To the best of my knowledge there is considerable evidence to suggest that all life on this planet can be traced to a common ancestor. Are there any credible alternative hypothesis.

Nope.

Mononoke wrote:Are there any credible alternative hypothesis.

More than one ancestor.

hackenslash wrote::nono:

Does it seem incredibly unlikely that there could be more than one ancestor? That is, that life "started" more than once?

There is more than one ancestor, but that's a different proposition than common descent. I have more than one ancestor, and so do you, but we also have a shared common ancestor (indeed, we have at least two of them, although almost certainly not contemporaries), and that's true of all life. When you get back to before the evolution of sex, you come to a point at which all the life currently extant on the planet has one single MRCA. That's not even to say that there weren't other organisms at the time, only that our lineage can be traced back to a single ancestor. The same is true of that point that is true now, namely that it can be traced back to a single ancestor of all life.

As for life starting more than once, that's problematic, as a brief perusal of the literature will readily reveal. It is certainly possible that life began more than once, but whether any of those other instances could have led to organisms extant in the biosphere now is a thorny problem, not least because of the aerobic atmosphere produced by life. Oxygen is highly corrosive, and would be hugely preventative to other life arising.

In any event, the response you quoted was to the usual failure to spot the word 'credible' in the question by the navel-gazer. It isn't that having life arise twice is implausible, but there are no credible hypotheses aside from UCD. There are plenty of rectally extracted blind assertions, such as 'special creation', but there are no hypotheses that are credible.

ughaibu wrote:

Mononoke wrote:Are there any credible alternative hypothesis.

More than one ancestor.

ORLY?

Actually, Douglas Theobald recently subjected a range of models to formal test, to see which one reproduced phylogenies that were closest to those found in the real world. The universal single common ancestor model was pitted against several other models, including models involving multiple ancestors, and all of the models were tested to see which produced the most realistic phylogenies. The universal single common ancestor model won.

The paper in question is:

A Formal Test Of The Theory Of Universal Common Ancestry by Douglas Theobald, Nature, 465: 219-223 (13th May 2010) [Full paper downloadable from here]

Theobald, 2010 wrote:Universal common ancestry (UCA) is a central pillar of modern evolutionary theory1. As first suggested by Darwin2, the theory of UCA posits that all extant terrestrial organisms share a common genetic heritage, each being the genealogical descendant of a single species from the distant past3–6. The classic evidence for UCA, although massive, is largely restricted to ‘local’ common ancestry—for example, of specific phyla rather than the entirety of life—and has yet to fully integrate the recent advances from modern phylogenetics and probability theory. Although UCA is widely assumed, it has rarely been subjected to formal quantitative testing7–10, and this has led to critical commentary emphasizing the intrinsic technical difficulties in empirically evaluating a theory of such broad scope1,5,8,9,11–15. Furthermore, several researchers have proposed that early life was characterized by rampant horizontal gene transfer, leading some to question the monophyly of life11,14,15. Here I provide the first, to my knowledge, formal, fundamental test of UCA, without assuming that sequence similarity implies genetic kinship. I test UCA by applying model selection theory5,16,17 to molecular phylogenies, focusing on a set of ubiquitously conserved proteins that are proposed to be orthologous. Among a wide range of biological models involving the independent ancestry of major taxonomic groups, the model selection tests are found to overwhelmingly support UCA irrespective of the presence of horizontal gene transfer and symbiotic fusion events. These results provide powerful statistical evidence corroborating the monophyly of all known life.

Calilasseia wrote:Actually, Douglas Theobald recently subjected a range of models to formal test, to see which one reproduced phylogenies that were closest to those found in the real world. The universal single common ancestor model was pitted against several other models, including models involving multiple ancestors, and all of the models were tested to see which produced the most realistic phylogenies. The universal single common ancestor model won.

As pointed out on an earlier thread, Theobald used model selection theory, what this tells us is which model is likely to be most predictively accurate. It does not tell us which model is most likely to correspond to reality and, accordingly, has no ontological implications.

Shrunk wrote:

ughaibu wrote:As pointed out on an earlier thread, Theobald used model selection theory, what this tells us is which model is likely to be most predictively accurate. It does not tell us which model is most likely to correspond to reality and, accordingly, has no ontological implications.

Just like everything else in science.

This thread begins with the question "are there any credible alternative hypothesis". Clearly this usage of "hypothesis" is not that of an hypothesis in science, as no observation of life on this planet being traced to a single ancestor is possible. So, by "hypothesis" I take it Mononoke meant a conjecture as to what the fact was. In other words, this thread doesn't ask a scientific question.

Of course I'm not joking, and if you have some serious reason to suppose that I am, what is that reason?

Shrunk wrote:

ughaibu wrote:Of course I'm not joking, and if you have some serious reason to suppose that I am, what is that reason?

Well, to start with, in the post just before the one in which you say this:

Clearly this usage of "hypothesis" is not that of an hypothesis in science, as no observation of life on this planet being traced to a single ancestor is possible.

you say this:

Theobald used model selection theory, what this tells us is which model is likely to be most predictively accurate.

Whether or not you intend it as a joke, it's pretty funny.

If you find it funny, fair enough, there's no accounting for taste. But I still have no idea what your reason is.

Shrunk wrote:You say there is no possible observation that could support the idea of UCA, immediately after you describe an observation that supports the idea of UCA.

No I didn't.

A question just popped into my head - how long would it have taken for life to cover the glode from its starting point?