What came first, the chicken or the egg?

[Swetasuria]

I think it is the chicken. I can't understand why there is such confusion?

Why is it compared to the promblem over the first genetic material formed?

Is there any explanation on the basis evolution or mutation that supports the egg to be formed first?

 

[onomatomanic]

In the context of evolutionary biology, the standard answer is "egg", reasoning that if you trace the lineage of any chicken far enough back, you'll eventually end up with an ancestor that can't any longer be called a chicken - but the things it lays are nevertheless still eggs.

 

[Reptillian]

Wait...how can you have a chicken without an egg?...how can you have an egg without a chicken?...no chicken, no egg...no egg, no chicken...but one had to come first...chicken...egg...chicken...egg... BWAK BWAK BUKAWWW! *head explodes*

(Lol, sorry I know I'm nutty. I agree with onomatomaniac)

 

[Monique]

Wrong! it's neither the chicken nor the egg.

Publication The evolution of early animal embryos: conservation or divergence?
From the author:

The oldest, foundational stage of development is the phylotypic period that recapitulates the adult stage of a primitive multicellular ancestor (go Haeckel). Aristotle was asking the wrong question, neither chicken nor the egg were the first, it was the pharyngula. (http://www.fly-jedi.org/2012/04/chicken-and-egg-causality-dilemma/)

Dilemma resolved..

 

[Number Nine]

It's a bit of a flawed question; categories like "chicken" are taxonomic conventions created for our convenience. There is no instant at which a "chickens" appeared where there were none before; we just look at the categories that exist today and decide that a group of animals are similar enough (in features and in ancestry) to be placed within a single category.

 

[Dremmer]

Which were we humans eating first, the chicken or the eggs?

 

[onomatomanic]

Which were we humans eating first, the chicken or the eggs?

"The chicken (Gallus gallus domesticus) is a domesticated fowl", whereas eggs can be gathered from, inter alia, the nests of wild birds. Unlike domestication, gathering is as old as the human species. So the answer would again seem to be "egg".

 

[Darwin123]

Wait...how can you have a chicken without an egg?...how can you have an egg without a chicken?...no chicken, no egg...no egg, no chicken...but one had to come first...chicken...egg...chicken...egg... BWAK BWAK BUKAWWW! *head explodes*

(Lol, sorry I know I'm nutty. I agree with onomatomaniac)

Don't you mean ovamatermaniac?
The first chicken egg was laid by an animal that you wouldn't call a chicken. There was a an embryonic chicken in the egg, but the mother is considered a different type of bird.
I am not sure that you are quite serious, but I will expand on this anyway. It may be useful to quote this post when similar issues come up.
You made an error in logic. The "paradox" is based on a false hypothesis. The hypothesis is that a female animal has to lay an egg which is the same species as the mother. Obviously, if a dog could lay a chicken egg there would be no problem. However, we need not get so far. The line of discrimination between species can be somewhat arbitrary. A bird that is not a chicken, but very similar to a chicken, can lay an egg that hatches into a chicken.
Let me ask an analogous question. Which came first, the king or the prince?
A prince is the child of a king. However, the king had to grow up from a prince. So obviously, the theory of royal succession doesn't work.
At some point, there were no kings or princes. Far back, somebody was charismatic and aggressive enough to get everyone to declare not only him as boss, but all his children and his children's children. So he had himself declared as king. He did not grow up as a prince.
An example would be King Saul in the Bible. He was the first king. There was no prince and no king in Israel before him. So the king came first, in this example. In fact, the one that followed him was not a Prince. King David was not a prince. He replaced Saul as King.
In fact, that happened quite often in history. So the king always came first, not the prince.

 

[onomatomanic]

The "paradox" is based on a false hypothesis. The hypothesis is that a female animal has to lay an egg which is the same species as the mother. Obviously, if a dog could lay a chicken egg there would be no problem. However, we need not get so far. The line of discrimination between species can be somewhat arbitrary. A bird that is not a chicken, but very similar to a chicken, can lay an egg that hatches into a chicken.

