(ORDO NEWS) — Perhaps everyone has heard the age-old riddle: “Which came first: the chicken or the egg?”
Metaphorically, this is a reflection on the futility of determining the cause of a self-perpetuating cycle. Literally, this is a great question for evolutionary biologists.
Chickens come from eggs, and eggs come from chickens. So what came first?
Most biologists unequivocally argue that the egg appeared first. At the most basic level, eggs are simply female sex cells.
Hard external eggs that can be laid on land (also known as amniotic eggs) have been a game changer for vertebrates.
The egg is a very important step in the evolution of vertebrates because it allowed the amniotes to move farther and farther from the water.
Before the advent of hard-shelled eggs filled with nutritious yolks, vertebrates had to rely on bodies of water to reproduce. Most amphibians still face this water constraint – they need moisture to survive.
According to a study published in the journal Current Biology, true birds did not appear in the fossil record until the mid-to-late Jurassic, about 165 to 150 million years ago.
But scientists believe that the first eggs in the shell appeared long before that – about 325 million years ago.
This means that the egg appeared “long before the chicken. These very first eggs were likely malleable and leathery in texture, very similar to those laid by today’s reptiles and platypuses.
During the Carboniferous, Permian, and Triassic periods, there were many terrestrial vertebrates that laid amniotic eggs, but the most famous of these animals are the dinosaurs.
Stein studied some of the earliest known dinosaur eggshells, which come from the early Jurassic, about 200 million years ago.
These eggs had an extremely thin outer shell, only about 100 microns thick – that is, in fact, it is the thickness of a human hair.
However, based on their structure, these early dinosaur eggs would have been rigid rather than flexible, making them the earliest known example of an egg we know today.
This subtlety probably explains why researchers have had a hard time finding earlier eggshell specimens. When an egg encounters rich, acidic soil, it slowly begins to dissolve.
Thus, the egg definitely preceded the chicken. But if we talk specifically about the first chicken egg, then the story changes.
Chickens (Gallus gallus domesticus) probably evolved from a subspecies of red wild birds (Gallus gallus) about 50 million years ago.
According to a 2022 research paper published in the journal Proceedings of the National Academy of Sciences, people living in Southeast Asia first domesticated these birds sometime between 1650 and 1250 BC.
At some point in the process of domestication, the last ancestor of modern chickens laid an egg containing an embryo with enough genetic differences to distinguish it from the parent species.
This chick embryo had to develop into a not-quite-chicken egg before hatching. Then, upon reaching adulthood, he will lay the first real chicken egg. Thus, we can say that the chicken preceded the chicken egg.
But the evolutionary history is ambiguous, and there is evidence that chickens have bred with other jungle subspecies even after becoming their own genetically distinct subspecies.
Some of these traits are more (or less) evident in some modern chicken breeds.
Moreover, the domestication of chickens seems to have occurred several times independently in parts of India and Oceania over several thousand years. Therefore, it is extremely difficult to determine which chicken was the original.
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