|Aug2-12, 03:43 PM||#18|
What came first, the chicken or the egg?
Part of the development of the embryo is considered the phylotypic stage. The phylotypic stage is where the general anatomy of the embryo is characteristic of every species in that particular phylum. Sometime this idea that there is a phylotypic stage that slowly evolves is called the hour-glass model.
In chordates, the stage at the beginning of the tailbud is considered phylotypic. Recent studies have pointed out that the tailbud stage isn't identical in every class of chordate. Yes, you can distinguish the tailbud embryos of fish, birds and mammals. The number of pharyngeal slits vary quite a bit, as does the size of the yolk sac. However, the adult stages of development vary even greater than the tailbud stage. An adult fish is very easy to distinguish from an adult bird.
The trochophore stage of the development of mollusks is also considered a phylotypic stage. Adult mollusks are very different from each other. The adult stage of bivalves, gastropods and chitons are very different. However, the earliest larval stage of each class is the trochophore. Trochophores don't vary much. Some cephalopods don't have a trochophore stage. However, the resemblance of trochophores to each other despite class is rather amazing.
The hourglass model explains this resemblance by hypothesizing that the phylotypic stage evolves very slowly compared to the other stages in development. Evolution works slower in the phylotypic stage than on the adult stage. The reason is that the tailbud stage of the embryo is basically protected from harm. The adult has to make a living. The ova and sperm have to compete. However, the phylotypic stage is generally the most passive stage in the animals existence. The tailbud embryo just has to grow in the chordate mother. The trochophore stage of a mollusk is plankton, and just floats.
The hour-glass model has taken a beating in recent years. Haekel was a rather single minded scientist of the eighteenth century that pushed the hour-glass model to an extreme. Haekel may have even misrepresented some of his data. He claimed that the early embryo was identical within a phylum of animals. This is very wrong. However, the general structure of the phylotypic stage is similar within a phylum. The proportions of each phylotypic feature may vary greatly from class to class, but the same anatomical features are present in all classes for the animal in the phylotypic stage.
I was pointing out that the egg obviously came first since the general structure of the birds egg is the same from bird to bird. The colors on the eggs may differ, but a birds egg is a birds egg. The adult bird is something else. Adult chickens vary in appearance from other birds, and even from each other. Therefore, the egg of the first chicken was indistinguishable from the eggs of other birds. It wasn't until the first chicken grew up that anyone can say, "This is a weird type of bird. I will call it a chicken." Therefore, the egg had to come first.
The other examples that I gave were organisms that don't have a clear phylotypic stage. Even their individuality is questionable. Some don't even need eggs to reproduce. They occasionally lay eggs anyway, but don't critically need the egg in any one generation. Therefore, the question of "what came first" is considerably more complicated in their case. Which did come first, the coral adult or the coral egg?
|Aug2-12, 09:04 PM||#19|
|Aug2-12, 09:37 PM||#20|
Viviparity can be useful both in cold climates and in aquatic environments.
Egg laying is a real problem in cold climates. The egg can freeze solid without some means of keeping it warm. Perhaps the first placental mammals, or even the first marsupials, lived in very cold climates.
It is even possible that the cold weather during the KT extinction killed off most egg-laying mammals.
Viviparous lizards live tend to live in cold climates. It is thought that they evolved this to avoid getting frozen. Here is a link to an article about viviparity in squamate reptiles.
“Cold climates and the evolution of viviparity
in reptiles: cold incubation temperatures
produce poor-quality offspring in the lizard,
Evolutionary origins of viviparity among the squamate reptiles are strongly associated with cold climates, and cold environmental temperatures are thought to be an important selective force behind the transition from egg-laying to live-bearing. In particular, the low nest temperatures associated with cold climate habitats are thought to be detrimental to the developing embryos or hatchlings of oviparous squamates, providing a selective advantage for the retention of developing eggs in utero, where the mother can provide warmer incubation temperatures for her eggs (by actively thermoregulating) than they would experience in a nest.”
One bird that has a real problem with eggs freezing solid is the penguin. Emperor penguins have this very elaborate migrating behavior, practiced by both parents, that has evolved to keep the egg from freezing. This is a very risky behavior. The males stand a good chance of starving to death while incubating the eggs.
