The question of chickens and eggs has puzzled mankind for hundreds of years and there has been no clear answer. To explain this problem scientifically, we first need to study how the first egg was formed. Since the earliest vertebrates evolved to land, eggs have been used as a form of reproduction. So, the history of the first egg can be traced back to 300 million years ago, when the ancestors of vertebrates began to climb ashore, and the egg at this time was actually an amniotic egg, which is the way almost all reptiles, birds and mammals on Earth reproduce today.
Like us humans, we reproduce through amniotic eggs, but in a different way than eggs. Our embryos develop inside the female but still need the protection of the amniotic membranes. Before birth, our embryos are hatched in amniotic fluid.
So, when did we choose the viviparous way?
To find out, we need to observe the appearance of the first egg. About 3700 million years ago, in the Devonian period, flesh-finned fish climbed from water to land and successfully evolved into amphibians. However, amphibian eggs still need water or a moist environment to hatch. Until 3The appearance of the Qisi lizard in the early Carboniferous period 400 million years ago was one of the first vertebrates to reproduce on land, and it was also the first group of amniotes.
The ziss lizard begins to lay a new type of egg, which is encased in a soft, flexible shell that protects the embryo while also having enough pores to allow the embryo to breathe. The eggshell and amniotic fluid form a separate space that provides moisture to the embryo. With such conditions, terrestrial reproduction is possible.
Ziss lizards no longer rely on swamps for survival and reproduction, which gives them a huge advantage in occupying terrestrial niches. We have not yet found a definitive answer to the disagreement about when amnioviparous animals produce viviparity and oviparity.
Reptiles, mammals, and birds all belong to the group of amniots, and they reproduce in a variety of viviparous and oviparous ways. Thus, the divergence of amniotic eggs exists in their last common ancestor, 3Among forest lizards 100 million years ago. The forest lizard is the ancestor of all amniotes, and from it, amniotics are divided into two groups: sauriformes, including reptiles, dinosaurs, and birds;Zygosochidae are the ancestors of mammals.
As a result, the history of eggs has also diverged. Some reptiles in the sauromorphs still lay eggs like early amniovites, but the predecessors of birds and dinosaurs began to form a different type of egg with a calcium-containing outer shell that was stronger and more prone to fossilization, which is why we were able to find many fossilized dinosaur eggs.
The Zygokogena has taken a more complicated path. Early toxocarophodas also laid primordial amniotic eggs with soft shells, but over the next 100 million years, the mammalian ancestors made great progress. They transform from lizard-like creatures to warm-blooded, hair-covered creatures with mammalian characteristics.
Finally, in about 1600 million years ago, in the late Jurassic period, the ancestors of placental mammals appeared, such as the Jurassic. These animals are collectively known as the subclass Zodia. There is no conclusive evidence as to how our mammalian ancestors chose between laying eggs and giving birth. Due to the scarcity of fossil evidence, we can only speculate from modern mammals. One of the possible answers is genetic mutations. The amniotic sac of mammals, including chorionic and allantoic membranes, is present in animals that are able to give birth to young, and they are just used in different ways. Mammals still form an amniotic sac around the embryo, while the mother provides nutrients to the embryo through the chorion and allantoic membranes, which are actually the components of the placenta.
Thus, a mutation may have changed the way amniotic eggs are used, leading to entirely new viviparous reproductive strategies. Another idea is that the zonic subclass developed along an evolutionary path similar to that of the order Monofora. Monoforarea is an egg-laying mammal that diverged from the class Zygosus, and they kept the eggs in the mother's body for longer and longer periods of time, eventually developing viviparity.
The time the egg remains in the body is thought to be caused by changes in gene expression. There is a set of genes involved in the selection of whether the amniotic egg develops towards the embryo or the egg, which we call syncytin. In mammals, syncytin plays a role in developing the placenta. In fact, syncytin is an ancient glycoprotein, similar to the substance that makes up the outer shell or membrane of some viruses. However, they are also found in modern mammals.
Therefore, the researchers believe that about 1900 million years ago, some viruses infected our mammalian ancestors and fused part of their genetic material into the host's DNA. This part of the DNA produces viral proteins, which over time are repurposed into new mammalian genes for the construction of the placenta. Thus, this accidental viral infection leads to a process in which mammals no longer lay eggs. Therefore, all mammals, including us, carry this ancient viral gene to this day, which explains why our mode of reproduction is viviparous rather than oviparous.
In fact, both modes of reproduction, whether viviparous or oviparous, have been able to develop to the present day and are suitable for the vast majority of living things. This proves that both ways are the right path to evolution. As for which animal will choose which mode of reproduction, it is entirely up to natural selection. If our mammalian ancestors had undergone slight changes in the selection nodes of viviparity and oviparity, perhaps we would now reproduce in oviparous ways