Bruce Toback writes: > Denys writes: > > >Besides here in Houston, we consume spiders (also known as crawdads or > >mudbugs, but an arachnoid by any other name, like crabs and lobsters.) > > Wirt is no doubt composing a much more detailed reply, but meanwhile, > lobsters and true crabs aren't arachnoids. Horseshoe and king crabs are > cousins to spiders, but crustaceans such as lobsters and crayfish are not. > > I look forward to Wirt's exposition of the precise evolutionary > relationship among the three groups :-). I really doubt that anyone is TRULY looking forward to this, but I'll answer Bruce's comment anyway. To begin with, arachNOIDs are not living organisms. They are human-engineered, giant, eight-legged metallic Japanese transformer-like creatures that live in the glens just outside Osaka and ravage the cities and the citizenry of Japan at night. ArachNIDs, on the other hand, are spiders, mites, scorpions and solpugids (sun spiders) -- but crawdads nor crabs are not, exactly as Bruce states. Arachnida is the scientific name of the Class name given to these animals. Arthropoda (jointed-leg animals) is the name of their Phylum, a more inclusive set of animals yet again (which does include crawdads, lobsters, insects, and horseshoe crabs, among others). We are now virtually 100% convinced that life arose on this planet just once (or if it didn't, whatever other forms that may have temporarily existed early on were outcompeted by the form that we find present here now). We've come to this conclusion because all of the code, for every living species, is essentially identical and can be arranged in a nearly perfect hierarchy. The mechanism that has allowed such a great diversity of speciation and diversification is nothing more complicated than the invention of object-oriented, hierarchical code reuse. To quote the great biologist, Art Bahrs, the architecture of life is much like: "32 bit extensions and a graphical shell [on top of] a 16 bit patch to an 8 bit operating system originally coded for a 4 bit microprocessor, written by a 2 bit company, that can't stand 1 bit of competition." Every cell in your body contains all of the information to build another you (with a few minor exceptions, of course. Red blood cells, for example, are so sacrificial that they lose their nuclei, where the DNA of inheritance resides, soon after maturity). Nonetheless, every piece of dandruff you slough contains all of the code that defines not only you, but also your essential mammalian nature, that was built on a reptilian platform, originally coded for a far simpler chordate (an animal with a spinal chord), built out of a differentiated colony of originally free-living nucleated cells -- that could never stand one bit of competition, at any prior stage of complexity. The oldest code in your tissues is now estimated to be at least 3.5 billion years old and is associated with your cells being simply a eucaryote (a fully nucleated, extremely complex cellular structure). And nature, just like Bill Gates, makes do with what he has available to him at the time. Genes come in families -- and occasionally superfamilies. One of these superfamilies is the opsin family. These genes encode the proteins that are the bleachable pigments that allow you (and every other animal) its vision. However, the invention of the intitial opsin gene is now believed to be about a billion years old, a quarter-billion years before multicellularity itself (the Metazoa/Metaphyta/Fungi) was invented and half a billion years before the first primitive, image-forming eye was evolved. This code re-use hierarchy is noted in the most basic Linnean classification scheme (which Linneaus only guessed at -- but which we can now verify with near certainty). Linneaus' basic hierarchy is (using, for example, HP's former spider-of-the-month): Kingdom (Animalia) Phylum (Arthropoda) Class (Arachnida) Order (Araneae) Family (Araneidae = Argiopidae) Genus (Argiope) Species (aurantia?) This path makes the process seem unusually short and direct -- but there are 2 million to 100 million species of animals currently present on the planet (most estimates range in the 8-20 million numbers, but we really have no idea. We're racing to at least catalog most species before we finish paving over the planet). All of these animals are in the same Kingdom. And there are five Kingdoms that are generally recognized (Animalia, Plantae, Fungi, Monera, and Protista). There is an extraordinary resource available on the web that directly demonstrates this process of hierarchical code re-use. Its URL is: http://www3.ncbi.nlm.nih.gov/htbin-post/Taxonomy/wgetorg?id=2759 The human genome is not the only genome that is being sequenced. For every species shown in the National Institute of Health's taxonomic hierarchy above, at least some fraction of the species' genome exists and can be read out. What you're looking at is a database, organized by commonality of descent -- but one where all of the code is similar and branches from one common point of origination. You may find that may have some trouble navigating the hierarchy simply because Linneaus' hierarchy is broken into a great many more divisions than I've shown above. But don't be discouraged. Most biologists couldn't navigate it with ease either (in part because people keep changing the names). The pointer above begins at the point of commonality of code that is the Eukaryotes (the fully nucleated cells that are characteristic of plants, animals and fungus). The earth was only ca. 1.5 billion years old at the time of the formation of this cell type. To get to HP's spider (or as close as we can for the species represented), find the Fungi/Metazoa group and click on it. At this level of code commonality, we, as a species, are indistinguishable from mushrooms. On the following screen, click on Bilateria (a subdivision of Metazoa, which is all of the bilaterally symmetric animals that you can see with your naked eye -- and a lot that you can't). At this level, we're indistinguishable from Denys' crawdads. On the next screen, click on Arthropoda (the jointed legged animals). This is the point where we, as chordates, diverge from the crawdads (about 500-530 million years ago). On the next screen, click on Araneae, the order of true spiders -- and we're basically home: if HP's spider were listed. The end point is a binomial name for a species (e.g., Homo sapiens, Rattus rattus, or Argiope aurantia). To get to Denys' crawdads, back up to the Arthropoda and click on the Eucarida, of which the Decapoda (ten-legged animals) are a subdivision. It's important to note, however, that this NIH hierarchy is a little misleading. Of all of the animals large enough to be seen, 97% of the 10 million species are in the Phylum Arthropoda. Only a very small portion of all of these animals are listed here. Wirt Atmar (If anyone finds any of this upsetting or significantly off-topic, I have set up a special complaint department just for this purpose. Please mail your complaints to <[log in to unmask]>, where they will handled with courtesy and dispatch).