Did the reptile or the egg come first? Scientists have changed their minds

In a pioneering study, researchers from Nanjing University and the University of Bristol have discovered that the early ancestors of reptiles, birds, and mammals might have given birth to live young. This counters the prevailing belief that the secret to the evolutionary success of amniotes – a group of vertebrates whose embryos develop within a protective membrane inside an egg – was the hard-shelled egg (see image here).

The research team analyzed a mix of 51 fossil species and 29 contemporary species. They classified these species as either oviparous (laying hard or soft-shelled eggs) or viviparous (birthing live offspring). 

The findings, published in the journal Nature Ecology & Evolution, present an intriguing twist. Evidence of viviparity and extended embryo retention (EER) cropped up across all major evolutionary branches of Amniota. This included Mammalia, Lepidosauria (lizards and their kin), and Archosauria (dinosaurs, crocodilians, birds).

EER refers to a process where the mother retains the young inside her for varying periods. This duration likely depends on optimal survival conditions. Despite the conventional notion that the hard-shelled egg represents a pinnacle of evolutionary innovation, this study proposes EER as the primary protective mechanism for these species.

The evolutionary journey toward the egg

Michael Benton, a professor in Bristol’s School of Earth Sciences, described the evolutionary journey. He noted that the first tetrapods, which evolved limbs from fish fins, were generally amphibious in nature. They depended on proximity to water for feeding and reproduction, akin to modern amphibians like frogs and salamanders.

The advent of amniotes around 320 million years ago marked a significant shift. With the evolution of waterproof skin and mechanisms to control water loss, they could break free from water bodies. 

“But the amniotic egg was the key. It was said to be a ‘private pond’ in which the developing reptile was protected from drying out in the warm climates and enabled the Amniota to move away from the waterside and dominate terrestrial ecosystems,” explained Professor Benton.

Challenging the current textbooks

However, this traditional perspective has now been challenged, according to project leader Professor Baoyu Jiang. Biologists had observed that many lizards and snakes exhibit flexible reproductive strategies, shifting between oviparity and viviparity. 

“Sometimes, closely related species show both behaviors, and it turns out that live-bearing lizards can flip back to laying eggs much more easily than had been assumed,” said Professor Jiang.

Fossil evidence, too, has indicated a predilection for live birth. “Many of them were live-bearers, including the Mesozoic marine reptiles like ichthyosaurs and plesiosaurs,” noted Dr. Armin Elsler.

He also mentioned the case of a choristodere from the Cretaceous of China, which underscored the recurring interplay between oviparity and viviparity across various groups, not just in lizards.

Dr. Joseph Keating elaborated on the pervasiveness of EER among today’s vertebrates: “Their young can be released, either inside an egg or as little wrigglers, at different developmental stages.” 

Dr. Keating also speculated on the potential ecological advantages of EER. The strategy could potentially allow mothers to birth their young when conditions are most favorable – when temperatures are warm enough, and food supplies are abundant.

The “reptile egg” model once lauded in textbooks now seems to be obsolete, according to Professor Benton. He argued that the earliest amniotes likely employed EER instead of laying hard-shelled eggs. This enabled them to safeguard the developing embryo within the mother for an adjustable duration, thereby postponing birth until conditions were favorable. 

“Whether the first amniote babies were born in parchment eggs or emerged as live, quick little insect-eaters remains a mystery,” said Professor Benton. “However, this adaptive parental protection gave them an edge over the earlier tetrapods.”

Study upends long held beliefs

This research provides a profound revision of our understanding of early vertebrate evolution. It replaces the idea of the hard-shelled egg as the key protective measure with the concept of EER. 

The latter not only provides the offspring with a safeguarding environment but also allows for birth to be delayed until the most suitable conditions are met. This significantly enhances the offspring’s chances of survival and, consequently, the successful continuation of the species.

The evidence from both fossil and living species sheds new light on the reproductive flexibility in the animal kingdom. It underlines how nature’s strategies for survival can be far more diverse and adaptable than previously assumed. 

Shaking the foundations, but still much to learn

This aspect, which seems to be particularly relevant among lizards and snakes, suggests that the reproductive methods of these creatures are much more plastic and reactive to environmental conditions than we understood before.

In conclusion, the research shakes the foundations of the conventional belief surrounding the evolution of amniotes. The study highlights how early reptiles, birds, and mammals might have leaned more toward live birth than egg-laying. 

Although it raises as many questions as it answers, one thing is certain: this finding injects fresh perspective into our understanding of the complex journey of life on Earth. The story of amniotes and their reproduction methods is evidently more intricate and fascinating than we ever imagined.

More about amniotes

Amniotes are a group of tetrapods (four-limbed animals with vertebrae) that includes reptiles, birds, and mammals. The defining characteristic of amniotes is the presence of the amnion during embryonic development—a thin, tough membrane that surrounds the embryo, enclosing it in an amniotic cavity filled with fluid.

Here’s a deeper look into what makes amniotes unique:

Evolutionary origin 

Amniotes first appeared during the Carboniferous period, roughly 312 million years ago. The earliest amniotes, like Casineria and Westlothiana, were small, lizard-like animals. They were among the first land-dwelling vertebrates, evolving from amphibian ancestors.

Amniotic egg

The name “amniote” comes from the amniotic egg, which is a key evolutionary adaptation. The egg contains several extraembryonic membranes: the amnion, chorion, yolk sac, and allantois. The amnion, as mentioned before, surrounds the embryo in a fluid-filled cavity, protecting it from desiccation and physical shock. The chorion facilitates gas exchange. The yolk sac provides nourishment from the yolk, while the allantois deals with waste management and also aids in respiration.

Water-independent reproduction 

The evolution of the amniotic egg allowed amniotes to reproduce away from bodies of water, unlike their amphibian ancestors which needed to return to water to lay their eggs. This was a significant development that helped amniotes spread across the globe and colonize a variety of terrestrial habitats.

Internal fertilization 

Amniotes also exhibit internal fertilization, meaning the egg is fertilized inside the body of the female. This is another adaptation that allowed amniotes to become fully terrestrial.

Diverse classification

There are two main classes of amniotes: the synapsids and the sauropsids. Synapsids include mammals and their extinct relatives. Sauropsids include birds, reptiles, and their extinct relatives, such as dinosaurs. This division is based on differences in the number and arrangement of temporal fenestrae (openings) in the skull.

Thermoregulation 

Amniotes can be ectothermic (relying on external sources to regulate body temperature) like most reptiles, or endothermic (regulating body temperature internally) like birds and mammals.

Variety in size and habitats

Amniotes can range from very small creatures such as shrews and hummingbirds, to some of the largest animals ever to have existed, like the blue whale and the long-necked, long-tailed dinosaurs known as sauropods. They have colonized nearly all habitats on Earth, from the deepest oceans (like whales) to the air (like birds), and from tropical rainforests to polar regions.

Anatomical features 

Other common features of amniotes include a more advanced jaw structure compared to amphibians, a high metabolic rate, a well-developed nervous system, and complex social behaviors.

More research is being done to understand the evolution and adaptation mechanisms of amniotes, including studies related to their reproductive strategies.

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