So why don't we lay eggs?

Like, seriously.

The vast majority of animal species on this planet lay eggs, most insects, most fish, most amphibians, most reptiles, all birds, and even a few mammals lay eggs to reproduce.

And if you go back far enough, you can see that our ancestors laid eggs for millions of years too.

So what happened to us?

Why do humans keep their young inside instead of laying eggs?

And what would it be like if we did lay eggs?

Can you imagine?

Hi, I'm Niba.

We're cracking open the case on why eggs are so cool by putting them through feats of strength and taking a close look under the shell.

Welcome to Big Ideas, a new show from the team behind Deep Look.

While Deep Look zooms in on one small animal, Big Ideas zooms out, answering the big questions about how animals survive.

When most people think of an egg, they picture this: A chicken egg.

This modern day bird marvel of evolution is surprisingly complex with an impressive set of features.

And yet it's just the tip of the iceberg when it comes to eggs, because eggs are also this and this and this.

Like the animals that make them eggs are constantly evolving over time.

Eggs were around for millions of years before chickens even existed.

So in the age old question of chicken versus egg, the egg wins.

By hundreds of millions of years.

The strategy of animals reproducing using an egg was first hatched not in a nest on land, but actually in the sea.

Researchers think the first animals to make eggs were ancient marine organisms, like sea sponges or possibly comb jellies.

They were broadcast spawners, meaning they release their sperm and eggs right into the water where they meet, and the eggs get fertilized.

Lots of creatures still reproduce this way.

Check out these sea urchins.

They release millions of eggs at a time.

These eggs don't have a hard shell for protection, but being soft allows water to flow into the eggs, bringing oxygen with it, helping the embryo breathe.

The embryos grow into larvae and they're on their own from day one, but they're self-sufficient feeding on microscopic algae as they go.

To us it might seem cold for these parents to just send their offspring off to fend for themselves.

Tiny fish and other predators do gobble up a lot of the unprotected babies, but they make so many of them that chances are at least some will survive.

What if parents want to give their offspring a bigger headstart in life?

Take a mama salmon, for example.

It lays thousands of eggs at a time which the males fertilize thoroughly.

The salmon packs each egg with more yolk compared to an urchin, that means more proteins, fats, carbs, minerals, and vitamins to feed that growing embryo.

That yolk gives the young salmon a jumpstart on growing when they're at their most tiny and vulnerable stage.

Thanks, mom.

Not all animals that lay eggs in water are broadcast spawners, or lay eggs for that matter.

But overall, laying eggs in water?

Massive success.

We don't live in water, so we would need a different strategy if we were to lay eggs, or would we?

These California newts spend most of their adult lives here along the forest floor, but during their mating season, they make a pilgrimage back to the exact pond where they were born.

Amphibian eggs can breathe through their jelly-like covering, but only in moist environments.

The vast majority of amphibians are tied to the water like this 'cause their eggs would shrivel up and die if they were left in the air.

But reptiles evolved a way to make eggs that survive out of water.

Their eggs have a shell, which includes a layer of calcium carbonate to keep the egg from getting dried out.

And they have an added feature, an albumin, you might know it as the egg white, a gel-like substance that provides extra protein and also hydration.

Like a drink for the baby lizard or turtle.

Plus, it provides padding in case the egg gets jostled around and helps regulate temperature by holding onto heat during warm periods and then releasing it during colder times.

Most snakes, most lizards, and most turtles have flexible leathery shells.

These eggs can survive out of water, but they still need to be kept in damp environments like buried in wet sand.

The shells of bird eggs are even more rigid and resistant to drying out due to high concentrations of calcium carbonate drawn from the mother's bones and arranged in tightly packed, layered crystals.

And if we did lay eggs, this is probably the kind that you'd want.

While you ponder that, take a look at this.

These shells are still thin, but they're surprisingly strong.

How strong?

Well, birds have a huge diversity of egg shapes and sizes.

Check out these chicken eggs for example.

I'll balance these four eggs pointing up on these bottle caps on the top and on the bottom.

