/xenforo/smilies/old/kawaii.gif" class="smilie" loading="lazy" alt="^_^" title="Kawaii ^_^" data-shortname="^_^" /> As to the billion years, there isn't really good evidence that animals <em>existed</em> 1.5 billion years ago, is there? They sort of had to be around for them to radiate explosively.</p></blockquote><p></p>
[QUOTE="Naraoia, post: 60230071, member: 202689"] Are you talking about [URL="http://www.ncbi.nlm.nih.gov/Complete_Genomes/AnalEnzymes.html"]analogous enzymes[/URL]? Because the whole point of that list is that these are NOT the same protein. They have no sequence similarity beyond that expected from chance, and they have completely different structures. This is why we think they evolved independently under similar selective pressures. Note: [I]parallel[/I] evolution does have a lot to do with common ancestors. The way I understand the term, it refers to cases where (1) a feature evolved indepedently in two related lineages (2) from a common precursor (3) in a [B]similar[/B] way One example I can think of is the loss of pelvic spines in three- and nine-spine sticklebacks. Because a gene called [I]Pitx1[/I] is a master regulator of pelvis development in both (and, in fact, in all vertebrates with a pelvis), switching it off in the pelvic area is a straighforward way of getting rid of the bones, and in fact [URL="http://www.pnas.org/content/103/37/13753"]both stickleback species followed this route[/URL]. Convergence refers to cases where features with similar functions evolved independently, but in [I]different[/I] ways. The wings of insects and birds are a clear-cut example - different ancestral structures, totally different wings. The wings of the three groups of vertebrate fliers are slightly less so, since the starting material is the same forelimb - but the actual details of the resulting wings are quite different. (Incidentally, this is a point worth remembering: there is not a sharp line between parallelism and convergence.) Once more, no one was trying to hide information. Walcott didn't think the Burgess Shale creatures were that special, and preparing and describing tens of thousands of specimens is a hell of a lot of work. For all people knew, there wasn't much [I]point[/I] in going into all that trouble. No one quite realised how strange some of the creatures were until a bunch of crazy grad students (again, IIRC Gould didn't actually have a major part in reexamining Walcott's fossils) decided to fiddle around with them. But you read [I]Wonderful Life[/I], didn't you? Where else would you be getting these ideas from? (I should add that Gould's view of the Burgess Shale is pretty radical. Being radical was his modus operandi.) Oh, right. To the best of our knowledge, the last common ancestor of [I]animals[/I] may have looked something like this: [IMG]http://www.choano.org/choanowiki/images/6/63/Dayel_Protero_colony_100x.jpg[/IMG] (Colony of [I]Proterospongia[/I]/[I]Salpingoeca rosetta[/I] by Mark Dayel from the King lab. Source: [URL="http://www.choano.org/wiki/Choanoflagellate_Gallery"]ChoanoWiki[/URL].) This is a colonial choanoflagellate - choanoflagellates are the closest living relatives of animals based on molecular phylogenies, and the fact that [URL="http://www.ucmp.berkeley.edu/porifera/pororg.html"]sponges[/URL] (which are animals) possess food-capturing cells very similar to these confirms the link. The most recent common ancestor of [B][I]bilaterian[/I][/B] animals (which could be described in a very simplified way* as "anything with a head and tail end"), which most of the Cambrian explosion was about, is a tough nut. The outgroup (sponges, jellyfish, comb jellies and... [URL="http://en.wikipedia.org/wiki/Placozoa"]Blobs[/URL]) are a diverse bunch each with their own derived quirks, and there is no living bilaterian that is an obviously good proxy for the last common bilaterian ancestor (or the Urbilaterian for short). We don't have a direct fossil record of the creature either, but there are several lines of evidence we might follow to narrow the possibilities: (1) We know that it [URL="http://www.sciencedirect.com/science/article/pii/S0092867408000500"]possessed several genetic pathways[/URL] that organise the body plans of distantly related modern bilaterians. Examples include Hox genes for defining regions along the head to tail axis, BMP signalling to define the back and belly sides, [I]Pax6[/I] and [I]Six[/I] to organise simple eyes, and possibly, Notch/Delta signalling to generate repeated body sections or repeated structures like the "rungs" of the prototypical ladder-like bilaterian nervous system. (Here's a [URL="http://www.nikonsmallworld.com/images/gallery2007/fullsize/Bergter-10331-3.jpg"]very nice image[/URL] of an annelid embryo with its nervous system stained in yellow - you can see the repetition here very clearly. While annelids are segmented animals with pretty much everything in them repeating like that, ladder-like nervous systems also occur in animals no one would call "segmented", for example, [URL="http://www.cdb.riken.jp/jp/04_news/annual_reports/2004/webhelp/fig/lab1_01fig2.htm"]flatworms[/URL].) (2) We know that possessing the genetic pathways "for" something doesn't necessarily mean that you also possess a full-blown version of that something. Case in point: the gene expression pattern that defines hands and feet actually originated [URL="http://www.ncbi.nlm.nih.gov/pubmed/17522683"]well before[/URL] [URL="http://www.ncbi.nlm.nih.gov/pubmed/17710153"]hands and feet[/URL] [URL="http://onlinelibrary.wiley.com/doi/10.1002/jez.b.21197/abstract"]themselves[/URL]. (3) We know that the Urbilaterian lived quite far into the Precambrian - certainly before definitive bilaterians like [I]Kimberella[/I] (~555 Mya, mid-Ediacaran). Clearly, the last common ancestor of a group had to live before any recognisable [I]sub[/I]groups of that group appeared, and [I]Kimberella[/I] is generally considered to be a relative of molluscs, many branches into the bilaterian tree. (4) We know that the Precambrian is very poor in trace fossils like trackways and burrows, which quickly become abundant in the Cambrian. [URL="http://dev.biologists.org/content/129/13/3021.abstract"]This suggests[/URL] that the ancestral bilaterian was likely relatively small and simple - large, complex, active bilaterians tend to leave traces. Putting all of that together, I think this Urbilateria guy was most likely a simple soft-bodied "worm" with a head and tail end and perhaps a flatworm sort of nervous system. *Very simplified because of the inevitable exceptions. For example, starfish and other echinoderms are undisputed bilaterians, but their adults have (more or less) radial symmetry and not much of a head. *** ... by the way, that's what you get when you bring up a subject I've actually read a lot about. Thank (mostly) a third-year essay assignment for the length of the above :D If anything went over your head, I'm happy to explain. Just ask ;) But both are utterly irrelevant to whether life on earth (or animals, or anything on earth) had a common ancestor. The evidence for common ancestry - nested hierarchies and fossils - is equally valid regardless of the detailed mechanisms. When you are only [I]hypothesising[/I], anything goes (if you're willing to risk being laughed at :P). Not once you start collecting evidence. We're certainly far less confused about our beliefs than you are :P I don't have to blend opposites. The reason there are opposites is because the evidence isn't sufficient to decide either way. (The determinism of evolution is not a black and white issue anyway; it's a continuum from "nothing would turn out the same" to "everything would turn out the same".) I'm sure that's part of it, but I'm very reluctant to believe we've already found "the" basic mechanism ^_^ As to the billion years, there isn't really good evidence that animals [I]existed[/I] 1.5 billion years ago, is there? They sort of had to be around for them to radiate explosively. [/QUOTE]
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