I will see if I can find it. But the basic argument has to do with Convergent & Parallel evolution. These are cases where there was no common ancestor. In the case of the protein you had it evolve in at least 25 independent organisms. Again no common ancestor. Although this is considered to be a example of Convergent evolution.
Are you talking about
analogous enzymes? 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.
You are the evo, so go ahead and explain it away as to how you can have evolution without common ancestors that you find in Convergent and Parallel evolution.
Note:
parallel 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
similar way
One example I can think of is the loss of pelvic spines in three- and nine-spine sticklebacks. Because a gene called
Pitx1 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
both stickleback species followed this route.
Convergence refers to cases where features with similar functions evolved independently, but in
different 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.)
Because like the Cambrian Explosion they like to try and hide this information away. Until Gould came along and blew the lid off of it.
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
point 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
Wonderful Life, 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.)
I thought it was clear that we were talking about the common ancestor for the animal phyla that showed up in the Cambrian explosion. There were perhaps 50 of them and around 30 were said to have survived.
Oh, right.
To the best of our knowledge, the last common ancestor of
animals may have looked something like this:
(Colony of
Proterospongia/
Salpingoeca rosetta by Mark Dayel from the King lab. Source:
ChoanoWiki.)
This is a colonial choanoflagellate - choanoflagellates are the closest living relatives of animals based on molecular phylogenies, and the fact that
sponges (which are animals) possess food-capturing cells very similar to these confirms the link.
The most recent common ancestor of
bilaterian 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...
Blobs) 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
possessed several genetic pathways 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,
Pax6 and
Six 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
very nice image 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,
flatworms.)
(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
well before hands and feet themselves.
(3) We know that the Urbilaterian lived quite far into the Precambrian - certainly before definitive bilaterians like
Kimberella (~555 Mya, mid-Ediacaran). Clearly, the last common ancestor of a group had to live before any recognisable
subgroups of that group appeared, and
Kimberella 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.
This suggests 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
If anything went over your head, I'm happy to explain. Just ask
So what? There are two theories and they can not both be true.
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.
Does that mean anything goes?
When you are only
hypothesising, anything goes (if you're willing to risk being laughed at
). Not once you start collecting evidence.
Do evolutionists even know what they believe in?
We're certainly far less confused about our beliefs than you are
You must have more confidence then I do that you can somehow blend opposites. Maybe we can get you a job as a marriage counselor if your so good to bring reconciliation between extreme viewpoints.
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 think the main thing overlooked in the Cambrian explosion is the basic mechanism driving it, which were significant changes in atmospheric and oceanic chemistry. Prior to that there were several significant and long snowball earth periods. Otherwise, the Cambrian explosion could have occurred a billion or so years earlier.
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
existed 1.5 billion years ago, is there? They sort of had to be around for them to radiate explosively.