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You might think that an animal either lays eggs or has live young. But these species prove it's a lot more complicated than that.

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In January of 2014, an Australian  three-toed skink laid three eggs, which is a pretty normal thing for a lizard to do.

Several weeks later, one of those eggs  hatched and a baby skink came out! Then, about a week after that,  the same parent gave live birth to one more bouncing baby skink.

That’s right. Two baby lizards, one born from  an egg and one from live birth, from the same parent and in the same clutch! This indecisive lizard demonstrated  something incredible but true: the line between live birth and  egg-laying is blurry and complicated. [♪ INTRO] Animals tend to have babies one of two  ways: they can give birth to live young, which is technically called  viviparity, or they can lay eggs that young later hatch out  of, which is called oviparity.

Both options have pros and cons: egg-layers can have more babies more  often with less of a strain on the parent, but the eggs can be susceptible  to dangers out in the open. Meanwhile, live bearers keep the embryos  nice and safe inside the parent’s body, but it can be pretty taxing for the parent. Evolutionarily, egg-laying is the default setting, the version that bony animals started out with.

And many groups stuck with it. All turtles and all birds,  for example, only lay eggs. On the other hand, our mammalian  ancestors made the switch to live birth one time many millions of years ago, and nearly all living mammal species  inherited this live-bearing habit.

Some groups are less clear-cut. For example, live birth has  evolved from egg-laying ancestry several separate times among fish and amphibians. And then there are squamates, the group of reptiles that  includes lizards and snakes.

About one-fifth of all squamates are live-bearers, including rattlesnakes, some skinks,  certain spiny lizards, and more. In fact, the transition from egg-laying  to live birth looks to have happened more than 100 times in different branches  of the lizard and snake family tree. And not only are there plenty of egg-laying  species and live-bearing species, there are also in-betweeners.

Some species make eggs but  don’t lay them right away. They hold them in the body while  the embryos continue to grow, and then lay the eggs later on in development. This is called egg retention.

Other species produce parts of the egg,  such as the yolk, but never actually fully form and lay an egg – instead they  give birth to those yolk-fed young. Some squamates give birth both ways! One population might tend to lay  eggs while another population of the same species might tend to give live birth.

And then of course there’s that Australian  skink I mentioned earlier, the first lizard ever known to give birth both ways  within the same litter of little lizards. This SciShow video is supported  by the gourmet chefs at Factor. Factor is meal delivery that’s simple, flexible,  and scratches the itch to treat yourself.

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The ancestors of humans underwent an  evolutionary transition from egg-laying to live-bearing, but that happened in the  distant past, which makes it hard to study. So researchers can turn to our reptilian  cousins to find answers to key questions, such as how this transition affects  the way embryos are nourished. See, inside an egg, the growing  embryo gets nutrients from a yolk.

If the egg evolves away, the  embryo still needs to be fed. In us live-bearing mammals, we have a special nutrient-bearing  tissue called the placenta. In many live-bearing lizards and  snakes, the solution is simple: they’ve lost many parts of the egg,  such as the eggshell, but kept the yolk.

The yolk still provides food for the  embryo inside the body of the parent. A few groups of live-bearing  lizards have taken it a step further and evolved new forms of specialized  tissues to nourish their embryos. They’ve done this by repurposing some of  the existing tissues within lizard eggs, such as the yolk sac or a gas-exchanging  membrane called the chorioallantois.

These tissues, once evolved to  best support an embryo in an egg, have been converted into a  new structure specialized for nourishing an embryo inside a parent instead. If that sounds familiar, it’s  because our mammal ancestors modified similar egg tissues during  the evolution of the placenta! Amazingly, by studying  reproduction in lizards and snakes, we can put together the picture of how  our own reproductive strategies evolved.

So that Australian skink isn’t just a  quirk of nature – it represents a bridge between two reproductive habits that  aren’t as separate as we tend to think. That skink and its cousins provide  an incredible opportunity for us to understand the evolutionary processes  that allowed all of us to be born. [♪ OUTRO]