Here is an article on emperor penguins. Yes, they lay eggs. However, that is an accident of history. I conjecture that emperor penguins wish they were viviparous. Here is a free link on the hard life of emperor penguins.
“The Emperor Penguin is perhaps best known for the sequence of journeys adults make each year in order to mate and to feed their offspring. The only penguin species that breeds during the Antarctic winter, it treks 50–120 km (31–75 mi) over the ice to breeding colonies which may include thousands of individuals. The female lays a single egg, which is incubated by the male while the female returns to the sea to feed; parents subsequently take turns foraging at sea and caring for their chick in the colony. The lifespan is typically 20 years in the wild, although observations suggest that some individuals may live to 50 years of age.”
Air breathing tetrapods that return to the sea have to do something about their eggs. If the eggs are laid in water, the embryo could drown. Maybe that is why many marine snakes are viviparous. Here is a link (buy on line) on viviparous sea snakes.
The viviparous sea snakes (Hydrophiinae) comprise ∼90% of living marine reptiles and display many physical and behavioral adaptations for breathing, diving, and achieving osmotic balance in marine habitats.”
|Aug2-12, 10:31 PM||#21|
It seems (most) birds have survived very well by doing the sensible thing; heading south (or north) for the winter. As I said, birds have evolved an optimal set of adaptations (IMO) that promote family life, reduce sexual tensions and provide a relatively simple, safe and generally painless means of reproduction.
* long distance migration, not just out on the pack ice.
|Aug3-12, 02:03 PM||#22|
If there is a reason for the class of birds not having viviparity, then it has to have something to do with the calcium carbonate in their egg shells. Other classes of animals have both viviparous species and oviparous species. However, these classes of animals generally do not have calcium carbonate in their egg shells. Very often, what these classes have in their egg shells is a tough form of keratin, which is a protein.
I already posted some links on viviparous lizards and viviparous snakes. Reptile species that lay eggs have tough egg shells with keratin. Monotremes have keratin egg shell cases. The shark/ray class also has both viviparous and oviparous species. Again, the egg-laying sharks have keratin egg shells. It seems reasonable that a keratin shell is less of a barrier to viviparity then a carbonate shell.
The calcium carbonate in a bird's egg shell may prevent the embryo from "invading" the mothers circulatory system. I conjecture that it is easy for an embryo to evolve a placental-like organ if it is in control of the egg shell ontology.
Crocodilians (alligators, crocodiles) also have calcium carbonate shells. They are no like other reptile shells. There are no viviparous crocodilians. So that is extra evidence consistent with the idea that the calcium carbonate causes a developmental constraint against the evolution of viviparity.
The first stages in the evolution of viviparity has to alter the egg shell case. So in this case there is a trade-off. A selection process that favors one does not have to favor the other in terms of the statistics of genes.
In terms of gene-selection, the question is whether the mother or the offspring gets the most benefit.
1) Does the calcium carbon shell really benefit the mother or the offspring?
-Those calcium carbonate shells crack so much more easily keratin, making it difficult to see why birds have calcium carbonate shells.
2) Does a keratin shell case benefit the mother or the offspring?
3) Whose genes statistically benefit more from viviparity, the mother or the offspring?
4) Are the keratin shells in egg laying species of any class deposited by the embryo or the mother?
5) Is the calcium carbonate deposited by the embryo or the mother?
I only brought up the emperor penguin to show that oviparity is not always easier on the parent then viviparity. In the case of the emperor penguin, the male takes most risk taking care of the egg. The male has to cradle the egg with his feet of many weeks, to prevent it from touching the snow. He has to congregate in large groups of males, huddled together to keep warm. They have to rotate their positions, making sure that each male gets only a few days at the edge of the huddle. Then, the eggs hatch. They have to protect the chicks still longer until the females get back. Then, they switch off.
The series of steps necessary to evolve such a behavior rivals in complexity the series of steps necessary to evolve viviparity. However, there is one major advantage that such penguins have for staying on a barren block of ice while breeding.
The penguins are safe from predators. There are no predators on the ice. The males could die of exposure or starvation. However, no bear is going to eat them. No leopard seal will eat them. So the risk of predation is minimized.