Yes, I've just been thinking along similar lines, partly in reference to this thread. I'd say it quickly becomes more an issue of semantics than anything else, if one takes this far enough. Let's try a thought experiment inspired by the basilisk legend:

"A [chicken/snake] lays [an avian/a reptilian] egg, from which hatches a [chicken/snake]."

("Avian" and "reptilian" is meant to refer only to the appearance and architecture of the egg.)

Consider each of the eight sentences which can be formed by selecting one word from each of the pairs, and decide whether you'd call the egg in question a "chicken egg" under those conditions.

 

[jackmell]

How about archaeopteryx:

http://en.wikipedia.org/wiki/Archaeoptryx

wouldn't that work? That's a (proposed) transition species between dinosaurs (which laid eggs) and birds and I assume archaeopteryx is also laying eggs and then by continued evolutionary change, its lineage includes a branch which gradually (or sometimes abruptly) evolved into the chicken lineage. So from that perspective one could argue the egg came before the chicken, i.e., archaeopteryx was laying eggs long before the chicken emerged. However, not quite sure the question is referring to just eggs or bonifide chicken eggs.

 

[Darwin123]

How about archaeopteryx:

http://en.wikipedia.org/wiki/Archaeoptryx

wouldn't that work? That's a (proposed) transition species between dinosaurs (which laid eggs) and birds and I assume archaeopteryx is also laying eggs and then by continued evolutionary change, its lineage includes a branch which gradually (or sometimes abruptly) evolved into the chicken lineage. So from that perspective one could argue the egg came before the chicken, i.e., archaeopteryx was laying eggs long before the chicken emerged. However, not quite sure the question is referring to just eggs or bonifide chicken eggs.

It doesn't really matter. In both cases, the egg came first. So long as the species is defined in the adult form, the egg came first. Since the morphology of embryos doesn't evolve as fast as the morphology of the adult form, scientists will continue to define a species in terms of the adult form.
I assume that the species called chicken is defined in terms of the anatomy of the adult. The anatomy of embryos in the tail bud stage vary across class lines. However, I assume that the tail bud stage is conservative within any class of vertebrates. If so, one has to wait until the chick grows up to decide whether it is a chicken. By then, the egg has hatched.
If he meant any eggs whatsoever, then clearly the egg came before the chicken. Purported eggs presumably from some invertebrate have been found in rocks 700 million years old. Thus, "eggs in general" preceded the vertebrates.
If he meant the eggs of a "proper chicken", there is the question of how that egg is defined. When I look in any dictionary, the "chicken species" is defined in terms of its adult form. Thus, one would have to wait until the egg hatched before one decided that it was a chicken. Thus, the egg came first.
If the evolution of the embryo wasn't conservative, then we would have a harder time. However, the greatest taxonomic detail occurs in the adult stage. Know one can tell what the species is from a blastula.
Yes, I know that Haekel was wrong. Embryos even in the tail bud stage vary with taxon. You can tell the embryo of a fish from the embryo of a chicken in the tail bud stage. Haekel fudged his illustrations. However, I seriously doubt that you can tell the embryo of a chicken from the embryo of a duck at any embryonic stage. I suspect that it would be difficult to tell a newly hatched chicken from a newly hatched duck. Therefore, one has to wait for the corresponding chicks to grow up.
The "proper chicken egg" had to proceed the "proper adult chicken". However, I wonder about those rare cases where the embryo evolution is not so conservative.
It may be especially hard in the those animals that can reproduce both sexually and asexually. For instance, coral can reproduce by budding or by releasing eggs. Which came first, the coral adult or the coral egg?
Then there are the organisms that form embryo-like stages by fusing individuals. Such as fungi. Such as slime molds.
Which came first? The amoebic stage slime mold (asexual reproduction of protozoa-like cells), the slug stage slime mold (fusion of amoeboid individuals), or the spore (asexual reproduction of the multicellular slug)?
Please take into account that a typical slime mold species has five independent genders!

