Four and a half billion years ago was a  violent and chaotic time on planet Earth.

During this period, the Hadean,   a Mars-sized planet called Theia  collided with our young planet.

The massive impact partially ripped Earth  apart and shattered Theia, fusing the planets   together and ejecting so much rock into orbit  that it eventually coalesced into the moon.

Scientists agree that there’s no way that early  life could have survived such a collision.

But we do know that life got its  start sometime around half-a-billion   years after this impact — not too  much later, in geological terms.

And this is pretty incredible because,   while Theia was the biggest, it was just  one of several big impacts around this time.

For hundreds of millions of  years, these large asteroids,   sometimes over 1,300 km in diameter,  repeatedly struck the Earth with such   force that they melted part of  the crust and vaporized oceans.

And it’s difficult to imagine how life could have   gotten started while Earth kept  receiving planet-sized punches.

Especially because the early  Earth was inorganic — lacking   the fundamental molecular building blocks of life.

So billions of years before life  evolved into chickens or eggs,   it faced a similar riddle.

How did the chemical  ingredients for life form in the first place?

It turns out that theses asteroid impacts  that made the Earth seem uninhabitable,   might have also been the secret ingredient  in cooking the planet’s primordial soup.

We may have planet-shattering asteroids  to thank for the origin of life on Earth.

Before the first creatures, or first algae, or  even first cells, life emerged from chemical   reactions.

Reactions that replicated molecules  and passed on information, the precursor to genes.

Now, we’re not sure exactly  which reactions came first,   but RNA, peptides, and proteins  were all likely early players.

See, nucleobases are organic molecules that form  the ingredients for RNA.

And many scientists   think that RNA was the first self-replicating  molecule, and later life developed from it.

Amino acids are the building  blocks of peptides and proteins,   which are also essential for generating  many of the reactions that life depends on.

But Earth didn’t start with any  of these pieces of the puzzle.

Today, in one way or another, the building  blocks of life mostly come from… other life.

So the catch is… the first versions of these   organic molecules would need  to have been made without life.

How did it all get started?

The first clue came in 1952, when Stanley Miller,   a PhD student at the University  of Chicago, had an idea.

If the ingredients of life formed from the  basic materials found on the early Earth,   then we should be able to do the same thing  by recreating those conditions in a lab.

So he approached Harold Urey, a  chemistry professor who had already   won a Nobel prize and had also speculated  about this sort of pre-life chemistry.

At first, Urey didn’t think this was a promising  idea, but eventually, Miller convinced him.

So Miller set up in the chemistry  lab, filling a large glass flask   half with water and half with gas — using a  mixture of methane, ammonia, and hydrogen.

This created a miniature ocean and atmosphere,   made up of his best guess at what  materials were around on the early Earth.

He then added some heat and  two sparking electrodes to   simulate ancient lighting and just left it.

Over the next week, the mixture in the  flask turned from clear to reddish-brown.

When he analyzed what he’d produced, Miller  found 5 amino acids had formed in the flask,   3 of which are used to build proteins.

And these weren’t just a few  lucky combinations – the amino   acids were produced in high concentrations.

Since then, modern researchers have re-analyzed  specimens from a number of Miller's experiments   in the 1950s with more advanced techniques,  and they found over 40 amino acids.

So the Miller-Urey experiment demonstrated  that a few simple bolts of lightning   in the right atmosphere could organize inorganic  substances into the building blocks of proteins.

There was just one problem:   Miller was probably wrong about what  the Hadean atmosphere was made of.

See, the potential mixture of  gasses that make up atmospheres   lie on a spectrum from reducing on  one end to oxidizing on the other.

A reducing atmosphere has lots of  gasses that donate electrons to others,   like hydrogen, methane, and ammonia.

Reactions that form complex molecules, like those   that life depends on, tend to happen  more easily in a reducing atmosphere.

An oxidizing atmosphere, on the other hand,   has lots of gasses that take electrons from  others, like carbon dioxide and oxygen.

And an oxidizing atmosphere can break down  those crucial complex organic molecules.

Since we know that oxygen wasn’t around for  at least another billion-and-a-half years,   the atmosphere couldn’t have been strongly  oxidizing until after life really got going.

But for the formation of those  early organic molecules, there’s   a big difference between a strongly  and a weakly reducing atmosphere.

And back in the 1950s, Miller thought that  the ancient atmosphere was strongly reducing   and filled with gasses like methane and ammonia.

But we’ve learned a lot since then.

