Azerbaijani | English | Persian | Turkish

Blog categories

How did we get here?

How did we get here?

Earth teems with life. But billions of years ago, our planet was just a ball of molten rock.
Did the first Earthlings rise from a chemical soup bubbling in a primordial pond? Or did the seeds of life crash down from outer space?
Now, at last, science may be on the cusp of solving that most enduring mystery.
"How did we get here?"
Life is full of mysteries, but the most compelling mystery is life itself.
The ancient Greeks believed the gods shaped man from clay.
The Vikings tell of two great continents, one made of fire, one of ice. When they met, sparks flew, and the first living beings were born.
Scientists are still trying to solve this age-old conundrum, how simple chemicals were somehow transformed into living molecules, molecules that eventually evolved into you and me.
But the answers may at long last be close at hand, and more surprising than we could have ever imagined.
As a kid, I used to go around with a magnifying glass, trying to set things on fire.
Was I being destructive?
I don't think so.
I was seeing what I could create with the power of the sun.
There was something magic to me about that spark.
Is this what happened on Earth billions of years ago?
Did a spark turn inanimate matter into something that can grow, reproduce, and evolve, something we would define as alive?
It's a puzzle that science has been struggling to piece together.
We have a very good theory of life's evolution, but we have no agreed theory of life's origin.
We don't know how a mix of nonliving chemicals turns itself into a living thing.
That's a very good question.
Because no one's actually experimentally converted the nonliving into living, we don't know recisely
what we need.
It's a question that may never find an answer.
But the scientists who dare to probe our moment of creation are leading us on a fascinating journey to an unexpected destination.
To solve the mystery of our genesis, we have to rewind evolution, go back to the time and the place where the first living things came to be... .
Our solar system more than 4 billion years ago. It was a very different place from the solar system we know. The sun was a young star, cooler than it is today. Earth was much hotter, having just solidified from a molten ball of hot rock. Comets and large meteorites were whirling all around it. Fiery impacts were frequent and often devastating.
It's a period of earth's geological history called the "Hadean," the age of hell.
Geologist Stephen Mojzsis, from the University of Colorado, is traveling back in time to the Hadean, trying to discover evidence of life there.
It's interesting to visualize what you might see standing on the surface of the Hadean Earth some 4 billion years ago.
The moon would fill the sky because it was much closer to the Earth at that time. But the sky itself would appear different. So rather than a beautiful sky blue, it must have been red and blasted by meteors and comets.
The oceans would also have looked different in the Hadean. They would not be blue and clear, but dark green, filled with iron minerals.
To us, it would appear as an alien planet, incapable of sustaining life. But Stephen is convinced it could have harbored primitive life, just like today's most extreme environments, where microscopic organisms find a way to survive.
We know, looking at our planet today. whether in the driest desert, the coldest glacier, the deepest ocean, the tallest mountain ..., life exists.
Why should we expect otherwise for the earliest earth?
Paleontologists look for evidence of ancient life-forms fren in rocks.
There are fossils of sea creatures and plants going back half a billion years.
But the more primitive, microscopic life that existed before that is much harder to detect, and finding rocks that date back to the Hadean is next to impossible.
Almost all of early earth is now gone, buried under lava flows and oceans.
But a few rare outcrops of 4-billion-year-old rock do remain.
Stephen tracked one down in a remote region in the heart of Greenland.
The traditional wisdom was that there could not be a record of the first half billion years of earth history.
Instead, what we have found by careful searches is that not only is there a record from this time period, but the record reveals to us an eminently habitable world.
The rocks Stephen found in Greenland came from an ocean that formed 3.8 billion years ago. They were peppered with a series of black dots. They were lumps of ancient carbon, and there was something very unusual about them.
Carbon comes in two forms:
normal carbon, called carbon 12,
and a much rarer form called carbon 13, or heavy carbon.
