r l, that "argument" fails on at least a couple of levels. First, implied is that never has "bacterium A" been observed to become "bacterium B". The fact is, that isn't how evolution works. Our freinds "bacterium A" and "bacterium B" both are the end result of the evolutionary biomorphological processes which led to their development; each is what it is, the descendents of each will identifiably be of the species from which they've descended. That "they don't change into something else" is a red herring, it means nothing beyond they are what they are. Presenting an objection so based "proves" only the ignorance of the one posing that specious objection.
A critter which morphologically is half-way between single-cell organisms and plants is
Euglena, "pond scum", a single-celled flagellate (they propel themselves about - are "motile" - with an appendage which is a whip-like "flagellum"), organisms which, while they do "eat" smaller critters, also contain chloroplasts, allowing them, in the manner of plants, to produce food for themselves through photosynthesis of less-complex chemical compounds into life-supporting nutrients.
Another set of critters which may, perhaps very likely does, represent a developmental stage intermediate between one type and another are gliding mammals, such as flying squirrels, a genus which appears to show signs of becoming more bat-like (not implying they are becoming bats, but rather that indications are they are evolving into another flying mammal) and another possible intermediate critter would be aquatic snakes, some of which present indications of incipient gill development. Then, of course, there are the amphibians, such as frogs, which as they mature from birth through juvenile stages into adulthood loose their gills, become airbreathers, and develop limbs.
A clear example of an intermediate step between single-cell organisms and multi-cell organisms is to be found in
Myxococcus Xanthus. Bacteria are not spores, spores are not bacteria. M. xanthus is an otherwise typical thin-walled, motile bacterium normally given to reproduction through cellular division in the manner common to single cell organisms, but which in response to certain environmental stimulii - specifically "starvation", lack of nourishment - spontaneously aggregates into a complex multi-cellular "social organism", forming among other cooperative multi-cellular structures a "fruiting body" within the interior of which individual thin-walled, rod-shaped, motile bacterium differentiate into spherical, non-motile, thick-walled spores, which remain intact even as the "parent", the assembled "social organism", disrupts and disintegrates as its un-nourished component cells die. The spores, in the manner of spores, can and do remain dormant untill such time as nourishment again is available, whereupon the spores redifferentiate into thin-walled, rod-shaped motile bacterium.
Going a step beyond, by extension of the specious objection r l presented in this instance, there is the absurd claim that "no transitional fossils have been found". As demonstrated
HERE, in layman-freindly fashion, that simply is not true. Anyone claiming there are no transitional fossils is either ignorant or lying, there are no other options.