I I am right, then the reason penguins didn't develop viviparity has something to do with the calcium carbonate. Instead of developing viviparity, the penguins developed elaborate courtship and breeding instincts that protect the egg from freezing.
|Aug5-12, 07:26 PM||#23|
Seriously, you've never seen racoons dig into a gator nest on Animal Planet or one of those Discovery animal shows or something?
|Aug6-12, 02:47 AM||#24|
Anyway, most birds seem to be literally "above it all" by virtue of flight. They can escape the cold and also reduce predation by nesting in trees. Also, as I said, many operate in pairs so that eggs and chicks can be guarded and fed 24-7 until they can leave the nest.
|Aug6-12, 05:41 PM||#25|
1) We can not tell from the fossils which of the Mesozoic mammals were viviparous and which were oviparous.
-Maybe most of the Mesozoic mammal species were oviparous.
-Maybe there were only a few viviparous species before the KT extinction.
2) During the Mesozoic, there were temperate areas that grew very cold during the winter.
-There were land masses at both the North and South poles with extremely cold winters and cool summers. This is clear from the vegetation and even some dinosaur fossils.
-Hypothetically, viviparous mammal species developed in the cold temperate regions.
-Viviparity could not evolve during the KT extinction because it was so short.
3) The cold period of the KT extinction would have "enriched" the mammals with viviparity by wiping out most oviparous species of mammals.
-The Miocene and Pleistocene could have further enriched the mammals by wiping out the few remaining oviparous species of mammals outside the Australian continent.
-"Enrichment" doesn't explain how viviparous mammals evolved, just why there are few oviparous mammals.
-There are no viviparous species of birds, including the ones that don't fly.
-There are no oviparous bats (order chiroptera), which are also "above it all".
-There are viviparous squamata (lizards and snakes), who are "below it all".
-There is a oviparous subclass of mammals called the monotremes.
|Aug6-12, 06:31 PM||#26|
As far as flying mammals and flightless birds go, they probably were later adaptations that did not necessarily entail changing their inherited means of reproduction. I have no idea about viviparous squamata. Monotremes are oviparous mammals.
|Aug6-12, 09:09 PM||#27|
There is no bird subclass that corresponds to the monotreme subclass in birds. That is, there is no viviparous birds. Furthermore, there are no flying monotremes.
Oviparity in monotremes is often hypothesized to be a primitive trait. If it is primitive, then the common ancestor of all extant mammals must have been oviparous. Thus, one can reasonably ask, "How did the ancestors of the viviparous mammals lose their oviparity?"
I don't see how animals that live in trees can escape climate. Maybe you mean that birds, by flying, could evolve migration behaviors to escape cold climate. However, the common ancestor of monotremes probably couldn't fly. So one wonders why extant monotremes remain oviparous.
Bats developed flying much later than birds, long after viviparity became common among mammals. However, most bats leave their offspring back in the cave or in the trees. Even if their is a developmental constraint against oviparity, one would think that bats would develop analogous behaviors. Carrying ones very young live offspring isn't much different from carrying an egg.
One could easily imagine a marsupial bat-analog. However, there are no marsupial bats. So what do eutharia have concerning flight that marsupials don't? Why aren't there monotremes adapted for flight?
There are probably a lot of answers to the question of how mammals evolved viviparity. Climate is probably only one factor out of many, if it is a factor. However, the articles proposing that cold climate lead to viviparity have some evidence. Vivaparous squamata seem to be correlated to cold climate. Whether this has any relevance to mammals remains to be seen.
|Aug7-12, 06:48 PM||#28|
I was surprised to learn that fossils do not allow us to determine the reproductive mode. If we could, we could look at the correlation of fossils with the expected climate of different regions at different times. Many oviparous mammals that survived the KT event might well have been killed with the arrival of the Pleistocene. However, if we can't determine the reproductive mode of extinct genera, we might never determine the main driver(s) of mammalian viviparity. I don't completely discount that egg predation also played a role as a driver of viviparity, especially in the early periods when all mammals were small, possibly non- arboreal, creatures.
|Aug7-12, 07:24 PM||#29|
This question relies on a misunderstanding of the complexity of evolution. What came first prototypes, endless ones, nothing came first, it is all a matter of iteration, something came first it was neither a chicken nor did it lay chicken eggs, but it laid a something, a proto something laid an egg. :)
Generally a problem of human intellect we cannot perceive millions of iterations, so we ask stupid questions, that have stupid answers because we are constantly involved with finite time periods. It would be nice to be an elf who lived for a billion years and observed evolution likewise, but we are stuck in our fast paced generational concerns for which questions like this seem ill suited. I or rather my ancestors laid an egg once, and that is the point. It was an egg but I was not a chicken and the process of me doing so was never so simple as that.