Normal geological deposits of carbon contain a precise ratio of carbon 12 to carbon 13, but not these lumps.
Life does something interesting. It discriminates against carbon 13. So the biological matter is profoundly enriched in carbon 12.
In the world's oldest known sedimentary rocks, it's very clear.
There's a carbon isotope signature of early life.
Stephen can't tell what the primitive life-form that left this telltale signature was like, but it must have been able to survive in a brutal environment, a planet constantly pummeled by giant space rocks, a world where most geologists believe the deluge of impacts would have melted huge parts of earth, boiled its oceans dry, and sterilized the entire planet.
Yet Stephen Mojzsis is sure that somewhere on this hellish earth...
There was life.
Stephen has developed a computer simulation to prove our planet could have remained hospitable to life even during the intense bombing campaign it endured in the Hadean 4 billion years ago.
Each of these episodes here is an individual asteroid or comet coming into the earth at the time of the late heavy bombardment.
We find that impact melt pools here --
basically lava lakes, some of which are the size of the continent of Africa. But this blue region here, which represents cold temperatures, are areas where liquid water is still stable. Even in the intense bombardment epoch of the early solar system, the earth would have remained a habitable place.
Microbes deep down in the earth --
or deep in the ocean, like those that today gather around volcanic vents --
would have the best chance of surviving these devastating blows.
What these bombardments do is favor organisms that can find a sanctuary to ride out the raining storm of space debris until the epoch of bombardment is over and the whole world is left for them to colonize. And we think that happened on the Earth about 3.8 billion years ago, when the bombardment ceased.
Stephen's discovery may have pinpointed the time and place for the origin of life, but it tells us nothing about how life actually started.
To do that, scientists must re-create early earth in their labs and try to catch a glimpse of that first magical spark.
Planet Earth, 4 1/2 billion years ago. If we stepped on its surface, molten lava would incinerate us immediately.
One breath of its atmosphere would kill us.
How could life have formed in this bubbling, poisonous hell?
In 1953, two intrepid chemists try to answer this question.
Stanley Miller and Harold Urey designed an experiment to simulate our planet soon after its birth. The results would turn out to be so groundbreaking that the apparatus has been preserved at the Scripps Institution in San Diego by their former student Dr. Jeffrey Bada.
At first glance, it looks like just an assortment of flasks and tubes, but this was carefully designed to, first of all, have a flask that would represent an evaporating ocean, and that was connected to a flask that represented the atmosphere. And in this atmosphere, you see these electrodes, and you can apply an electric discharge to these electrodes to simulateatmospheric lighting. And the products would condense out of the atmosphere via this condenser, run into this tube, and then back into the water flask.
Perhaps inspired by the book of Genesis, Miller and Urey left their experiment on the origin of life running for seven days. Then, as now, the flask representing the ocean slowly started to turn dark brown, filling with a seemingly toxic sludge.
It is a tricky experiment.
If you're not careful, you can blow it up. And moreover, this solution is highly toxic. It contains large amounts of hydrogen cyanide. You'd never want to try and drink this thing, or you'd be dead in a second. But this brown goo also contains something remarkable. Among the toxic chemicals are amino acids. Amino acids are the basic building blocks of proteins, and living things are built from proteins.
They make up bones, hair, and skin.
It was a major breakthrough.
Up to this time, people had tried to make organic compounds simulating an early atmosphere, but they'd always fail. But it was only a baby step toward life. In all these samples saved from Miller and Urey's experiments, Jeffrey Bada has never been able to detect amino acids joined together to make proteins.
What you're making is simple molecules, what we call monomers.
Life as we know it is made up of polymers, complex molecules that are made up of monomers. And the challenge still remains how we can assemble these simple molecules into complex molecules that have a biological function.
That crucial step towards life may not come from a bubbling flask in a lab.
Jen Blank is sure of that because she believes life needed something else to get started on earth and it came from the sky...
At 20,000 miles per hour.
