[Lewis Thomson (OP)]

Adam has been puzzled over this question for a while and would like some help finding an answer.

"Scientifically speaking, what came first the chicken or the egg?"

What findings do you have? Discuss them in the comments below...

[Halc]

Rigid eggs were around since perhaps the Cretaceous, long before there were chickens, so any designated 'first chicken' most assuredly came from an egg laid by its mother that was not a chicken by definition.

One might alternatively ask what came first, the chicken or the chicken egg, which simply comes down to how one defines a 'chicken egg'. Is it an egg laid by a chicken, or is it an egg containing a chicken? The answer depends on this definition, but is obvious either way.

[Deecart]

The chicken/egg paradox only appear when someone use conceptual thinking.

Chicken do eggs.
Eggs do chicken.
What do what ?

But eggs, like previously said is not a specificity of chicken.

Ancestor of chicken do eggs.
Eggs do chicken (evolution)
Chicken do eggs.

And before there was an egg, surely there was an ancestor with no egg.

Ancestor of ancestor of chicken do "vivid" (like some snakes, the egg remain inside the mother).
"Vivid" do ancestor of chicken (evolution)
Ancestor of chicken do eggs (eggs go out of the mother instead of giving birth inside the mother)

Take a look on those eggs :

[alancalverd]

All animals come from eggs (ova). The only difference is whether the ovum develops to independence inside (viviparous) or outside (oviparous) the mother.

What is baffling, however, is how the perfectly sound and efficient reproductive process employed for billions of years by single-cell creatures became the complicated and phenomenally inefficient mess of sexual reproduction. This is surely the strongest evidence against "intelligent design".

[Bored chemist]

What is baffling,

Not all of us are baffled.

[alancalverd]

Then your explanation of the apparently obvious would be welcome!

All you have to do is explain how evolution got from asexual amoebic mitosis to splitting species simultaneously into at least two anatomically and physiologically distinct sexes, one of which generates and broadcasts zillions of wasted sperm whilst the other produces a few ova and waits.

Then you can explain how exactly the same process evolved both in plants and animals.

You may not mention god, and in view of the second requirement, beware of relying too heavily on chance. The Y chromosome needs a lot of explanation because it seems to have evolved and thrived long before it was needed!

[Bored chemist]

https://en.wikipedia.org/wiki/Evolution_of_sexual_reproduction

[alancalverd]

The fascination of that article is in that it begins by stating that nobody knows the answer. Worse, one of the hypotheses it offers starts with

Organisms need to replicate their genetic material in an efficient and reliable manner.

which is exactly contrary to the random, inefficient and unreliable business of sexual reproduction! Zillions of sperm compete to fertilise one ovum whose resultant genes do not replicate those of either parent, and the entire process is predicated on A meeting B and inseminating her before she eats him.
 

[Bored chemist]

which is exactly contrary to the random, inefficient and unreliable business of sexual reproduction!

If you were right, we wouldn't be having this discussion.
The "purpose" of evolution is to find ways of making successful living things, and sex is good at that.
It doesn't care if it wastes zillions of sperm since they are "cheap".

[Deecart]

The fascination of that article is in that it begins by stating that nobody knows the answer. Worse, one of the hypotheses it offers starts with

Not exactly.
It was a mystery, but we actually have some better insight.

Since hypotheses for the origin of sex are difficult to verify experimentally (outside of evolutionary computation), most current work has focused on the persistence of sexual reproduction over evolutionary time. The maintenance of sexual reproduction (specifically, of its dioecious form) by natural selection in a highly competitive world has long been one of the major mysteries of biology,

https://en.wikipedia.org/wiki/Evolution_of_sexual_reproduction

Unlike other wikipedia articles, this on is not very good in my opinion, and should not be taken as a reference.
In this article per example we dont really talk about "sexuality" and like very often when we talk about genetic we forget to talk about the "plants" .
And other lifeforms have many other sexual gender, not only two like the "blob" (Physarum polycephalum,)

Scientists in France believe an intelligent blob expresses some 720 different sexes. The Paris Zoological Park is showcasing the ultimate gender fluid.