Like, sunlight breaks down  methane in the atmosphere,   so a methane atmosphere probably wasn’t  stable over a long period of time.

And gasses bubbling up from the mantle  through volcanoes give us a clue about   how the mantle would have influenced  the atmosphere of the early Earth.

So most scientists now think that the  early atmosphere was similar to what   this combination of gasses coming up from the  mantle through volcanoes looks like today.

This means the Hadean atmosphere was  mostly carbon dioxide and nitrogen,   which is only weakly reducing.

This change means that the Miller-Urey  experiment doesn’t work out nearly as   well.

It’s not impossible to form complex  molecules in a weakly reducing atmosphere,   but they just don’t form as easily.

And to complicate things further,  a 2022 study even found that a less   reducing atmosphere would have changed  how electricity discharges through it.

In this atmosphere, an electric field would need   to be about 28% stronger to  generate a lightning strike.

So the sparks simulating the lightning that the  experiment relied on would have been less common.

But Harold Urey may have actually  come up with the solution to this   conundrum – before he even knew it was an issue.

Way back in 1952, he’d proposed that  these asteroid impacts that made the   Hadean so tumultuous may have temporarily  changed the chemistry of the atmosphere.

But their famous experiment didn’t  simulate asteroids.

Even in the 1950s,   throwing rocks at glass lab equipment was  probably against workplace safety rules.

It wasn’t until 70 years later that a 2023  study created a model of early Earth chemistry   that revealed how cosmic blows set off a  series of events that changed everything.

Picture a big iron asteroid, hurtling  through space toward a young Earth.

It’s around a third the size of Pluto,  so, much smaller than the collision that   formed the moon – but still big  enough to pack a cosmic punch.

When they collide, the impact  instantly vaporizes the early ocean,   melts part of Earth’s crust, and scatters iron  from the asteroid throughout the atmosphere.

Over the next few months, that steam  and iron react to form iron oxide   and importantly, hydrogen gas in the atmosphere.

As things cool over time the steam  condenses and the ocean reforms.

But the hydrogen gas stays in the  atmosphere for longer.

It reacts with   nitrogen and carbon dioxide to form methane  and ammonia, creating a reducing atmosphere.

So thousands of years after impact, the  right conditions emerge for sparks of   lightning to make amino acids -- just  like in the Miller-Urey experiment!

Even more than that, scientists have since found  that sunlight passing through this atmosphere   causes reactions between the methane  and nitrogen to form reactive nitriles.

And these are the first step towards building  nucleobases, the building blocks of RNA.

The models suggest that this altered  state lasted for millions of years,   long enough for organic molecules to  proliferate and seed the surface of the Earth.

But that hydrogen gas was light  – so light that Earth’s gravity   wasn’t strong enough to hold on to it forever.

It slowly escaped into space and eventually,   losing this hydrogen brought the  reducing atmosphere to an end.

Until the next major impact, that is.

Looking at the pattern of craters on the moon,   it’s likely this sequence of events happened  as many as seven times during the Hadean.

And while each impact was big,   they probably weren’t big enough  to reset things back to square one.

We think this because a 2009 study  used a computer model to simulate   asteroids hitting the early Earth  and found they probably wouldn’t   have sterilized the planet like that big  Theia collision that formed the moon.

So they seem to have provided the conditions  for life without wiping the slate clean.

Maybe, then, life didn’t arise in spite   of the constant bombardment of  asteroids, but because of it.

And there’s a growing body of  research that’s beginning to   reveal just how these impacts from outer  space were crucial to the origins of life.

For example, a 2015 experiment showed the  plasma created during the smaller asteroid   impacts can turn the simple organic molecule  formamide into the nucleobases used in RNA.

And in 2023, scientists analyzed samples  taken from an asteroid by a spacecraft and   found amino acids as well as one of the  nucleobases used in RNA on the asteroid!

So they could have even brought  some of these to Earth directly.

Comets crashing into Earth might be  part of the story, too.

When they hit,   the ice in them receives a  sudden shockwave of pressure.

And lab experiments have found that this can  link amino acids together to make peptides,   which is another step towards making proteins.

So while we don’t know exactly what  caused the right conditions for the   building blocks of life to form, we  know they did.

After all, we do exist.

And it’s becoming increasingly likely that, even  though these impacts at one point rendered the   Earth uninhabitable, at the end of the day they  are precisely what made it… ultimately habitable.

Life, that thing that Earth  is uniquely famous for, may