We know that in the early history of the solar system, comets were slamming into the planets, and maybe this would have been a vehicle for delivering prebiotic materials to the early earth.
Comets --
mountain-sized lumps of ice and dust circling the sun.
In 1999, NASA sent a spacecraft called Stardust to snag a piece of a comet and bring it back to earth. When scientists analyzed the material, they discovered that it contained amino acids, the building blocks of protein, the very tissue of life.
One of the big outstanding questions is whether or not organic compounds coming in on a comet
and slamming into the earth could survive the harsh conditions of that delivery experience. And so we set out to test this in the laboratory.
Jen Blank works at SETI, the Search for Extraterrestrial Intelligence, but she's not interested in E.T. so much as alien molecules. So she's developed a computer simulation to see what happens to them on impact.
Comets that hit earth head on are not very promising.
The collision almost completely incinerates them.
In this movie, the blue colors are cold, and as you go toward the red, it gets hotter and hotter. And so we expect the normal impact to have the most extreme conditions.
You can see there's no blue color, so essentially the material's all volatilized or vaporized.But when a comet makes a glancing blow, it doesn't incinerate.
It melts and dumps massive amounts of water and amino acids on our primeval planet.
This time, we're coming in at a 15-degree angle from the horizontal.
Watch the blue, which will correspond to the liquid water.
So you can see a lot of the water's going away, but you still are retaining somewhere on the order of 20% that's being delivered as liquid water.
But these simulations only track the temperature of the comet as it crashes to earth.
To see what happens to the amino acids, Jen needs to get her hands on some serious firepower.
Here's where you might want to have a picture of a gun or something.
This is the shockwave lab at Caltech. Its pride and joy -- a 65-foot-long gun capable of firing projectiles at over 16,000 miles per hour.
And this is Jen's bullet.
Let's imagine this is a comet.
It's a metal cannister with a liquid fill volume that contains water and dissolved amino acids. And, in our experiment, we want to test the the response of this to a collision with an earth.
This is actually a two-stage gun, and at the far end,we use gunpowder to compress gas.
The gunpowder's ignited, and the compressed air slammed into the projectile,and sends it traveling down the barrel of the gun at velocities of around 2,000 to 3,000 miles per hour.
This corresponds to an oblique-angle impact between a comet and a rocky Earth.
To simulate this glancing interplanetary blow, Jennifer dials down the gun from its maximum muzzle speed and prays her cannister and its payload will survive.
Temperatures in these impacts would be essentially thousands of degrees centigrade.
So these are really extreme conditions.
300 volts. Ready.
Smell that?
That's the gunpowder, the gunpowder smell.
The capsule is sent into this larger tank, which is called the recovery tank.
It weighs about 2 tons, and so it really muffles the collision. And, really, at the end of the experiment, if all things go well, it looks much the same as when we first started.
Now Jen breaks open the bullet capsule to retrieve its smashed contents and discover what happened to this cometary soup of organic chemicals.
And here's an example with two
different amino acids in it --
glycine, which is the simplest amino acid, and proline, which is another one. And here's the initial solution. You can see just two.
Here's an analysis of the solution afterwards.
The most high-abundance byproducts turned out to be all combinations of the first two amino acids. So we were actually very excited because the reactions that were occurring were actually forming larger biologically relevant molecules.
Jen's high-powered experiment suggests comets may have bludgeoned early Earth one step closer to life.
They helped amino acids join together, perhaps even forming primitive proteins.
We're actually harnessing the power of the impact to build larger biologically relevant molecules.
So this could have been a dominant source of the building blocks that led to the origin of life.
We used to think our planet was a hellhole 4 billion years ago, where any spark of life would have been instantly incinerated.
Now we know the chemicals of life were inside comets, raining down on our planet. And they didn't crash and burn.
They thrived.
But other ingredients of life are still missing. At the top of the wanted list is DNA, the molecule that carries our genetic identity.
