ReggieM citing Simmons << “Velvet worms are thought to be descended of insects, but the evidence for this is scanty…” >>
OK, now the evolution of the Velvet Worm. I’ve already done whales, giraffes, now worms.
HONESTY ALERT: There appears to be a genuine difference of opinion here. Prothero says the velvet worm are the transitional links between the arthropods (insects, etc) and the group they split from in the Cambrian, the nematodes, relying on Brusca/Brusca
Invertebrates (Sinauer Assoc, 1990):
“As outlined by Brusca and Brusca (1990:683), there are many other features that unite onychophorans [velvet worms] with arthropods and make them outstanding transitional forms between the phylum Nematoda and the phylum Arthropoda.” (Prothero,
Evolution: What the Fossils Say, page 193).
More recent research (since 1990) appears to put the velvet worms into the arthropods group. See below.
Prothero’s book covers the velvet worm on pages 193-194, which was recently summarized in his article
"Evolution: What Missing Link?:
“A classic example of a transitionary form links the arthropods [insects, spiders, scorpions, crustaceans and their relatives] to the lineage they split from in the Cambrian, namely, the nematode worms.
These are the ‘velvet worms’ or Onychophora. In many respects, the velvet worms resemble nematodes, but they also have key attributes of the arthropods – most notably segmented legs that end in hooked claws. They also have many other features found in arthropods but not nematodes, including an outer layer made of chitin, which they moult on a regular basis, antennae, compound eyes and arthropod-like mouthparts…
You could not ask for a better ‘missing link’ between the nematodes and the arthropods, except it’s not missing – we’ve known about velvet worms for over a century in both the living fauna and the fossil record.” (Prothero,
New Scientist, 8 March 2008)
But we’re not limited to Prothero. For the evolution of worms and insects in general, we’ll want to refer to:
Evolution of the Insects by Grimaldi/Engel (Cambridge Univ Press, 2005)
And this article in
Science (1993) by William Ballard, summarized by the
New York Times:
Summary: The velvet worm is genetically akin to the joint-legged arthropods and not part of the more primitive phylum of segmented worms, the annelids. In an analsis of DNA sequences of 31 types of animals Ballard’s team compiled a genetic tree showing arthropods as descending from a common ancestor, i.e. velvet worms are arthropods. DNA data confirms the velvet worm was not the link between the arthropods and the annelids, but genetically similar to the more advanced scorpions and spiders.
(see New York Times, “Science Watch: Velvet Worm’s Lineage”, 9 February 1993)
Same author in
Cladistics journal (2000):
“
Despite a flurry of research activity in recent years, the interrelationships between and within the major clades of arthropods remain a contentious issue. The monophyletic status of the Arthropoda is one of the few points of widespread consensus, although a few workers (e.g., Fryer, 1996) still endorse the view (Anderson, 1973; Manton, 1977) that arthropods are a polyphyletic group. Reviews of major competing hypotheses for the relationships between chelicerates, crustaceans, myriapods, and hexapods,
as well as the status of Onychophora and Tardigrada relative to the euarthropods, have been outlined by Wheeler et al (1993), Wills et al (1995, 1998), Regier and Shultz (1997), and Zrzavy et al (1997), among others.” (
"Arthropod Cladistics: Combined Analysis of Histone H3 and U2 snRNA Sequences and Morphology", Cladistics 16, 155-203 [2000])
Apparently there are “debates” going on with regard to the insects and worms.
And a more recent article from the journal
Proceedings of the Royal Society Biological Sciences (from 2006)
summarized by ABC Science (of Australia):
Summary: For many years velvet worms were believed to be the ‘missing link’ between worms and insects. Recent research has shown they are not a sister group of the arthropods, but lie within them and share a common ancestry. (see “Velvet worms spin web of intrigue”, 2 August 2006 by Jacquie van Santen)
From
Softpedia News:
Summary: Velvet worms are more closely related to spiders and scorpions than to butterflies, according to new research. Understanding the evolutionary origin of onychophorans (velvet worms) could be the key to understanding the evolution of arthropods. New molecular biology techniques allowed biologists to determine that the onychophorans share a common ancestor with all modern arthropods. (see “The Brain of a Living Fossil Surprises Scientists – Velvet worm brains reveal secret sisterhood with spiders” by Sci/Tech News Staff, from
Softpedia.com 25 July 2006)
You’ll want to look up the evolution of arthropods themselves, since the velvet worm are now considered within same. Simmons says they evolved from insects, but that is the older view. He needs to get up to date.
BTW, I didn’t say Prothero answered every evolutionary question there is. Some things we don’t know yet. I only said
Prothero’s book (which provides plenty of evidence of transitionals)
demolishes Simmons’ book (who does not accept “common descent” at all). Read the books side by side and you’ll see.
Phil P
answering questions about the evolution of the Whale, Giraffe, and Velvet Worm in response to questions in a thread about a movie that basically denies the evolution of the Whale, Giraffe, and Velvet Worm. 
And I was just about to explain the “origin of life” too (which is also a topic in the film). 
