Scientists are piecing together how and why live-bearing animals evolved from egg-laying ones — and why they might evolve in the other direction on rare occasions.
The old riddle, “Which came first, the chicken or the egg?” is relatively easy to answer as a question about the evolution of birth in animals. Egg laying almost certainly came before live birth; the armored fish that inhabited the oceans half a billion years ago and were ancestral to all land vertebrates seem to have laid eggs. But the rest of the story is far from straightforward.
Over millennia of evolution, nature has come up with only two ways for a newborn animal to come into the world. Either its mother lays it in an egg, where it can continue to grow before hatching, or it stays inside its mother until emerging as a more fully formed squirming newborn. “We have this really fundamental split,” said Camilla Whittington, a biologist at the University of Sydney.
Is there some primordial reason for this strict reproductive dichotomy between egg laying (oviparity) and live birth (viviparity)? When and why did live birth evolve? These are just some of the questions that new research — including studies of a remarkable lizard that can lay eggs and bear live young at the same time — is exploring, all the while underscoring the enormous complexity and variability of sexual reproduction.
Early female animals laid eggs in the sense that they released their ova into the world, often thousands at a time. Sperm released by males then fertilized some of these eggs in a hit-or-miss fashion, and the resulting embryos took their chances on surviving in the hostile world until they hatched. Many creatures, particularly small, simple ones, still reproduce this way.
But as animals became more complex, vertebrate species — including many amphibians, reptiles and even some fish, like sharks — turned to a less chancy strategy: internal fertilization. Females could then ensure that a higher percentage of their eggs would be fertilized, and they could get more selective about which males they would breed with. The embryo could develop safely inside its mother until she eventually released it inside a protective shell.
Live birth evolved later — and more than once. In reptiles alone, it has evolved at least 121 separate times. And although scientists don’t know exactly when the first live animal emerged from its mother, they do know what forces may have been driving the transition from egg laying and what evolutionary steps may have preceded it.
Both birth methods get the job done, of course, but they present contrasting advantages and difficulties. Crucially, egg-laying mothers can be physically free of their offspring sooner. Birds, for instance, have never evolved live birth, possibly because the energy cost of flying while pregnant is unsupportable. Egg layers can also generally have more offspring in a single litter, since the size of the mother’s body isn’t a constraint. This advantage may partially offset the risks of leaving eggs exposed to predation and the elements.
Live-bearing mothers, on the other hand, can house their embryos and protect them from predators and environmental dangers for longer. But they do so at their own peril: Being pregnant exposes them to more predation and puts them at considerable risk from the embryo itself. “The embryo is partially foreign, and its tissues are invading into the tissues of a mom,” said Chris Organ, a biologist at Montana State University. “It’s wild to think about.” For the length of her gestation, the mother balances on a tightrope, diverting resources to a foreign being while keeping herself healthy. Share this articleNewsletter
The Australian three-toed skink (Saiphos equalis) is doubly remarkable: Not only can it both lay eggs and bear live young, but it can do both within a single litter of offspring.
The major difference between oviparity and viviparity therefore centers on a strategic evolutionary decision about when the mother should deposit her embryos. If she deposits them early, she’s an egg layer, and if she deposits them late, she’s a live bearer. Most reptiles, for instance, deposit their embryos just a third of the way through their development.
“Between true egg laying and live bearing there’s a whole range of possible times [to deposit the embryo], but it’s probably disadvantageous to do that,” Whittington said. “We call it a fitness valley.” Animals that try to give birth somewhere in that fitness valley might incur all the risks of egg laying and live bearing without reaping the benefits of either. “We think that, evolutionarily, that’s quite disadvantageous,” she said.
(Marsupials found a novel solution to balancing these risks: The young they give birth to are practically fetal in their immaturity, but they then finish their development inside their mother’s pouch. In this way, the mother can provide the protective advantages of carrying her young to full term without needing to accommodate a full-size newborn inside her body.)
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According to recent research, the earliest mammals, birds, and reptiles might have given birth to live offspring instead of laying eggs.
The hard-shelled egg was previously believed to be essential to the development of amniotes, a class of vertebrates that develop as embryos or foetuses inside an amnion, a membrane that serves as a protective shell inside the egg.
However, a recent study of 29 extant species and 51 fossil species that can be classified as viviparous (giving birth to live young) or oviparous (laying hard or soft-shelled eggs) suggests differently.
The traditional “reptile egg” model found in textbooks has been relegated to the trash by our work and the work of numerous others in recent years. Professor Michael Benton, University of Bristol
The results show that giving birth to live young and prolonged embryo retention were common in all three of the major evolutionary branches of Amniota, which are Mammalia, Lepidosauria (lizards and relatives), and Archosauria (dinosaurs, crocodilians, and birds).
The term “extended embryo retention” (EER) refers to the practice of a mother keeping her offspring for a variable period of time, most likely in response to optimal survival conditions.
Experts point out that while the hard-shelled egg is frequently cited as one of evolution’s greatest inventions, this research suggests that EER provided this specific group of animals with the best possible protection.
“The first tetrapods to evolve limbs from fishy fins were broadly amphibious in habits before the amniotes,” according to Professor Michael Benton of the University of Bristol’s School of Earth Sciences.
As with contemporary amphibians like frogs and salamanders, they had to reside in or close to water in order to feed and procreate.
“Amniotes were able to escape the water when they first appeared 320 million years ago because they developed waterproof skin and other strategies to prevent water loss.
“But the amniotic egg was the key.
It was rumored to be a “private pond,” where the growing reptile was shielded from the warm weather and allowed the Amniota to disperse from the water and take over terrestrial ecosystems. ”.
He went on, “Our work and a lot of others’ in recent years have relegated the traditional’reptile egg’ model found in textbooks to the trash.”
“In order to protect the developing embryo for a longer or shorter period of time inside the mother, the first amniotes evolved extended embryo retention rather than a hard-shelled egg, allowing birth to be postponed until conditions become favorable.”
It’s unclear if the first amniotes were born as live, snapping little insect-eaters or in parchment eggs, but this adaptive parental protection gave them an advantage over tetrapods that spawned earlier. ”.
Professor Baoyu Jiang has discovered that laying eggs is a much easier transition for live-bearing lizards than previously thought.
According to Professor Baoyu Jiang, the project leader, “this conventional view has been challenged.” Numerous lizards and snakes exhibit a flexible reproductive strategy throughout both oviparity and viviparity, as observed by biologists.
Occasionally, closely related species exhibit both behaviors, and it turns out that lizards that bear life can return to laying eggs far more readily than previously thought. ”.
The study, which was carried out by scientists from the Universities of Bristol and Nanjing, was published in Nature Ecology.
Until now, the hard-shelled egg was thought to be the key to the success of the amniotes.
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