Now at last, one man may have cracked the code to the origins of DNA itself.
It's taken 4 billion years for life to evolve into organisms as complex as you and me.
We're at the tip of a tall branch of the tree of life.
Down in the deepest roots are microbes whose bodies are just a single cell.
But each microbe's biology is just like ours.
No matter how different it looks, its identity resides in a strand of DNA. Every living organism we know belongs somewhere on this tree. But why did the tree of life grow in the first place?
To solve that mystery, we have to find the seed. Scientists don't know muchabout this seed, but they are sure about one thing. Every living organism on earth shares one common feature, a tough outer layer that separates it from the world outside.
Every cell has a membrane. The first seeds of life must have had one, too.
Well, we think we need this kind of primitive cell membrane to keep the genetic molecules trapped inside.
You can't just have everything diffusing around.
You have to have this compartmentalized.
Jack Szostak at Harvard Medical School is on a quest to solve one of life's biggest mysteries --
how the earliest life-forms walled themselves in, defined "me" from "not me." So, modern cell membranes are really tough, they're stable, they're great barriers, they allow cells to control everything that gets in and out. But that requires a lot of fancy, highly evolved machinery, which wasn't around by definition for the first cells. So those membranes had to be really different. But what might these primitive skins around the cells have looked like?
Jack found inspiration...
...In soap bubbles.
So, these are delicate.
They really illustrate the idea that there's an encapsulated space.
That was pretty good.
Soap bubbles are made from molecules called fatty acids, primitive chemicals that Jack thinks were produced on the primeval Earth inside hydrothermal geysers.
The right kinds of minerals could catalyze the assembly of fatty acids from simple things like carbon monoxide, methane, water.
One kind of nice way that that could happen is in a hydrothermal-vent system, and then they could bubble up to the surface.
So Jack set about re-creating the chemistry of a geyser in his lab and eventually created fatty acids.
Now he mixes them into a primordial chemical soup made of water, salt, and amino acids, and he watches as a remarkable transformation takes place. So, membranes form in sheets, and they're kind of wavy, and the edges are kind of high energy.
So what happens is they close up on themselves, and they make little round structures.
So they're closed structures, like tiny soap bubbles.
These little dots are actually hollow bubbles, less than a thousandth of an inch across.
This is what the first living cells may have looked like 4 billion years ago.
Jack may have re-created the seeds of life right under his microscope. But it is impossible for life to evolve unless cells can pull off one crucial task.
They must be able to grow and divide.
This is how seeds blossom into flowers, how a caterpillar becomes a butterfly, and how a baby becomes an adult.
Soap bubbles grow and divide with nothing more than a puff of air, so Jack slightly jiggles the vesicles and watches something incredible happen.
The membrane will start to grow spontaneously.
What we see in the microscope is that it grows in a very peculiar way.
The whole initial vesicle turned into a long, flexible tube.
These fatty-acid membranes are achieving one of evolution's most essential jobs --self-replication. And they appear to do it automatically.
To Jack, this is a sure sign he's getting close to understanding how the miracle of life began.
So we have a cycle of growth and division that's very much like primitive cell growth and division.
Jack may very well have found the recipe for life's earliest cell structure, but to be truly alive, those cells need one vital ingredient -- genes.
Genes are the molecular identity that can pass from a cell to its copy.
All modern life does this with a double helix of DNA.
It's the most complex chemical molecule we know, made up of tens of billions of atoms.
DNA controls every detail of every living thing -- the color of our eyes, the shape of the leaves on a tree, the way even the most simple bacterium swims. And every time a cell divides, it places a copy of its DNA in both cells of the new generation.
At the University of Manchester in England, chemist John Sutherland is trying to discover how DNA came to be ..., the key to discovering how we came to be.
For me, the really interesting point here is the transition between chemistry and biology.
It's always been assumed that you need to have an informational molecule.
