[MUSIC] Let's imagine extraterrestrials land on Earth tomorrow, only they're not here to enslave and destroy humanity, they're just galactic census takers.

Then let's imagine instead of our generals, or world leaders, they want to speak to our scientists.

You know, the important people.

Their first question, might be "How many life forms reside on this planet?"

And the answer is... um, uh, well... umm.

We don't know.

[MUSIC] Hey smart people, Joe here, and as a biologist, this is kind of embarrassing to admit.

We don't know how many species there are on Earth.

5 million?

10 million?

Unless you want to count microbes and extinct species too, then maybe a trillion?

It's the answer to one of the most basic questions about life, and those aliens will probably think we're a bunch of galactic dummies, but it's not like we haven't been trying to find the answer.

Aristotle.

He was big on putting things in order.

Aristotle's universe was made of spheres, one set inside another, with Earth and us in the center.

This was basically all wrong, but for almost two thousand years, this is how most western "science" was approached.

If you just define and organize all the things, you will understand *the secrets of the universe *.

Enter Linnaeus.

He agreed with that Aristotle approach: if he found everything on Earth and gave it a name, he'd unlock *the secrets of the universe * Linnaeus was a Swede, and naming things turns out to be an old Swedish pastime.

His filing system for organizing all those names gave birth to the one taxonomists still use today, but Carl, like most scientists back then, was wrong in a big way, and it took these two guys to show the world why.

In our last video, where we talked about discovering that weird hermit crab caterpillar, we learned that even defining what a species is might be impossible.

And this is a big reason why.

Nature has a pesky habit of changing.

Like counting sand on a beach on a windy day.

It's not only annoying, it's impossible.

We identify maybe 18,000 new species every year, and we've counted about one and a half million living species so far.

That's a lot, but it's still a fraction of what's out there.

One recent estimate says there's 8.7 million non-bacterial species on Earth.

At our current rate it'd take 400 years to ID them all.

Catching them all, just isn't realistic.

Even if we did somehow find all the species that live on land, there's two and a half times as much area covered by water, and stuff lives there too.

We can only make estimates, and over the years those have ranged from 2 to 100 million.

Why are our guesses all over the place?

For one thing scientists have only been able to globally share data through the magic of the internet for like 25 years.

To make things worse, we have a bias towards feathers and fur.

Of the 10,000 or so bird species we know of, we'd found almost half by 1850, because all the hippest Victorians were into birding.

Compare that with insects and arachnids, where half of their known species were found in just the past few decades.

Luckily, the more species we find, the better our guesses get.

Estimates seem to be converging on one number: about 5 million living species, plus or minus 3 million because who needs precision!

That doesn't count microbes, of course, because that number I mentioned earlier, 1 trillion?

That's the real estimate when you count the little guys.

I'm beginning to think maybe we don't run this planet after all.

It may sound impossible to identify, name, and describe all 5 million of Earth's species or whatever the real number is, but consider the Library of Congress holds nearly eight times as many books, each of them written by a person.

If more of us take the time, this is a story we can write.

Because you can't read the story of life, if you don't have all the pages.

You can't understand a system if you don't know all its parts.

And you can't conserve what you don't know exists.

Plus I really don't want to make a fool out of myself in front of the aliens.

Stay curious!