# Common ancestor of vertebrates and arthropods? **URL:** \ **Category:** Factual Questions\ **Created:** [August 7, 2013, 1:55pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584 "2013-08-07T13:55:52Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 7, 2013, 1:55pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/1 "2013-08-07T13:55:52Z")
Arthropods have an exoskeleton; vertebrates have an endoskeleton. I’m not a biologist, but these body forms are so diametrically opposed that I’m wondering what creature is regarded as their common evolutionary ancestor. Anyone know? ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [August 7, 2013, 2:08pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/2 "2013-08-07T14:08:42Z")
I’m not at home right now, so I don’t have it with me, but I highly recommend you read _The Ancestor’s Tale_ by Richard Dawkins. I’ve read dozens of books about evolution and that is one of the best there is. It takes you back in time from us to the very beginnings of life, and at each stop he teaches something new about evolution. [Link to Amazon.com](http://www.amazon.com/Ancestors-Tale-Pilgrimage-Dawn-Evolution/dp/061861916X/ref=sr_1_1?s=books&ie=UTF8&qid=1375884489&sr=1-1&keywords=the+ancestor%27s+tale) ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [August 7, 2013, 2:17pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/3 "2013-08-07T14:17:43Z")
Also, don’t be deceived by superficial differences. You may think you’re a lot different from a fruit fly, but we share many of the same genes and their different body plan doesn’t look quite so different once you understand the genes that regulate it. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [August 7, 2013, 2:18pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/4 "2013-08-07T14:18:22Z")
They also have profound differences in central nervous system configuration. Vertebrates have their spinal cord on their dorsal side (their back / top) while arthropods have their ganglion string ventrally (front / bottom). This indicates a last common ancestor being a worm with a neural net rather than a central/ peripheral nerve configuration. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [August 7, 2013, 2:42pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/5 "2013-08-07T14:42:51Z")
Whatever it was, it predates the Cambrian Explosion. The basic body plans were already in place then. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 7, 2013, 2:44pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/6 "2013-08-07T14:44:04Z")
> [@Machine\_Elf](#): > > Arthropods have an exoskeleton; vertebrates have an endoskeleton. I’m not a biologist, but these body forms are so diametrically opposed that I’m wondering what creature is regarded as their common evolutionary ancestor. Anyone know? There is no clear candidate for a common ancestor, either in living forms descended from that ancestor, or in fossil forms. Since the skeletal system is so different, the ancestor probably did not have any kind of skeletal system (which would make finding a fossil ancestor unlikely). Arthropods and vertebrates both share a segmented body plan (less obviously in vertebrates, but it can be seen in primitive chordates like [Amphioxus](http://www.blc.arizona.edu/courses/schaffer/182/chordate/Amphiox-1.jpg)). The development of segmentation is governed by similar genes, suggesting that the ancestor of both may also have been segmented. However, the actual process of the development of segmentation differs, so that it is possible it arose independently in both groups. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [August 7, 2013, 2:46pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/7 "2013-08-07T14:46:36Z")
There’s nothing in the fossil record that appears to be the “[ur-bilatarian](http://en.wikipedia.org/wiki/Urbilaterian)”, the ancestor of arthropods and vertebrates (and molluscs). Hypothetically, however, the ur-bilaterian is probably some sort of tiny soft-bodied worm like thing that lived before the Cambrian. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [August 7, 2013, 2:51pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/8 "2013-08-07T14:51:23Z")
> [@DrFidelius](#): > > This indicates a last common ancestor being a worm with a neural net rather than a central/ peripheral nerve configuration. Or just a worm-like thing with a central nerve cord, but without a strong front-back distinction externally. Some later evolved to have a preference of holding their bodies nerve-cord-side-up and some of those became vertebrates, while others evolved to have a preference of holding their bodies nerve-cord-side-down and some of those became arthropods. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [August 7, 2013, 3:04pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/9 "2013-08-07T15:04:38Z")
> [@leahcim](#): > > Or just a worm-like thing with a central nerve cord, but without a strong front-back distinction externally. Some later evolved to have a preference of holding their bodies nerve-cord-side-up and some of those became vertebrates, while others evolved to have a preference of holding their bodies nerve-cord-side-down and some of those became arthropods. Possible, but molecular and development evidence indicates otherwise. Both arthropods and vertebrates use the same molecular signals to determine which end is “up”. It’s a less parsimonious explanation to say that the ur-bilatarian flipped its posture as well as its whole dorsal/ventral patterning system relative to the nerve cord. IIRC, one common hypothesis is that the ur-bilatarian used the same dorsal-ventral patterning signals, but had both dorsal and ventral nerve cords. One lineage the dorsal cord and lost the ventral, and the other lineage did the opposite. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 7, 2013, 10:41pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/10 "2013-08-07T22:41:51Z")