You can't really have life unless you can have inheritance, so you need to have something which you can inherit the information stored in a molecule.
A molecule as complex as DNA could never have formed all by itself in a primordial pond.
But there is a simpler version of it -- a single-stranded, informational molecule called ribonucleic acid, or RNA.
Scientists studying the origin of life have long believed that RNA might be the precursor of DNA, a simpler carrier of life's genetic code.
And so the name of the game, if you like, is to make RNA from very, very simple precursor chemicals, using simple organic chemistry under conditions which could have prevailed on the early earth.
RNA is a giant molecular string made up of four different basic building blocks. The order in which these blocks are arranged forms a genetic code. When you look at RNA, as a chemist, you're in sort of astonishment, really, at just what a wonderful molecule it is.
It's complex. It's a really beautiful structure. And you inevitably wonder ..., how on earth did that structure arise?
How on earth did chemistry produce it?
RNA's structure looks simple, but looks can deceive. Each building block is actually made of two parts: a sugar molecule and a nuclear base.
Chemists found they could make the nuclear bases, and so, when they then realized they could actually make the sugars, they just thought, "we must be able to join them together." And so they tried for many years, but the problem was, chemically, you just can't join them together.
For years, scientists tried and failed to whip up some RNA by placing a sugar and a base in a pot and heating it up. But John realized that the primordial soup metaphor was too narrow.
Early Earth's kitchen had more than just a stovetop.
It had an oven, a steamer, and a freezer.
This is like a pond that's sitting there for a while, and then the temperature goes up, and the pond starts to evaporate. And the residue that's left after the pond's evaporated is then heated for a period. And then it rains again. And after it rains, the sun comes out.
So it's a sequence of events, rather than one static set of conditions.
So John and his team re-create the sequence of wetting, drying, heating, and cooling that would have taken place on early Earth. And incredibly, for the first time, these chemists achieve what none before them ever did..., they create two of the four basic building blocks of RNA.
Well, now we have to find a way to make the other two, and I think we're pretty close to doing that. And then we want to string them together to make an RNA polymer molecule. And we've actually recently found some ways in which we think we can string them together.
John may be hot on the trail of the first genes, but there's no guarantee he will ever stumble upon the answer in a lab.
Now a few renegade scientists are taking another path to the origin of life.
They think the most primitive life-forms might still be alive and lurk right under our feet.
All the life we know today, even the simplest of microbes, shares the same biology.
We're all from the same tree of life.
But we know our incredibly complex biology could not have been the first form of life on earth.
What was life like before the life we know?
Finding that life might help us understand how we got here, and the answer could be right under our noses.
All life so far started as the same life.
But it's never been clear to me that you can't have more than one form of life on the planet at the same time.
I think it's entirely likely that we share this planet with a genuinely alien type of life..., alien not because it necessarily came from space, but because it belongs to a different tree of life from you or me.
Remarkably enough, it turns out that nobody has really thought to look for life on earth as we don't know it.
Paul Davies is one of the world's leading cosmologists.
He's the first to admit he's not a biologist, but he's not afraid to venture into their territory and ask questions no one else thought to ask.
We don't know how a mix of nonliving chemicals turns itself into a living thing. We don't even know whether this is a very likely sequence of events or very unlikely sequence of events.
But let's suppose it's very likely. Then shouldn't it have happened many times over right here on earth?
Paul's term for possible homegrown alien life is "the shadow biosphere." And he has a plan for how we might discover it.
We could look at places on earth where conditions are so extreme, so harsh, they're beyond the reach of life as we know it to see if there's some hardy alien type of microorganism living there.
One of Paul's colleagues, Felisa Wolfe-Simon, is looking for a shadow biosphere by digging through the mud. So, the life that we might find in, let's say, this much mud..., we could have billions of different microbes that are as different as you and I are to a mosquito.
In fact, we are more closely related to mosquitoes than they are to each other.
