Is this true and will we survive?

First of all, let's define what a mass extinction event is.

It's a large die-off of species and it's only happened five or six times

in the 4 billion year history of life on this planet.

The most famous extinction event happened 65 million years ago

when an asteroid struck what's now the Yucatan Peninsula in Mexico.

The asteroid kicked up a lot of dust,

creating a shift in climate and wiping out the dinosaurs and many other species.

Of course, this event had a happy ending for us

because it killed off the big reptiles and created space for mammals

and ultimately, humans.

We may not be so lucky next time.

Mass extinction events can wipe out as many as 90 percent of all species.

Humans would most certainly not survive an event of that magnitude.

But there's no iron-clad proof that we're in a mass extinction event now.

The only thing we know about past extinction events

is what we see in the fossil record.

Even though today, we're losing, by some estimates, 200 species a day,

that still doesn't add up to what we know happened in the past.

The passenger pigeon is a case in point.

It once numbered in the billions in North America and is now extinct.

But there are only two known examples of this species in the fossil record.

What has scientists worried, though, is a similarity between what's happening now

and what we see in past extinction events.

For example, the Permian Extinction Event, about 250 million years ago,

was started by a massive volcanic eruption in what is now Siberia.

Lava vaporized a huge cold deposit, causing a spike in carbon emissions

on the order of what we're seeing today.

Another example is the Great Oxygenation Event of 2.4 billion years ago.

Here, cyanobacteria, a precursor to modern-day plants,

began emitting oxygen, gassing much of the anaerobic bacteria into oblivion.

We're currently changing Earth's atmospheric chemistry in similarly large ways.

Only time will tell if we're causing another mass extinction.

For Scientific American's Instant Egghead, I'm Fred Guterl.