Evolution of Placental Mammals

I've been wondering about this for a while. It seems hard for me to imagine how such a complex system of the placenta in mammal embryos and the female uterus and the hormonal cycle that governs it could have evolved, since it involves the evolution of two different complementary structures that exist and become useful at different times in an organism's life. How is the advent of this system described by pure evolutionists? How can the female uterus with it's thickened lining and the embryonic placenta both evolve at the same time through natural selection when neither is useful without the other?


[edit on 7/19/2005 by djohnsto77]

Originally posted by djohnsto77
since it involves the evolution of two different complementary structures that exist and become useful at different times in an organism's life.

Consider the bombardier beetle. It has evolved a system in which two different chemicals are mixed within its body and released in a small explosion. The system by which the chemicals and the system for handling them and keeping them seperate (to prevent death by explosion), seems extraordinary, and yet biologists can deal with the system.

Its one of the consequences of natural selection that systems would appear to have evolved in tandem. Structures and organ systems will arise seperately, and thru natural selection when and where they happen to work together will be re-inforced.

How is the advent of this system described by pure creationists?

Literally, "God did it". I don't think IDists consider it to be "irreducibly complex" and therefore 'designed'. IDists, in fact, consider almost nothing to have evidence of design, and most of them, like I think Behe and Dembski, think that man evolved from lower primates and that the different kinds of animals have all evovled thru time. But YECists and OECists don't have any explanation, indeed, their entire idea is that there simply is no explanation and that god dreamt the whole thing up.

How can the female uterus with it's thickened lining and the embryonic placenta both evolve at the same time through natural selection when neither is useful without the other?

What makes you say its not useful?

The placenta is an evolution of a structure that is contained within the vertebrate egg, like in shelled eggs. In a sense, a womb is like a shelled egg. I don't think it requires anything extraordinary for a system of live birth, marsupial birth, and even internal birth to arise from such a setup, nor for the various sacs that are already in eggs to develop into all sorts of different 'organs', like a placenta.
Here is a neat little paper I just google scholared up.

Evolution of placental specializations in viviparous African and South American lizards.
[...]South American lizards of the genus Mabuya exhibit several reproductive specializations that are convergent on those of eutherian mammals, including viviparity, long gestation periods, ovulation of tiny eggs, and placental supply of the nutrients for development. Studies of placental morphology and development indicate that New World Mabuya share several other derived features, including chorionic areolae and a "Type IV" epitheliochorial placenta with a villous, mesometrial placentome. Some characteristics of these lizards are shared by two African skinks, M. ivensii and Eumecia anchietae, including minuscule eggs, placentotrophy, an absorptive chorioallantois, and features of the yolk sac.

And with phylogenetic and evolutionary methods they can even make hypotheses about how the pattern of evolution here

Available evidence is consistent with two explanations: (1) placentotrophy originated in Africa, predating a trans-Atlantic colonization by Mabuya of the New World; and (2) placentotrophy arose two or three separate times among these closely related skinks. As illustrated by analysis of these animals, not only can data on fetal membrane morphology yield phylogenetic information, but phylogenetic evidence in turn provides a valuable way to reconstruct the evolution of fetal membranes in a biogeographic context. When appropriately interpreted, morphological and phylogenetic evidence can be combined to yield robust evolutionary conclusions that avoid the pitfalls of circular reasoning

This abstract looks at the relationships of montremes, marsupials, and eutherian mammals via cladistical analysis of proteins within the organisms, and finds that monotremes are the 'sister-group' to marsupials (ie are not properly groups in with marsupials, to the exclusion of eutherian mammals), which is interesting because monotremes are of course egg laying mammals. And, notably, monotremes have very primitive milk glands. This shows that there can be odd combinations of hormones, glandular activity, and all sorts of physical activities that are coordinated with the development and birth of offspring in mammals, even tho the 'complete' placental system hasn't evolved 'yet'. So if there are milk glands that can be activated in coordination with the hatching of the offspring, we can also see that its reasonable that there'd be coordination of other hormones, even ones that aren't strictly 'meant' for the development and birthing process originally.

Here is a that looks at the origin of placenta like structures in fish.

abstract
The evolution of complex organs is a source of controversy because they require the contributions of many adaptations to function properly. We argue that placentas are complex, that they have evolved multiple times in Poeciliopsis, and that there are closely related sister taxa that have either no placentas or intermediate stages in the evolution of a placenta. Furthermore, placentas can evolve in 750,000 years or less, on the same ime scale as suggested by theoretical calculations for the evolution of complex eyes. Independent origins of such complexity, accompanied by ister taxa that either lackor have intermediate stages in the evolution of the trait, present an opportunity to study the evolution of novelty and omplexity from a comparative, evolutionary perspective.

Here is an interesting page.

There are numerous accounts of philosophical, anatomic and evolutionary kind that discuss the placental evolution. For instance, Mossman (1987), in his extensive classic on comparative placentation, devotes much time to explain the possible derivation of various placental structures. Ramsey (1989) traces the "History" of the human placenta and Soma (1978) provided a review in Japanese with many color illustrations of unusual species. Kaufmann & Burton (1994) have also published an excellent review that is replete with literature citation. It is primarily directed to the understanding of the human placenta and, i.a., it traces the origin of "chorion" to Aristotle and "placenta" (cake) to Realdo Columbus.

Which could make some interesting follow-ups and details.

This looks like an interesting pdf on the compartive biology of the many different types of placenta in the mammals.

And this web essay on The Study of Large Changes thru evolution looks informative too.

Here's an intersting sample, looking at how the descendant can 'loose' fitness over its ancestral type, and how fitness can be subjective. Here the ancestor is 'less fit' becuase its not as well represented in the later stages (representation in later generations is what fitness is all about in a sense).

Over such huge distances, for what humans perceive as "improvements" (increase in complexity) fitness will fall. We can demonstrate (Fig 2.2) that fitness will fall as a relative measure, against an ancestor from a line that widely diverged from original type. But there is evidence that fitness falls absolutely for increase of complexity too.
Sexual reproduction is more complex than earlier evolving asexually reproduction. But for reproduction by sex genotype fitness falls by 50%, over 100% for the less complex asexual forms. (Fitness falls by 50% of DNA unique to an individual. But expressed genes can be less than 3% of total DNA, and mostly these will be 100% conserved. See Smart Gene Hypothesis.) We suspect (there are no agreed ways to measure it) that earlier, prokaryote life, is reproductively fitter than more complex eukaryotes, which evolved later.

Which is an intersting way to think of it, but I am not too certain that this is the way most people look at it. THere is an idea that organisms can 'cross' from one 'adaptive peak' (such as a well adapated monotreme) to another (such as a placental mammal) by going thru 'maladaptive lowpoints' simply thru the general randomness of genetic mutations and genetic drift that occur faster than natural selection can act on them (tho I don't think that anyone would say it can move from monotreme to placental in one directed step of course).

Good info Nygdan, thanks!

This was a mistake, I meant pure evolutionists:

Originally posted by Nygdan

How is the advent of this system described by pure creationists?

Literally, "God did it". I don't think IDists consider it to be "irreducibly complex" and therefore 'designed'. IDists, in fact, consider almost nothing to have evidence of design, and most of them, like I think Behe and Dembski, think that man evolved from lower primates and that the different kinds of animals have all evovled thru time. But YECists and OECists don't have any explanation, indeed, their entire idea is that there simply is no explanation and that god dreamt the whole thing up.

I fixed it in my original post now.

[edit on 7/19/2005 by djohnsto77]