That's how different these microbes are.
Felisa works at the U.S. Geological Survey in Menlo Park, California, but it's NASA's astrobiology program that pays her to study mud. So, one of the things I always do when I go to a new environment ..., regardless of where it is in the world.
I take samples to set up a Winogradsky Column.
A Winogradsky Column is like a potted history of the earth, a breeding ground for all kinds of strange microbes.
You take your sample of mud and you just fill, say, a glass jar, and you put it in the window.
You'll see over time, beautiful colors evolve in this Winogradsky Column.
The progression of colors reveals distinct types of microbes that are inhabiting the column. Every type of microbe needs specific elements to survive. Some feed on sunlight, others on carbon.
More exotic bugs feed on sulfur. But one mud sample Felisa took in 2009 revealed bugs stranger than she could have ever imagined. It came from a place that is highly toxic to almost all life on earth -- Mono Lake in California.
You can't talk about Mono Lake without being a little wistful. You feel like you're on another planet.
It also contains very interesting compounds.
Of particular interest to me was very high levels of arsenic.
There's roughly 40,000 times or so the recommended arsenic from, say, the E.P.A. So it seemed to me logically that it could harbor potentially the vestiges of a shadow biosphere.
Felisa was not disappointed when she dug in the mud of Mono Lake.
She did, indeed, find bugs that could survive these highly toxic doses of arsenic.
And her newfound interest in this poison didn't just cause ripples with her scientific colleagues.
I came home one day, and I brought all these books in, and I put them on my counter.
My husband said, "so, what are you interested in arsenic for?" And I said, "well, I'm interested in how it dissolves, where you might find it." And my husband, a little disturbed, said, "but you don't study arsenic." And then I started to giggle 'cause I realized he was getting a bit nervous.
Arsenic is an effective poison to most organisms because it closely resembles the element phosphorus.
It tricks our cells into substituting one element for the other.
Since phosphorus forms the backbone of DNA, its effects are devastating. But not the bugs that Felisa found.
No matter how big a dose of arsenic she gave them, even many times more than the sky-high levels in Mono Lake, the microbes just kept on growing. So these are microbes using what seems to be poison or toxic substances, and this biology is thriving.
It can cope with hundreds of thousands of times what would be, say, an okay level of arsenic for a human to be exposed to.
Could this microbe be part of the shadow biosphere, our own homegrown alien life?
Could its rules of biology be different from ours?
So, perhaps, maybe they use a similar kind of DNA -- only it's a little different.
Maybe parts of it are different. Maybe they use similar proteins to ours, but maybe they use different amino acids than we do.
If arsenic atoms are somehow replacing phosphorus atoms in these microbes, then these bugs do not fit on our tree of life.
They may not look any different from life as we know it, but these bugs could be the descendants of an entirely separate genesis. So if we found something that even did something a little different, it could mean that here on Earth, there was not just one tree of life, but there could be multiple trees of life.
Humanity has spent an eternity thinking about the loneliness of being us.
It would provide for us an example of something else, some other form of life that was also successful.
If we found those microorganisms, then bingo.
We could say life on Earth has happened at least twice.
Two out of two on one earth-like planet surely means that the universe is teeming with life.
It would be inconceivable life had happened twice on one earth-like planet and not at all on all the other earth-like planets.
If Felisa's bugs are the offspring of a second genesis here on Earth, then life could be a cosmic norm.
We would not be alone in the universe.
But that leads us to an even more intriguing possibility -- life on Earth may not be from Earth at all.
The scientific quest to discover the origin of life has revealed something totally unexpected...
There might have been more than one genesis.
Our planet might harbor not one, but two or more trees of life, each growing from a separate seed. Where did these seeds come from?
That question is forcing us to reassess who we really are because the answer could be out of this world.
Planetary scientist Ben Weiss has pieces of another world in his lab.
They are rocks that have traveled from Mars to Earth, and he thinks microscopic Martians may have hitched a ride on some of them.