 

[Darwin123]

Which were we humans eating first, the chicken or the eggs?

I read a theory that humans first started raising chickens for cock fighting. I think it was in a recent issue of Smithsonian Magazine. The popularity of chickens as a food came about only within the last four centuries, according to this article.

 

[Swetasuria]

If the evolution of the embryo wasn't conservative, then we would have a harder time.

"...evolution wasn't conservative...", What do you mean by that? Is it similar to the examples of fungi and coral you have given?

 

[bobze]

The first chicken egg was laid by an animal that you wouldn't call a chicken. There was a an embryonic chicken in the egg, but the mother is considered a different type of bird.
I am not sure that you are quite serious, but I will expand on this anyway. It may be useful to quote this post when similar issues come up.

It would be good not to confuse people with this. Parents of 1 species don't give birth to offspring of different species. Organisms bequeath organisms so similar we consider them the "same species". Its important to remember here also that species in this context are arbitrary boundaries put upon the gradations of successive generations, for the ease of human convenience. Not natures.

There is also another very, very important point to make here that may confuse readers reading your post. Its that evolution happens to populations. There is no "first chicken" or "first person" or "first of any species". Evolution is the change in allele frequencies over time for a population. It is incorrect, both conceptually and in reality, to think of a not-quite chicken giving birth to a chicken. At no point did such a finite line exist where a species "pops up". What did happen is a population of organisms reproduced and their alleles over time were sufficiently altered for us to label them as a distinct species. That is an easily misunderstood point--even often by students of the biological sciences. Think about it like a color bar;

Its like asking where yellow becomes green. There is no finite point in the transition, only "populations of pixels" (think vertical lines) with changing RGB values (think of alleles). The "point" at which we consider 1 a "new" species in this case is going to be an artifact of human preference.

For chickens and the rest of real life, this distinction between "species" in context becomes even harder to detect. A domestic rooster;

A representative of that ancestral population, the red jungle fowl;

A population of, according to us, almost chickens---but not quite chickens.

I agree with the rest of your post though. I like the royal analogy--Just remember though that evolution is populations!

 

[SW VandeCarr]

Regardless of the changes in allele frequencies in populations over time, it's scientifically undisputed that birds evolved from reptiles and reptiles laid (and continue to lay) eggs. So, with respect to chickens (which are birds), the egg came before the chicken.

EDIT: My question is why mammals had to develop a new way. The avian egg is relatively simple and quite elegant IMO. The mammalian way is too complicated, messy and unnecessarily hard on the mother. Has a chicken ever died from the complications of pregnancy?

 

[bobze]

Regardless of the changes in allele frequencies in populations over time, it's scientifically undisputed that birds evolved from reptiles and reptiles laid (and continue to lay) eggs. So, with respect to chickens (which are birds), the egg came before the chicken.

EDIT: My question is why mammals had to develop a new way. The avian egg is relatively simple and quite elegant IMO. The mammalian way is too complicated, messy and unnecessarily hard on the mother. Has a chicken ever died from the complications of pregnancy?

Having live young has its advantages. Ever seen a lowly raccoon raid a giant alligator nest, destroying all the young? Having your babies with you circumvents that problem.

 

[SW VandeCarr]

Having live young has its advantages. Ever seen a lowly raccoon raid a giant alligator nest, destroying all the young? Having your babies with you circumvents that problem.

Well I've never seen a lowly raccoon raid a giant alligator nest, so I have to either take your word for it or demand a peer reviewed reference. Seriously, there are certain advantages to gestation and live birth, but our avian friends are doing just fine. They can fly south for winter without having to take a plane or get hotel reservations. And they can hang around the pool without feeling self-conscious because they're obviously pregnant. Besides we are talking about avian eggs, not reptile eggs. The birds perfected the egg IMO. They usually guard their eggs 24/7 until they hatch. I don't have a ready reference for it, but I've read that with many bird species, the female forms a monogamous relationship with the male to share egg minding duties. If you insist, I will find a reference for this. But you will then need to find a reference for the lowly raccoons (and not non-lowly raccoons) and giant alligator nests (it's not clear to me whether you mean giant alligators or giant nests, so you will need to cover both).