Ovipermacimusy, sheez, if only we all lived for ever, all could understand the increasing amounts of reiteration, this all would be so simple.
Maths it up, it's not a chicken and an egg problem, it's a long winded prototype prototype laying a prototype egg, until and ill defined special boundary that cannot be defined makes a chicken a chicken which is ill defined, and its egg a chicken egg. We can be certain at some point a chicken laid an egg, but not when. But it did even if we don't know exactly when. The egg was laid, but not by a distinct chicken, it was laid by a re-iteration at an indistinct moment by an undefined prototype or phenotype or genotype. It's like a probability problem: there is no defined boundary to what makes a chicken a species, hence there is no exact moment that makes an egg a chicken egg, and no chicken that came first. Evolution is hard, stop asking imprecise questions, and start asking how exactly the iterations make something indistinctly.
|Aug7-12, 07:51 PM||#30|
|Aug7-12, 08:55 PM||#31|
This reference presents the hypothesis that both viviparity and milk secretion started in the middle to early Jurassic. Node 3 in the diagram shown in the link branches off in the early to middle Jurassic. This would place viviparity, milk secretion and the evolution of angiosperms (i.e., flowers) at nearly the same time.
“Since the mammaliaforms evolved from egg-laying therapsids and the monotremes still preserve egg-laying, it’s likely that the early mammals reproduced by laying eggs up until node 3 on the diagram. The skeletal structure of the multituberculates suggests that they bore immature live young similar to those of the marsupials. Since the monotremes produce milk, but secrete it through pores, it’s likely that nipples evolved at node 3 or sometime after. This may have happened as late as the divergence of the eutherians and marsupials, but probably came earlier. Some researchers think that milk secretion originally evolved as a means of hydrating the thin, leathery shells of basal mammals’ eggs, and was only secondarily used as a means of feeding hatchlings. With viviparity there would be a strong selective drive towards formation of a nipple to more efficiently feed the young. Evidence for this hypothesis will be found if it is discovered that monotremes secrete milk to coat their eggs.”
Hey, there is a correlation between viviparity and flowers! The first pollen is found from about the time that some mammals developed viviparity.
Suppose that viviparity has some sort of causal correlation with flowers! Maybe the birds took to the trees too soon, before there were fruit trees. They had to nest in gymnosperms, which are basically pine trees. The pine needles protected the eggs from egg predation.
The early mammals were feeding on pollinators for the angiosperms, which mostly lived in river valleys as small weeds. The flowering plants did not provide as much protection against egg predation as pine trees. So some mammals were more likely to develop viviparity than birds.
I present this hypothesis just for amusement. It is only slightly more serious than the lame joke that started this thread.
It does appear that a lot of innovations occurred during the Jurassic period. The evolution of the stapes in the middle ear started this period. Viviparity started this period. Milk secretion started this period. Birds started flying during the Jurassic period. And yes, flowers started during the Jurassic period.
Birds, bees, flowers, and mammals. All during the Jurassic. And it doesn't appear that the dinosaurs noticed anything!
|Aug8-12, 12:36 AM||#32|
Laying an egg in such a disturbed region would be very risky. If an egg were layed on the inner banks of a river, the egg could be washed away during a flood. So a viviparous animal in such a disturbed environment would have an advantage over oviparous animals living in such an environment.
Viviparous behavior could have first evolved in cold regions, to prevent the eggs from being “frozen” or “maimed” by the cold. However, the access of disturbed regions could explain how it spread out of cold areas.
This conjecture places mammals right next to early angiosperms. Early angiosperms were weeds that specialized in disturbed regions. Most angiosperms grow quicker than other plants because angiosperm leaves can draw water and carbon dioxide faster than other leaves. In the Mesozoic, most angiosperms lived in temporary areas where they had to grow quickly.