About a ton of Martian rocks lands on earth every year, and over the history of the solar system, billions of tons of materials have been transferred.
So it's possible that we, in fact, are Martians.
4 billion years ago, when Earth was being pounded by meteorites and comets, so was Mars.
Shrapnel from those impacts was flying all over the early solar system.
Scientists have found one Martian rock that dates back to those days of interplanetary violence.
It's called ALH84001. ALH84001 is a Martian meteorite that formed on the surface of Mars and then was knocked off the planet.
It wandered around in space, and it landed on the earth about 11,000 years ago.
It was found in Antarctica by some U.S. scientists in 1984.
This rock is very special.
In the 1990s, the discovery of tiny, wormlike structures in the rock turned ALH84001 into an international celebrity.
The claims that these were fossilized remains of Martian microbes have since been discredited, but Ben's investigation of this meteorite might still offer proof of life on Mars because the rock is magnetized.
You see all these little dots, little features.
That's magnetization that originated on Mars.
In fact, it's 4-billion-year-old magnetization. So that must mean that there was a magnetic field, a global magnetic field on Mars 4 billion years ago or earlier.
A global magnetic field acts like a protective cocoon. It has kept Earth's atmosphere safe since day one. Without a magnetic field, the planet has no protection from the solar wind, an intense stream of particles from the sun.
Over hundreds of millions of years, it can blow a planet's atmosphere clean away.
Mars has no magnetic field now and almost no atmosphere.
The magnetization imbedded in ALH84001 proves that 4 billion years ago, Mars had both.
ALH84001 contains some trapped, we think, atmospheric gases from early Mars.
The composition of these gases do not resemble the composition of the Martian atmosphere today.
But they do resemble what you might think early Mars had in its atmosphere.
Even though today it's cold and dry and not very hospitable for life on the surface, we think in its early days that it had a climate which was much more like the Earth's today.
It was presumably significantly warmer and wetter.
There might even have been standing bodies of liquid water on its surface.
Its atmosphere was thicker.
And it might have been a better place for life to originate.
Since Mars is only half the size of Earth, it would have cooled from a molten ball of lava much quicker than Earth.
In other words, Mars could have harbored life sooner than earth. And Ben's most recent study is closing the link between the biology of two planets.
He's discovering that microscopic Martians could, indeed, have survived the hot and bumpy ride on a space rock from Mars to Earth. So, what you see here is a slice of the meteorite.
In fact, on the very outside of it over here on the left, there's a little melted zone. And that's the zone that got heated to high temperatures when it passed through Earth's atmosphere. But most of the meteorite was barely heated at all by passage through the atmosphere.
As mars was slowly cooling and losing the ability to support life, the last Martians may have jumped ship to our warmer, wetter planet.
Organisms could have hitched a ride on this material that was being exchanged between mars and the Earth that even potentially seeded the planet on which they landed on. And there's every reason to think that, you know, if there was an origin of life on Mars billions of years ago, that it probably made it to the Earth multiple times.
If Ben is right, the best place to look for clues to the origin of life might be on the surface of planet that's now dead.
Fossils of primitive life on Mars dating back billions of years could still be there.
Perhaps the next space probe we send to Mars will stumble across them, and we'll be able to study our long-lost ancestors.
But it's also possible the first living things on Earth are still here, lurking in the shadow biosphere.
You and I are the latest chapter of a story that's been unfolding for billions of years.
How that story begins is still unknown.
Did comets seed the Earth with the raw ingredients of life?
Was Mars our original birthplace before we jumped to a new planet? Or are we the Earth's second or third incarnation of life?
Aliens might be living among us.
We might all be Martians.
In the end, the stuff of science fiction might lead us to a cosmic truth and answer that eternal question...
How did we get here?

fa ما چگونه به اينجا آمده ايم؟

tr Buraya nasıl geldik?