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Slimebeast on 10 September 2010
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superchunk said:
Slimebeast said:
 

It's a demonstration of one possible step, but it doesn't show any proof of links between steps. Proof would be to show some more examples of further steps in the same evolutionary path. Since you also seem to believe this article demonstrates evolution right here, right now - and that it's also happened several times in other species, from lizards to mammals - it's only logical that there should be living examples of steps that are closer to the live birth of mammals. Or else you need to ask yourself why there are several examples of lizards that retain their eggs until they dissolve and give live birth, but no example of an evolutionary step further down the line towards mammalian full live birth where the embryo gets full nutrition from a placenta without having a shelled egg at any point in embryonal development.

Ever study the birthing a platypus? Its a mammal that lays eggs.

Point is there are animals all accross the spectrum, but her is the same species that utilizes two distinct paths. In the long run one version will become the sole path as the other is far more fuitful. i.e. evolution by natural selection.

More food for thought.

http://ncse.com/creationism/analysis/mammal-eggs-reptile-placentas

"The marsupial mammals have a rudimentary and short-lived placenta which is, in most marsupials, structurally and functionally different from the typical eutherian placenta. Placental nourishment of marsupial young is negligible compared to nourishment from the milk obtained in the pouch. Furthermore, there are mammals which lay eggs and have no placenta. These creatures, the monotremes, share with other mammals the characteristics of fur and the ability to lactate, but they lay eggs with leathery shells, which the females then incubate in a pouch."

and

"Some reptiles (e.g. garter snakes) are viviparous and develop a rudimentary placenta (see Stewart, JR, American Zoologist 1992 32(2):303-312, "Placental Structure and Nutritional Provision to Embryos in Predominantly Lecithotrophic Viviparous Reptiles" for a not-so-recent discussion of these facts)."

Also, http://icb.oxfordjournals.org/content/32/2/303.short has more info on placenta having reptiles.

Basically, the info is there if you actually look. Reptiles appear to be evolving (obviously very slowly and based on environmental conditions, typically cold weather) to live birth as its better for the survival of the species.

The platypus is a curious case, I'll give you that. But it's also very unique and tricky to draw conclusions from.

Marsupials are not an evolutionary step between reptiles and mammals.

Garter snakes I already included since they belong to the "several examples of lizards" category. (a snake that is very similar to these snake-like lizards, skinks, in the article)