If life arrived on space rocks, as some scientists have proposed, then what arrived may have been quite similar or even identical, may have been “seeded” abundantly throughout the earth and thus, arose from all over the earth and, is so complexly cross-integrated at root levels, as to only appear to be singular in origin.

Another perhaps more likely reality, is that what causes life to arise came out of the big bang, is refined in stars along with the majority of elements, is spread througout the Cosmos within zillions of planetary environments, as so much “fertilizer” waiting for conditions to arise favorable for it to “arise”. This life “fertilizer” may be identical or nearly identical in structure, which would also explain similarity of life, which may indeed have arisen all over our planet, from zillions of tiny identical or nearly identical life forms, as well as arisen abundantly throughout the Cosmos.

More Information on why modern evolutionary theory is mainly a poorly contrived “human construct”, grossly inadequate to actual explain how, when, where or why life came to exist on our planet:
http://freedomtracks.com/500/theory.html

Two completely separate evolutionary lines would make it possible to test all kinds of hypotheses that can’t be tested now– because all known organisms are too closely related.

People who think that evolution can be “disproven” by some minor, new discovery just don’t understand the huge weight of evidence supporting evolutionary theory. It’s not like sinking the Bismarck– it’s like sinking the whole continent of Europe.

(Bom dia, Marta. 🙂 )

So I’ll quote now what Darwin said in his book:

“In considering the origin of species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties, from other species. Nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified, so as to acquire that perfection of structure and coadaptation which most justly excites our admiration. Naturalists continually refer to external conditions, such as climate, food, etc., as the only possible cause of variation. In one very limited sense, as we shall hereafter see, this may be true; but it is preposterous to attribute to mere external conditions, the structure, for instance, of the wood-pecker, with its feet, tail, beak and tongue, so admirably adapted to catch insects under the bark or trees.” (DARWIN, p. 11)

“Now let us turn to nature. When a part has been in an extraordinary manner in any one species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of modification, since the period when the species branched off from the common progenitor of the genus. This period will seldom to be remote in any extreme degree, as species very rarely endure form more than one geological period.[â?¦] And this, I am convinced, is the case. That the struggle between natural selection on the one hand, and the tendency to reversion and variability on the other hand, will in the course of time cease; and that the most abnormally developed organs may be made constant, I can see no reason to doubt.” (Darwin, p. 172-173)

“If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case. No doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, round which, according to my theory, there has been much extinction” (Darwin, p. 210).

(DARWIN, Charles. The origin of species. London: Collectorâ??s Library, 2004)

I think calling it a “first premise” is a post hoc thing you’ve added. Darwin never said that. If I were to infer a first premise from Darwinian Theories I’d say it was descent with modification.