For centuries, the question of whether the chicken or the egg came first has puzzled minds young and old, intriguing scholars across generations.
Finally, scientists are becoming more confident in revealing the answer based on a study around amphibians and lizards.
According to the study, which examined 51 fossil species and 29 existing species, the earliest ancestors of modern reptiles, birds, and mammals likely gave birth to live young instead of laying eggs. This groundbreaking finding challenges the notion that all ancestral species exclusively laid eggs.
Published in the journal Nature Ecology & Evolution, these new insights shed light on the long-debated question, offering a glimpse into the reproductive practices of ancient creatures.
In the published paper, the researchers published their findings, stating that, “The discovery of oviparity in this assumed viviparous extinct clade, together with existing evidence, suggests that EER (Extended Embryo Retention) was the primitive reproductive mode.”
EER refers to the extended retention of embryos by the mother for a variable amount of time, likely influenced by favorable environmental conditions for the survival of offspring.
The researchers further elaborated that prior to the emergence of amniotes, a group of vertebrates that undergo embryonic or fetal development, the initial tetrapods that develop limbs from fish-like fins were predominantly exhibited amphibious habits. These early creatures depended on water for feeding and breeding, resembling modern amphibians such as frogs and salamanders.
“With the appearance of amniotes around 320 million years ago, they gained the ability to break away from water by evolving waterproof skin and other mechanisms to regulate water loss. However, the crucial factor was the advent of the amniotic egg. Acting as a self-contained environment, it shielded the developing reptile from dehydration in warm climates, allowing the Amniota to venture away from water and thrive in terrestrial ecosystems,” explained Professor Michael Benton from the Bristol School of Earth Sciences.
This view was challenged by researchers as several lizards and snakes display flexible reproductive strategies, exhibiting both oviparity and viviparity. Fossils revealed that many of these species were live-bearers, indicating a transition between giving birth to young and laying eggs.
“EER is common and variable in lizards and snakes today. Their young can be released, either inside an egg or as little wrigglers, at different developmental stages, and there appear to be ecological advantages of EER, perhaps allowing mothers to release their young when temperatures are warm enough and food supplies are abundant,” explained Dr. Joseph Keating.
While the conclusion has yet to be fully established, the research team suggests that adaptive parental protection provided early animals with an advantage.
