We all know that chickens lay eggs, but did you ever wonder how their ancestors managed to survive on land without water? A new study suggests that the earliest reptiles and birds may have had a different strategy: they gave birth to live young.
The study, published in the journal Nature Ecology and Evolution, challenges the long-held view that hard-shelled eggs were the key to the success of amniotes, a group of vertebrates that includes mammals, reptiles and birds. Amniotes have a special membrane inside their eggs that protects the developing embryo from drying out.
The researchers analysed 51 fossil species and 29 living species of amniotes and found evidence that many of them were viviparous, meaning they directly gave birth to their young. They also found signs of extended embryo retention (EER), a flexible reproductive strategy where the mother can keep the young inside her body for varying periods of time, depending on the environmental conditions.
This means that instead of relying on a hard egg shell to protect their offspring, these ancient amniotes may have used EER as a way to adapt to different habitats and climates. EER could also allow the mothers to control the timing and location of birth, ensuring better chances of survival for their young.
Michael Benton, a co-author of the study from the Bristol’s School of Earth Sciences, explained that this finding overturns the traditional view of how amniotes evolved. “Before the amniotes, the first tetrapods to evolve limbs from fishy fins were broadly amphibious in habits. They had to live in or near water to feed and breed, as in modern amphibians such as frogs and salamanders,” he said in a press statement.
“When amniotes appeared around 320 million years ago, they were able to break away from the water by evolving waterproof skins and other characteristics that allowed them to control water loss. But the amniotic egg was key here. In a way, the amniotic was thought to be a ‘private pond’ in which a developing reptile was protected from drying out in warm climates. This was believed to be what allowed Amniota to move away from waterbodies and dominate the land.”
However, the new study shows that this classic model is not supported by the fossil and living evidence. Baoyu Jiang, leader of the project and a professor at Nanjing University, said that biologists had noticed that many lizards and snakes display flexible reproductive strategy across oviparity (egg-laying) and viviparity. “Sometimes, closely related species show both behaviours, and it turns out that live-bearing lizards can flip back to laying eggs much more easily than had been assumed,” he said in a press statement.
Armin Esler, co-author of the study, added that when they looked at fossils, they found that many ancient marine reptiles were live-bearers. Some other fossils showed the to-and-fro between oviparity and viviparity, not just lizards.
The researchers concluded that EER may have been an ancestral trait of amniotes that was lost or modified in some groups over time. EER may have also been an important factor in the evolution of mammals, which are mostly viviparous today.
The study sheds new light on the diversity and complexity of reproductive strategies among vertebrates, and how they may have influenced their evolutionary success. It also raises new questions about how EER affects the development and behaviour of the young, and how it is regulated by hormones and genes.
The next time you see a chicken laying an egg or a lizard giving birth to a baby, you may want to think about their ancient ancestors who may have done things differently.
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