On what testable, observable, and repeatable data is a universal common ancestor based on?

Question

The various alleged "missing links"? No, because if one does not first accept the Theory of Evolution, there is no compelling (although one might argue there is good) reason to accept them as transitional forms.
Homologies? No, because they are no more proof of common descent than they are or common design or archetypes.
"Shared" genes? No, for the same reason.

Furthermore, the sudden appearance of almost all current phyla during the Cambrian explosion is good evidence that they did not share a common ancestor.

"Some studies have tried to analyze the general relationships between animals and vertebrate groups through molecular data. One study analyzed molecular data from 10 different vertebrates and found that using different mitochondrial genes, twenty different disagreeing phylogenetic trees were produced, which differed at both recent and ancient divergence points."

http://teleological.org/WPblog/2007/01/20/common-descent-vs-common-design/

Correct me if I'm wrong, but does this not indicate that mitochondrial DNA is not compelling evidence for a universal common ancestor? And if not, just how is mitochondrial DNA compelling evidence?

"I'm sorry, how is your concept of common design different than common descent? You just say that X started it all, but this still agrees with common descent."
I never denied common descent (or evolution, for that matter). The question was about UNIVERSAL common descent.
As for common design: "The idea behind commin [sic] design is that all the archetypes created by God have varying trait in common because there was a single designer who used the same design module across a variety of archetypes. He started with a basic body plan of two fore limbs and two hind limbs, and modified that conceptual design to produce the design for a bird. The desigm [sic] for a bird was then further refined to give the actualised designs for chickens and eagles."
(see link above)

"'The various alleged "missing links"?'

Well, it didn't take you long to screw that up."

Perhaps this statement was a bit harsher than I intended, but I think that a little healthy skepticism is warranted considering how often a reconstruction of what scientists claim is a human ancestor is based on fossil evidence that is fragmentary and/or from disparate sources. A good example of this (but far from the only one) is Ardipithecus ramidus, the fossil evidence for which consisted of only "a fragment of the right mandible, one intermediate hand phalanx, the left humerus and ulna, a distal humerus, a proximal hand phalanx fragment, a left clavicle fragment, a proximal foot phalanx, and a few teeth", which "came from 5 different locations".
http://www.apologeticspress.org/articles/127

"(Just out of curiousity, are you a Creationist? Creationists believe in a common ancestors.)"

"a" common ancestor"s"? Not sure what you meant here, whether it "a common ancestor" or "common ancestors". If the latter, I would agree. I think I've stated several times that I have no problem with evolution (as descent with modification) or even natural selection. The problem is that proponents of the Theory of Evolution (at least every one that I've encountered) try to use evidence for descent with modification as evidence for universal common ancestry when they often could just as easily be evidence for common design.

"Common ancestry is actually not the theory of evolution."
Never said it was. Darwin himself seems to have wavered between "one primordial form" and "some few beings". However, whether it's a single organism or a single ancestral pool, all other living things are said to arise from it or them, so I fail to see how the difference is significant.

"Oh, but I hear you claim these as homologies. And yes, that's exactly what they are."
So you're surprised that I'm claiming that they are exactly what they are? Did you expect me to claim them to be something else?

"However, they do strengthen evolutionary theory as opposed to weakening it."
Sigh. I already covered this: "[T]hey are no more proof of common descent than they are or common design or archetypes."

"For example, we know the species that led to the whales because they share many of the same specific features as whales yet they have functional legs."

"In short, describing Pakicetus, which is clearly a land dweller, as ‘walking whale’ simply on the structural features in its ear bones and molars, is" simply untrue.
http://www.trueorigin.org/ng_whales01.asp

"The reason evolutionists are confident that mesonychids gave rise to archaeocetes, despite the inability to identify any species in the actual lineage, is that known mesonychids and archaeocetes have some similarities. These similarities, however, are not sufficient to make the case for ancestry, especially in light of the vast differences."

http://www.trueorigin.org/whales.asp

One last point before I throw this open for vote.... While one might attempt to disguise the abruptness of the Cambrian explosion by stating that it lasted 100 million years, this is still quite small compared to the amount of time between a) the supposed emergence of life and b) the supposed last common ancestor and the beginning of the Cambrian explosion (which are a) ~35x and b) ~30x greater than the length of the Cambrian explosion). But even more difficult to explain away is that so many of the phyla appear fully formed and fundamentally different from one another.