So I have this image of mammals grazing on flowers on an inner bank of a riverbed. The riverbed is eroding away. A pregnant mammal frantically runs to get to the next inner bank. The flower “tries” to grow and seed quicker. The “flower” may even trick the pregnant mammal to carry a seed.
Here is a 2004 article that shows that both birds and mammals helped to spread seeds from the earliest angiosperms. Also, this 2004 article makes the point that the earliest angiosperms grew in river valleys in areas of great erosion.
“(c) the Mesozoic was dominated by large herbivorous
dinosaurs, possible sources of diffuse, whole-plant dispersal; (d) simultaneously, several
groups of small vertebrates, including lizards and, in the later Mesozoic, birds
and mammals, could have established more specific vertebrate-plant associations, but
supporting evidence is rudimentary; and (e) the diversification of small mammals and
birds in the Tertiary established a consistent basis for organ-level interactions, allowing
for the widespread occurrence of biotic dispersal in gymnosperms and angiosperms.”
“Angiosperms appeared and diversified in the later portions of the Early Cretaceous
through the Late Cretaceous. The initial radiation involved shrubby or
perhaps herbaceous plants growing in disturbed sites, particularly along rivers
(Wing & Boucher 1998, Friis et al. 1999).”
There are viviparous plants. The seeds germinate while attached to the mother plant. One such plant is the mangrove. Interestingly, the mangrove lives in highly disturbed regions.
“In this harsh environment, mangroves have evolved a special mechanism to help their offspring survive. Mangrove seeds are buoyant and therefore suited to water dispersal. Unlike most plants, whose seeds germinate in soil, many mangroves (e.g. red mangrove) are viviparous, whose seeds germinate while still attached to the parent tree.”
This is a 1960 article that claims that angiosperms evolved in the early Creteceous.
It is concluded that no bona fide remains, either megafossil or microfossil, have yet been described from rocks older than early Cretaceous sediments.
However, here is an article from 1999 that claims that there are Jurassic microfossils.
“Two species of angiosperm-like pollen are described from an outcrop of lower Oxfordian Oxford Clay at Normandy, France. An Oxfordian age is established by ammonites (Quenstedtoceras mariae zone) and substantiated by associated age-diagnostic dinoflagellates. The angiosperm-like taxa are determined to be in place by their state of preservation and unique morphology.”
|Aug8-12, 12:44 AM||#33|
Which came first? The chicken or the soup? Just teasing. Egg laying critters were around long before there were any birds that could be identified as "chickens".
|Aug8-12, 05:02 PM||#34|
Clearly live birth is favoured by evolution in some conditions, to the extent that even some lizards give birth to live young. Look at why and there will be your answer although it is clearly more complicated than that; in the lizards case it is because the environment is often very cold, so that eggs may not ever hatch if young were not born live. The same thing applies to any reason live birth exists, it's not that hard to work out why many species give birth to live young, it's simply because environment favours it. All you need to do is work out what the selection pressures were and one very obvious one I have given you, the fact that eggs require incubation, but the environment is often not that forgiving is an obvious place to start. Temperature and reliance on it. Natural selection would favour a species that could live and breed in many environments over one that was only suited to a warm one. So why did the dinosaurs die out? Why did mammals survive, it seems somewhat easy to understand why given that abrupt changes environment favour young that are already developed enough to be motile, to move to a more favourable environment, and do not die when it gets too hot or cold because of that. Deversification is key, the more a species is able to adapt to adverse conditions the more likely it is to survive. Eggs have absolutely no ability to change their position in unfavourable conditions quite often, although not always particualrly in the sea, but even then they cannot chose where they end up, live animals can jump out of the fire and to somewhere that isn't a frying pan.
"It is not survival of the fittest that denotes what it is most likely to survive, but survival of those most able to adapt."
Paraphrasing Charles Darwin.
Personally I am not sure why this discussion even exists, as we can list many reasons why live birth although somewhat energy intensive, has a pay off beyond simply producing inactive unadaptable, intrinsically at least motionless and unreactive eggs. Further having live young means the adult is at the scene to protect live births where as eggs may well have no protection at all. It seems at least to me fairly obvious why mammals are if not dominant most widespread. Well beyond the asexual bacteria et al but one can argue that being single celled is not really that much of an adaptation that will matter at least to things we value.
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