https://www.advocate.com/news/2019/10/17/scientists-say-intelligent-blob-has-720-sexes

Diploidy is associated with sexuality and it gives very interresting adaptativ properties, right, so it says that it is possible to have 2 kind of gender.
But the existence of gender is not full sexuality.
There are at least two mean to have "gender" : Sexual dimorphism, or no sexual dimorphism (or between, light or strong dimorphism).
The dimorphism is very interresting for the population of one specie, because it permit symbiosis of two different life form within the same specie.
The stronger the dimorphism and the more the specie have "subpopulations" capable of having different and complementary capacities (gender can be seen like producing subpopulation within a population)

[alancalverd]

In this article per example we dont really talk about "sexuality" and like very often when we talk about genetic we forget to talk about the "plants" .
And other lifeforms have many other sexual gender, not only two like the "blob"

Beware that these words have very different legal meanings in today's English.

Sex is what is determined by genes. Sexuality is identified by behavior. Gender is whatever an individual wishes it to be.

[Deecart]

Sex is what is determined by genes. Sexuality is identified by behavior. Gender is whatever an individual wishes it to be.

You are probably right.
I was myself wondering if i was translating well.

"sexuality" remain a very complex domain, even the use of the words are not clear for eveyone...

So if we talk about what genes makes a living being become phenotypicaly belonging to one of many sexes we should call it sex.
Hermaphrodism is determined by genes (like every trait you would say), but then, hermaphrodites do not belong to a sex category ?

Also, we have some male and female sex differenciated species that can sometime also do parthenogensis, like the sharks.

Some shark species are capable of asexual reproduction.

To procreate, most species (sharks included) require the fertilisation of an egg by a sperm. But for some, it does not take two to tango. Through a natural form of asexual reproduction called parthenogenesis, some animals can produce offspring all by themselves.

The term is a portmanteau of the Greek words parthénos, meaning ‘virgin’, and génesis, meaning ‘creation’.

This form of asexual reproduction has also been recorded in other shark species such as whitespotted bamboo sharks, blacktip sharks and bonnethead sharks. According to a study published by Australian biologists, zebra sharks are also capable of making the switch from sexual to parthenogenetic reproduction after being separated from their male mates.

Scientists believe this phenomenon occurs as a last resort for females that have an extremely hard time finding a mate. It typically happens to animals living in captivity, such as aquariums, where there are no other options available. In nature, parthenogenesis may happen as a result of human activity such as overfishing or anthropogenic climate change, where a large number of males have been wiped out.

https://dug.com/some-shark-species-are-capable-of-asexual-reproduction/





 

[alancalverd]

Hermaphrodism is determined by genes (like every trait you would say), but then, hermaphrodites do not belong to a sex category ?

Hermaphrodite is indeed a category. The common earthworm is a fine example  as at maturity it carries ripe ova at one end and sperm at the other, thus making mating twice as much fun.

Also, we have some male and female sex differenciated species that can sometime also do parthenogensis, like the sharks.

Reproductive flexibility is pretty clever but I'd be surprised if genetically male sharks can be viviparous, and given the propensity of oviparous fish to fertilise the eggs outside the female body, I'd be a little sceptical of claims of true virgin birth among the viviparous sharks.

But just to add to the confusion, don't forget eels and barramundi (and others I haven't caught) that change sex with age.

[Deecart]

But just to add to the confusion, don't forget eels and barramundi (and others I haven't caught) that change sex with age.

Yes, like the butterfly where male sexual chromosomes are ZZ and female ZW

In butterflies, sex is determined by chromosome differences. But unlike in humans with the familiar X and Y, in butterflies, it is the females that determine the sex of offspring. Males are ZZ, while females are ZW. How do females compensate for the loss of genetic information? New research shows that, contrary to previously inconsistent findings, in all species studied, that the expression of Z-linked genes is consistently equalized between the sexes.