Answer 1

Mitochondrial DNA FTW!

Answer 2

Sigh.

There certainly is compelling reason to accept them as transitional forms. Why? Because the organisms they transitioned into did not live at the same time as they did in the fossil record, nor do they show up earlier. For example, we know the species that led to the whales because they share many of the same specific features as whales yet they have functional legs. What is even more interesting is that there are no actual whale fossils found alongside these ancestors, nor are there whale fossils before them.

Oh, but I hear you claim these as homologies. And yes, that's exactly what they are. Taken alone, they are not evidence for evolution. However, they do strengthen evolutionary theory as opposed to weakening it. Same with shared genes (I won't, however, get into inserted genetic virii, which are also very compelling evidence). You're looking for a smoking gun here, but there isn't one. Rather, there are many separate lines of evidence that only lead to one conclusion, and that is the evolutionary history of this planet.

As far as the Cambrian explosion goes, it was not really an "explosion." The length of the explosion was at least a 100 million years. In a geologic sense, that's an explosion. However, it is also equal to 4.5 million generations of humans. I won't write how many "greats" are needed here. Secondly, the fact that the phyla "appear" is exactly what evolutionary biologists would expect to find. I mean, the phyla had to come from somewhere, right? But don't get carried away by the word "phyla." There weren't birds flying around, or reptiles slithering, or even bony fish. The precursor animals to these existed. And finally, there are fossils of animals that lived before the Cambrian explosion. They were soft-bodied creatures, but we find the trails they dug in the earth.

Answer 3

the existence of DNA.

But I'm afraid you don't understand the Cambrian explosion.

(Just out of curiousity, are you a Creationist? Creationists believe in a common ancestors.)

But that, being said, is still a problem....
your definition of evolution is flawed. Common ancestry is actually not the theory of evolution. Evolution postulates that many ancestors are quite possible, that's why we say that other planets could have life.... we don't think the life flew from there to here.

Answer 4

I'm sorry, how is your concept of common design different than common descent? You just say that X started it all, but this still agrees with common descent. And Mitochondrial DNA, that works too.

Answer 5

"Missing links" speak to me! Then "Reunification" could be a debut .
According to Bohm, the apparent faster-than-light connection between subatomic particles is really telling us that there is a deeper level of reality we are not privy to, a more complex dimension beyond our own that is analogous to the aquarium. And, he adds, we view objects such as subatomic particles as separate from one another because we are seeing only a portion of their reality. Such particles are not separate "parts", but facets of a deeper and more underlying unity that is ultimately as holographic and indivisible as the previously mentioned rose. And since everything in physical reality is comprised of these "eidolons", the universe is itself a projection, a hologram.
In addition to its phantomlike nature, such a universe would possess other rather startling features. If the apparent separateness of subatomic particles is illusory, it means that at a deeper level of reality all things in the universe are infinitely interconnected.The electrons in a carbon atom in the human brain are connected to the subatomic particles that comprise every salmon that swims, every heart that beats, and every star that shimmers in the sky. Everything interpenetrates everything, and although human nature may seek to categorize and pigeonhole and subdivide, the various phenomena of the universe, all apportionments are of necessity artificial and all of nature is ultimately a seamless web.
In a holographic universe, even time and space could no longer be viewed..
.. a universe in which individual brains are actually indivisible portions of the greater hologram and everything is infinitely interconnected,

Answer 6

Genetic evidence. Evolution is a proven fact; details on request to any valid e-mail address.

Answer 7

"The various alleged "missing links"?"

Well, it didn't take you long to screw that up.

Mitochondrial DNA is a perfectly good answer to your question.