How many of these books do you think I can put on here?

Well, let's find out.

Each egg can support 50 to a hundred pounds.

That's between 22 to 45 kilograms.

As long as the pressure is applied evenly.

Why don't we try something a little heavier?

Let's try 20 pounds of cement.

Oh wow.

This burly shell means it's no sweat to have their parents sit on the eggs all day to keep 'em warm and protect them from predators, all without the egg cracking.

The extra warmth means the eggs can develop and hatch quicker.

But if the eggs parents have to take off for a bit, the shell keeps them from drying out in the sun.

And that albumin helps keep them from getting too hot or too cold.

How about two?

That's a huge advantage, especially for female birds that would otherwise have to carry that extra weight as they fly around getting food.

Maybe that's why every known species of bird lays eggs.

Okay, so the shell is really cool, but what's underneath?

If you soak an egg in vinegar for about a week, the acidic vinegar dissolves the calcium carbonate shell right off the egg.

And this here is the membrane.

Most animal eggs have this membrane to separate their insides from the outside world.

Crack open an egg.

And you can see the yolk, that mega food source, along with two chalazae, the two thin ropes that hold the yolk in the center of the egg, like little anchors.

That's surrounded by the clear albumin water supply.

And the yolk has a white spot called the blastodisc, which contains the ovum, the female reproductive cell.

But if the hen who laid this egg had mated with a rooster, that sperm would fertilize this ovum and grow into a tiny embryo.

If it's kept warm, that embryo feeds on the yolk and albumin and grows.

It also dissolves calcium from the inside of the shell to build the bones of the growing chick, making the shell easier to break out of until the chick is ready to make its grand entrance.

All in all, it's a beautifully efficient method for procreation.

And that said, if eggs are such a successful system, why did our ancestors give it up?

Paleontologists think that about 320 million years ago, a group of reptiles split off and some of them became our earliest mammal ancestors.

They laid eggs.

And today a few mammals still keep it old school.

I'm looking at you platypuses and echidnas.

They're members of a group of mammals called monotremes.

They've got soft leathery eggs, kinda like reptiles, but they also feed their young milk like mammals.

Later, some mammals evolved to be able to feed their developing offspring as they were nestled safely inside them: Marsupials, relatives of the kangaroos and koalas we see today were the first mammals to give live birth called viviparity.

They give birth to their young, early and the not really ready to go offspring need to make the perilous journey into their mom's pouch so that she can nurse them.

Instead of getting all their nutrients from the egg, these mammals get around the clock warmth and nutrition provided directly by their mom instead of relying on a finite amount of yolk inside an egg.

And pregnant moms might move a bit slower and be less agile, but they aren't tied to eggs that don't move.

Later, placental mammals evolve to carry their offspring longer and complete their development inside their mom, meaning they could nourish the growing embryo continuously while protecting it from danger.

Nowadays, the vast majority of mammals are placental mammals, from cats to whales to people.

There are some drawbacks, though.

Viviparous animals tend to have fewer offspring than egg layers, but their offspring typically have a better chance of survival because of the extra care and protection they receive.

Of the more than 5,000 known species of mammals, only five lay eggs.

And to add to that, there are different reptiles, fish and invertebrates and other non-mammals that also do live birth.

One size doesn't fit all.

An evolution is still happening today.

Who knows what new strategies might come about in the future.

And maybe my life would be better with big old people eggs instead of periods.

But I guess then I might have to eat like a bajillion calories to grow the eggs every month.

And fetus development would probably be way slower since a fetus couldn't get that constant supply of nutrition and warmth.

Unless I stayed at home, keeping the eggs warm for like months or years before it hatched.

Or maybe it would be fun then 'cause then I could just paint it, give it cute little outfits and egg-ccessories.

Would you want that?

Eggs or no eggs, to reproduce most animals first need to mate.

Sometimes it's cuddly, like earthworms.

Sometimes it's not, the poor praying mantis.

Barnacles reach out to visit with their neighbors, whereas newts have to travel.

Watch these four tiny romances blossom in this Deep look episode.

Check it out.