 

[Darwin123]

"...evolution wasn't conservative...", What do you mean by that? Is it similar to the examples of fungi and coral you have given?

I meant that there is a stage in the development of the embryo where evolution doesn't seem to move as fast as in other parts of the embryo.
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 anyone 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?

 

[Darwin123]

Having live young has its advantages. Ever seen a lowly raccoon raid a giant alligator nest, destroying all the young? Having your babies with you circumvents that problem.

One can carry ones eggs with you. Some amphibians do that.

 

[Darwin123]

Regardless of the changes in allele frequencies in populations over time, it's scientifically undisputed that birds evolved from reptiles and reptiles laid (and continue to lay) eggs. So, with respect to chickens (which are birds), the egg came before the chicken.

EDIT: My question is why mammals had to develop a new way. The avian egg is relatively simple and quite elegant IMO. The mammalian way is too complicated, messy and unnecessarily hard on the mother. Has a chicken ever died from the complications of pregnancy?

I will bet that there were penguins that did.

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.
http://www.faculty.biol.vt.edu/andrews/pdfs/QColdClimates.pdf
“Cold climates and the evolution of viviparity
in reptiles: cold incubation temperatures
produce poor-quality offspring in the lizard,
Sceloporus virgatus
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.
http://en.wikipedia.org/wiki/Emperor_Penguin
“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.
http://icb.oxfordjournals.org/content/early/2012/05/24/icb.ics066.short?rss=1
“Abstract
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.”

 

[SW VandeCarr]

I will bet that there were penguins that did.

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.

It seems there are no viviparous birds, which is somewhat surprising since egg retention is not that great an evolutionary leap as opposed to placental adaptations. As for penguins, I love them, but you have to wonder about big non-migrating* birds that "choose" to live on the Antarctic continent year around when almost no other plant or animal does. They have such a "human" quality that endears them to many people. Maybe that's the problem. The only other animal that lives in Antarctica year around is the human. The US maintains a station at the geographic south pole while the Russians maintain a station at the "pole of inaccessibility" where the world record low temperatures are recorded (-88 C). There are also a number of coastal stations maintained by several countries.

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.


http://www.jstor.org/discover/10.2307/2461788?uid=3739824&uid=2129&uid=2&uid=70&uid=4&uid=3739256&sid=21100958627763

 

[Darwin123]

It seems there are no viviparous birds, which is somewhat surprising since egg retention is not that great an evolutionary leap as opposed to placental adaptations.

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.


http://www.jstor.org/discover/10.2307/2461788?uid=3739824&uid=2129&uid=2&uid=70&uid=4&uid=3739256&sid=21100958627763

Saying that the conditions of the oviduct don't allow viviparity doesn't answer the question of why birds don't have viviparity. All the other amniote vertebrates have oviducts. Many of these classes produce species with viviparity.
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.

 

[bobze]

Well I've never seen a lowly raccoon raid a giant alligator nest, so I have to either take your word for it or demand a peer reviewed reference. Seriously, there are certain advantages to gestation and live birth, but our avian friends are doing just fine. They can fly south for winter without having to take a plane or get hotel reservations. And they can hang around the pool without feeling self-conscious because they're obviously pregnant. Besides we are talking about avian eggs, not reptile eggs. The birds perfected the egg IMO. They usually guard their eggs 24/7 until they hatch. I don't have a ready reference for it, but I've read that with many bird species, the female forms a monogamous relationship with the male to share egg minding duties. If you insist, I will find a reference for this. But you will then need to find a reference for the lowly raccoons (and not non-lowly raccoons) and giant alligator nests (it's not clear to me whether you mean giant alligators or giant nests, so you will need to cover both).

