Original subtitles

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.

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This incredible variability in birthing style makes lizards

and snakes an incredible resource for understanding reproductive evolution.

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.

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