They do so by either passing along either their Z (male) or W (female) chromosomes. Males are ZZ, while females are ZW. This ZW pattern is also prevalent in birds, some fish, and insects like butterflies. Similar to XY pairs, ZW pairs are different from each other in their shape and gene content: The Z chromosome is larger and has many genes, while the W consists mainly of repetitive DNA.

https://www.sciencedaily.com/releases/2017/07/170707070538.htm

or the clown fish.

Wait, what?
The Process of Sequential Hermaphroditism

To understand what Finding Nemo got wrong, you have to understand sequential hermaphroditism. Hundreds of species of fish undergo this biological process, which is a kind of sexual transition. For scuba divers, perhaps the most identifiable (and beloved) undersea hermaphrodite is the clownfish, which is born exclusively male. Not all marine sequential hermaphrodites begin life as males; moon wrasses in the Indian Ocean and western Pacific, for example, all start off as females and become males, and some fish, such as Asian sheepshead wrasse, spend a number of years as one sex (Asian sheepshead wrasses are born female) and then switch gender. The mangrove killifish is both male and female; it self-fertilizes for its entire reproductive life. In fact, about 2 percent of fish species display some kind of hermaphroditism, or roughly 500 different species worldwide.

Clownfish carry both female and male reproductive organs. In the female-dominated clownfish community, the female is the largest fish. She mates only with the breeding male, usually the second-largest and most aggressive male in the community. The rest of the community are made up of sexually immature males. When the female dies, the breeding male will get first choice of food and begin to gain weight, eventually becoming female.

The biological change inside the fish is governed by hormonal changes—chemical shifts that trigger a complex and cascading effect,” says Marah J. Hardt, Ph.D., founder and CEO of OceanInk, and author of Sex in the Sea. For the transitioning male, the testes dissolve and ovaries form.

https://www.sportdiver.com/what-happens-to-clownfish-when-it-changes-sex

What do we need to say about the seahorse ?

When they’ve decided they like each other, the seahorse females swim towards the surface of the water, and the males follow. The females then put their bright orange eggs into the pouch of the males through the hole at the top of the pouch. Once the eggs are safely inside, the males will add their sperm and shut the opening. The eggs are fertilised by the sperm, and then start developing into baby seahorses.

With that, the job of the seahorse mum is done! She swims off, and leaves the father to take care of the growing babies. Inside the pouch, the babies grow eyes, tiny snouts, and little tails. It takes about 20 days for the babies to develop, safely tucked away from other animals that might want to eat them.

https://theconversation.com/curious-kids-is-it-true-that-male-seahorses-give-birth-92843

[Deecart]

To add to the confusion...

The sex of most turtles and all crocodilians is determined by the temperature where the eggs developp.

Environmental Sex Determination

Temperature-dependent sex determination in reptiles

While the sex of most snakes and most lizards is determined by sex chromosomes at the time of fertilization, the sex of most turtles and all species of crocodilians is determined by the environment after fertilization. In these reptiles, the temperature of the eggs during a certain period of development is the deciding factor in determining sex, and small changes in temperature can cause dramatic changes in the sex ratio (Bull 1980). Often, eggs incubated at low temperatures (22–27°C) produce one sex, whereas eggs incubated at higher temperatures (30°C and above) produce the other. There is only a small range of temperatures that permits both males and females to hatch from the same brood of eggs. Figure 17.20 shows the abrupt temperature-induced change in sex ratios for the red-eared slider turtle. If eggs are incubated below 28°C, all the turtles hatching from them will be male. Above 31°C, every egg gives rise to a female. At temperatures in between, the broods will give rise to individuals of both sexes. Variations on this theme also exist. The eggs of the snapping turtle Macroclemys, for instance, become female at either cool (22°C or lower) or hot (28°C or above) temperatures. Between these extremes, males predominate.

https://www.ncbi.nlm.nih.gov/books/NBK9989/

[Petrochemicals]

Chickens are avian dinosaurs, not all dinosaurs where made extinct in the kt event and they survive today. As non avian dinosaurs, or birds as they are sometimes called, obviously evolved from some common dinosaur ancestor of dinosaurs and birds, or non avian dinosaurs. As such the egg laying and feathers preceded the chicken. The point at which a chicken was a new species distinct from other birds or non avian dinosaurs , or when non avian dinosaurs (birds) where no longer dinosaurs is a difficult question.