Couldn’t you also have the cord migrate through the body, with an intermediate form where the cord ran right down the middle? ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [August 7, 2013, 11:08pm UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/11 "2013-08-07T23:08:32Z")
> [@Chronos](#): > > Couldn’t you also have the cord migrate through the body, with an intermediate form where the cord ran right down the middle? There’s a gut in the way. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [August 8, 2013, 12:18am UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/12 "2013-08-08T00:18:47Z")
QUOTE=DrFidelius;16550866]There’s a gut in the way. [/QUOTE] Well, the arthropod nerve chord [passes \*around \*the gut](http://en.wikipedia.org/wiki/File:Arthropod_body_struct_01.png), so that’s not a barrier. In both arthropods and chordates, the nerve chord starts with the “brain” above the gut. In Chordates it then runs the length of the body, always above the gut. In arthropods it immediately descends, splitting into two strands to pass either side of the gut, and then runs the length of the body below the gut. This is usually assumed to be because the ancestral organism was a burrower that had both an ventral and dorsal nerve chord, with the “brain” wrapped around the start of the gut like a modern [roundworm](http://www.google.com/imgres?um=1&client=firefox-a&sa=N&rls=org.mozilla:en-US:official&biw=1536&bih=670&hl=en&tbm=isch&tbnid=qQTDib4ZKn3_6M:&imgrefurl=http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookDiversity_7.html&docid=XFprvc4zoWNZtM&imgurl=http://www.emc.maricopa.edu/faculty/farabee/biobk/nematode.gif&w=548&h=218&ei=E9kCUoiXIMfolAXanIBI&zoom=1&ved=1t:3588,r:16,s:0,i:127&iact=rc&page=1&tbnh=133&tbnw=336&start=0&ndsp=20&tx=239&ty=64). Those organisms that developed a crawling lifestyle required more tactile information about the substrate they crawled over, and relied on the eyes and antennae to provide any information about what was above them. [As a result the ventral nerve chord became better developed as the dorsal was progressively lost, yet the brain remained dorsal to collect information from the eyes and antennae.](http://micoleinahh.files.wordpress.com/2013/01/i10-82-annelid.jpg) In contrast, those organisms that were free swimming like our ancestors required tactile information from above as well as below. That could have led to equal development of both dorsal and ventral chords, but in the case of our ancestors the [hyperdevelopment of the foregut](http://www.reefkeeping.com/issues/2005-03/rs/images/image004.jpg) associated with filter feeding made it impossible to pass the nerve chord around the gut. As a result the ventral nerve chord was lost. Through good fortune, that also made it easier for our ancestors to develop intelligence. Without any need to pass around the gut, the brain could grow unconstrained. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 10, 2013, 1:06am UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/13 "2013-08-10T01:06:13Z")
And the turtle demonstrates that at least a partial exoskeleton can develop from and endoskeleton. (I think it is now thought that the shell comes basically from modified spinal bones. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 10, 2013, 1:41am UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/14 "2013-08-10T01:41:13Z")
> [@Hari\_Seldon](#): > > And the turtle demonstrates that at least a partial exoskeleton can develop from and endoskeleton. (I think it is now thought that the shell comes basically from modified spinal bones. The top shell is composed mainly of modified ribs. Turtles are the only vertebrates in which the [pectoral girdle and pelvic girdles are within the rib cage.](http://valuedminds.com/wp-content/uploads/Turtle-Shell.jpeg) The arthropod exoskeleton is entirely different in character of course, being made up largely of chitin instead of bone. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [August 11, 2013, 1:45am UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/15 "2013-08-11T01:45:32Z")
According to [Wikipedia](http://en.wikipedia.org/wiki/Protostome), “phylogenetic trees constructed using 18S ribosomal RNA” place arthropods in a Clade Protostomia along with several other phyla including mollusks, earthworms, flatworms, roundworms, and rotifers. Vertebrates are in a separate clade, Deuterostomia, which also includes starfish and acorn worms. The cited article mentions differences in embryo development between the two clades. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 11, 2013, 2:12am UTC](https://boards.straightdope.com/t/common-ancestor-of-vertebrates-and-arthropods/665584/16 "2013-08-11T02:12:19Z")
> [@septimus](#): > > The cited article mentions differences in embryo development between the two clades. Yes, in Protostomes the first opening from which the digestive tract develops eventually becomes the mouth, while in Deuterostomes the first opening becomes the anus. So our nervous systems are not only upside down relative to arthropods, we’re also ass-backwards.😃