Seriously, you've never seen racoons dig into a gator nest on Animal Planet or one of those Discovery animal shows or something?

 

[SW VandeCarr]

Seriously, you've never seen racoons dig into a gator nest on Animal Planet or one of those Discovery animal shows or something?

Maybe. I must not have been very impressed. I wonder how many raccoons have been caught doing that. Alligators are pretty fast. Egg predators apparently haven't curbed the alligator population in places like Florida. Darwin123 suggested that viviparity is a cold weather adaptation. However, viviparous mammals developed well before the KT boundary and the Pleistocene cold period only began 2-3 million years ago.

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.

 

[Darwin123]

Maybe. I must not have been very impressed. I wonder how many raccoons have been caught doing that. Alligators are pretty fast. Egg predators apparently haven't curbed the alligator population in places like Florida.

Which is one reason I don't think that egg predation is the main impetuous toward viviparity. It doesn't matter much to the embryo whether the mother is guarding it as an egg, or carrying it around with her. In fact, the mother may not be able to defend her offspring so readily if she has to carry it into battle. She has more mobility if she is separate from the embryo that she is trying to protect.

Darwin123 suggested that viviparity is a cold weather adaptation. However, viviparous mammals developed well before the KT boundary and the Pleistocene cold period only began 2-3 million years ago.

There are four qualifications to this objection to "cold weather viviparity".
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.

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.

4) Birds (class aves) are universally oviparous, .
-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.

 

[SW VandeCarr]

4) Birds (class aves) are universally oviparous, .
-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 viviparous subclass of mammals called the monotremes.

The thrust of my posts was to ask what drove mammals to substantially change their method of reproduction as compared to birds. The main difference the two classes would have encountered is that birds could fly and mammals had to deal with conditions on the ground. Early mammals were small rodent-like creatures. Both predation and climate (which flying birds could escape) are possible drivers. Are there any other likely drivers?

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.

 

[Darwin123]

The thrust of my posts was to ask what drove mammals to substantially change their method of reproduction as compared to birds. The main difference the two classes would have encountered is that birds could fly and mammals had to deal with conditions on the ground. Early mammals were small rodent-like creatures. Both predation and climate (which flying birds could escape) are possible drivers. Are there any other likely drivers?

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.

Sorry. This was a typo. Monotremes are by definition oviparous mammals. I edited my original post so that it is now correct. Oviparity in monotremes supports my point.
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.

 

[SW VandeCarr]

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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 think it's not so much losing oviparity but the transformation to viviparity. Even humans produce eggs. I think we are pretty sure the process begins with egg retention followed by a progression toward a placental viviparity. We also now believe that both early reptiles and mammals had a common amniote ancestor, which makes mammals very old indeed, but still later than the previous glacial period (the Karoo (Permian) ~260-350 mya). It's reasonable to believe that the early mammals were oviparous and that monotremes are a surviving remnant of oviparous mammals. I agree with you that viviparous mammals need not have been very numerous for a long time, perhaps just developing in those regions with seasonal sub-freezing temperatures. If there was a major short cold period at the KT boundary, many oviparous mammals may have been killed off.

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.

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.

Yes. I've been saying that right along. Arboreal nesting is a possible protection against egg predation. Migration is a possible protection against getting your eggs frozen.

However, the common ancestor of monotremes probably couldn't fly. So one wonders why extant monotremes remain oviparous.

They would be a surviving remnant of a class that once was probably entirely oviparous. If they are confined to Australia, that would be consistent with the cold weather theory.

 

[Cerlid]

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.

 

[SW VandeCarr]

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. :

The original question was dealt with. The oviparous mode of reproduction existed before chickens (or birds) evolved. That's all that needs to be said or can be said. The current discussion (which I think is relevant and more interesting) is about what the drivers of the evolution of viviparous (live birth) reproduction in mammals might have been. In what ways does this represent a useful adaptation for mammals? Do you have anything useful to contribute regarding this?