[Deecart]

First "birds" already had eggs.
But they could not sit on them, because they would have crashed them.

Early birds like Archaeopteryx were far too heavy to sit on their eggs without cracking them. The conclusion holds true for non-bird dinosaurs too, leading to fresh doubts about how to interpret spectacular fossils that appear to show dinosaurs brooding their eggs.

Most birds today lay eggs with strong, hard shells. This strength is necessary because many birds practice contact incubation – meaning the adult rests its body weight directly on the eggs. But just because modern bird eggs can support the weight of a brooding adult it doesn’t necessarily follow that ancient bird eggs could, says Charles Deeming …

https://www.newscientist.com/article/2163286-ancient-birds-couldnt-sit-on-their-eggs-without-smashing-them/

Or perhaps they were not so heavy as we think...

[alancalverd]

Parental mass isn't the critical parameter!

There's little point in a coldblooded creature sitting on eggs other than to protect them from predators, so we'd expect any brooding dinos to be warmblooded and/or vegetarian. But the mechanical parameters of interest are the maternal pressure (weight per unit area) and the contact area of the eggs. A big fat momma might weigh several tons but spread the load over several square meters, and the crushing failure force on a small egg with a thick shell can be very large -  particularly if applied at the pointy end.

[Deecart]

To avoid confusion (because many people think dinosaurs were like reptiles coldblooded), we actually think that most of the dinosaurs and especialy the "bird like", were warmblooded (previous studies reported they could be in the middle; cold and warm blooded).

Many dinosaurs were likely warm-blooded with high metabolic rates that resembled those of modern birds, according to a study published yesterday (May 25) in Nature. Comparing samples from more than 50 vertebrate species, some modern and some extinct, researchers found evidence that endothermy, or warm-bloodedness, was already widespread before the mass extinction event at the end of the Cretaceous period, challenging the widely held idea that differences in metabolism explain why birds fared so much better than non-avian dinosaurs.

https://www.the-scientist.com/news-opinion/most-dinosaurs-were-warm-blooded-after-all-70071

The studie published in Nature : https://www.nature.com/articles/s41586-022-04770-6.epdf?sharing_token=l2D7AjeyHhuVl246-M-oUNRgN0jAjWel9jnR3ZoTv0OWDuXm2XdCWi3gm57zLrCuWunVJEVtDATKXxJHXkZVaAaUZMdov_b3TYvEhgUbh5Lv3YRhSBfeyJq4ivXLIIoh397TVqs011bmi1TbcPKhS_0poHYdyuUnhWZUPuQ64qDrhQ7pSYiQVc9bas-XkIxKm9vDB6gnSOk1itwr17qhrRbt445T8Mq91rLAGBsXF7A%3D&tracking_referrer=www.the-scientist.com

But the mechanical parameters of interest are the maternal pressure (weight per unit area) and the contact area of the eggs. A big fat momma might weigh several tons but spread the load over several square meters, and the crushing failure force on a small egg with a thick shell can be very large -  particularly if applied at the pointy end.

The chicken eggs, altougth it is already very robust, break if you put, let say 4 chicken on it.
Even better if the "object" put on it is solid (like could be the skin of dinosaurs), so there is no distribution of the weight.
The exeption (it is much more robust) is when the egg is pointed upward, but this is unlikely to occur in nature.

[Bored chemist]

There's little point in a coldblooded creature sitting on eggs other than to protect them from predators,

So, there's little point- other than a really good point.
Also
"Why Turn Eggs?
Hens regularly turn their eggs, so it is important that we copy this habit in an electric incubator. But why? What will happen if we don’t? The first thing we need to look at is how eggs develop during incubation. Throughout the process the embryo grows into a baby chick, where improper temperature or humidity can spell disaster. "
 From

https://www.brinsea.com/Articles/Advice/TurningEggs.aspx
I don't know if dinosaurs did that or not, but their descendants do.