Human egg laying stories

Added: Danile Mull - Date: 11.01.2022 11:21 - Views: 36340 - Clicks: 8503

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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. 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. 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 Human egg laying stories 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.

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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 separate times. 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.

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Birds, for instance, have never evolved live birth, possibly because the energy cost of flying while pregnant is unsupportable. 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 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. Most reptiles, for instance, deposit their embryos just a third of the way through their development.

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. Scientists are still learning about the developmental constraints and requirements of these birth strategies.

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Consider, for instance, the thickness of an eggshell. In the outside world, though, a thicker shell is helpful to protect against predators. An egg laid too early, then, might be too thin to survive, and one laid too late might be too thick to meet the exponentially growing oxygen demands of the embryo.

In a paper published in Nature inOrgan and his colleagues demonstrated that before a species could evolve live birth, it probably had to evolve the ability to determine the sex of its offspring genetically. The sex of many creatures is circumstantial: Environmental factors, particularly temperature, can determine whether the embryo develops as male or female.

Consider sea turtles. If they laid all their eggs in the water, they would be less likely to get a variety of males and females because the temperature gradient there is much smaller than it is on land. But once Human egg laying stories marine species has evolved the ability to determine sex through genes, it no longer needs to venture onto land and can fully adapt to its aquatic life. The embryo of a three-toed skink just before it is laid in an egg is almost fully formed. Because the commitment to egg laying occurs so late in development, this species has the option for live birth instead.

At the time of that publication, scientists thought that live birth might have evolved among the reptilian ancestors of ichthyosaurs only after they moved from the land to the sea. But the discovery of a million-year-old fossil changed that. That position is telling: Most viviparous marine reptiles are born tail first so that they can continue to draw oxygen from their mother during labor.

The headfirst birth position indicates the ichthyosaur inherited live birth from an even more ancient land ancestor.

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Whittington and her team study the Australian three-toed skink Saiphos equalisa lizard with the remarkable distinction of being able to both lay eggs and give birth to live young. Recently in Molecular EcologyWhittington and her team describe the differences in gene expression — which genes are switched on or off — between a lizard mother that lays eggs and one that gives birth to live young.

Within a single species, there are thousands of such differences between a female with an egg and one without. Crucially, the specific genes that get switched on in these cases are very different. But in three-toed skinks, a lot of the genes that switch on when a mother makes an egg also get switched on in mothers with embryos. The finding implies that this lizard is in a transitional state between egg laying and live bearing. Which way the lizard is evolving is impossible to say and may still be undetermined.

The idea that the skink could be moving away from live bearing and back to egg laying is a relatively new development in the field. But a growing body of research since then has shown that it may be quite common. Recent analyses of genetic relationships between species revealed that certain egg layers are deeply nested within an evolutionary tree of live-bearing neighbors. Does it use the same genetic instructions when it evolved? Do [different species] have the same problems? The three-toed skink is not the only remarkable creature she studies as she searches for answers.

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Read Later. A lizard that both lays eggs and gives birth to live young is helping scientists understand how and why these forms of reproduction evolved.

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A Strategic Choice Early female animals laid eggs in the sense that they released their ova into the world, often thousands at a time. The Right Temperature for Males Scientists are still learning about the developmental constraints and requirements of these birth strategies.

Stephanie Liang. Show comments.

Human egg laying stories

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Transition from egg-laying to live-born