This ancient rock is changing our theory on the origin of life Tara Djokic
The Earth is 4.6 billion years old,
but a human lifetime often lasts
for less than 100 years.
So why care about
the history of our planet
when the distant past seems
so inconsequential to everyday life?
You see, as far as we can tell,
Earth is the only planet
in our solar system
known to have sparked life,
and the only system able to provide
life support for human beings.
So why Earth?
We know Earth is unique
for having plate tectonics,
liquid water on its surface
and an oxygen-rich atmosphere.
But this has not always been the case,
and we know this because ancient rocks
have recorded the pivotal moments
in Earth’s planetary evolution.
And one of the best places
to observe those ancient rocks
is in the Pilbara of Western Australia.
The rocks here are 3.5 billion years old,
and they contain some of the oldest
evidence for life on the planet.
Now, often when we think of early life,
we might imagine a stegosaurus
or maybe a fish crawling onto land.
But the early life that I’m talking about
is simple microscopic life, like bacteria.
And their fossils are often preserved
as layered rock structures,
called stromatolites.
This simple form of life
is almost all we see in the fossil record
for the first three billion years
of life on Earth.
Our species can only be traced
back in the fossil record
to a few hundred thousand years ago.
We know from the fossil record,
bacteria life had grabbed
a strong foothold
by about 3.5 to four billion years ago.
The rocks older than this
have been either destroyed
or highly deformed
through plate tectonics.
So what remains a missing
piece of the puzzle
is exactly when and how
life on Earth began.
Here again is that ancient
volcanic landscape in the Pilbara.
Little did I know that our research here
would provide another clue
to that origin-of-life puzzle.
It was on my first field trip here,
toward the end of a full,
long week mapping project,
that I came across something
rather special.
Now, what probably looks like
a bunch of wrinkly old rocks
are actually stromatolites.
And at the center of this mound
was a small, peculiar rock
about the size of a child’s hand.
It took six months before we inspected
this rock under a microscope,
when one of my mentors
at the time, Malcolm Walter,
suggested the rock resembled geyserite.
Geyserite is a rock type that only forms
in and around the edges
of hot spring pools.
Now, in order for you to understand
the significance of geyserite,
I need to take you back
a couple of centuries.
In 1871, in a letter
to his friend Joseph Hooker,
Charles Darwin suggested:
“What if life started
in some warm little pond
with all sort of chemicals
still ready to undergo
more complex changes?”
Well, we know of warm little ponds.
We call them “hot springs.”
In these environments, you have hot water
dissolving minerals
from the underlying rocks.
This solution mixes with organic compounds
and results in a kind of chemical factory,
which researchers have shown
can manufacture simple cellular structures
that are the first steps toward life.
But 100 years after Darwin’s letter,
deep-sea hydrothermal vents, or hot vents,
were discovered in the ocean.
And these are also chemical factories.
This one is located along
the Tonga volcanic arc,
1,100 meters below sea level
in the Pacific Ocean.
The black smoke that you see billowing
out of these chimneylike structures
is also mineral-rich fluid,
which is being fed off by bacteria.
And since the discovery
of these deep-sea vents,
the favored scenario for an origin of life
has been in the ocean.
And this is for good reason:
deep-sea vents are well-known
in the ancient rock record,
and it’s thought that the early Earth
had a global ocean
and very little land surface.
So the probability that deep-sea vents
were abundant on the very early Earth
fits well with an origin of life
in the ocean.
However …
our research in the Pilbara
provides and supports
an alternative perspective.
After three years, finally, we were
able to show that, in fact,
our little rock was geyserite.
So this conclusion suggested
not only did hot springs exist
in our 3.5 billion-year-old
volcano in the Pilbara,
but it pushed back evidence for life
living on land in hot springs
in the geological record of Earth
by three billion years.
And so, from a geological perspective,
Darwin’s warm little pond
is a reasonable origin-of-life candidate.
Of course, it’s still debatable
how life began on Earth,
and it probably always will be.
But it is clear that it’s flourished;
it has diversified,
and it has become ever more complex.
Eventually, it reached
the age of the human,
a species that has begun
to question its own existence
and the existence of life elsewhere:
Is there a cosmic community
waiting to connect with us,
or are we all there is?
A clue to this puzzle again
comes from the ancient rock record.
At about 2.5 billion years ago,
there is evidence that bacteria
had begun to produce oxygen,
kind of like plants do today.
Geologists refer to
the period that followed
as the Great Oxidation Event.
It is implied from rocks
called banded iron formations,
many of which can be observed as
hundreds-of-meter-thick packages of rock
which are exposed in gorges
that carve their way through
the Karijini National Park
in Western Australia.
The arrival of free oxygen allowed
two major changes to occur on our planet.
First, it allowed complex life to evolve.
You see, life needs oxygen
to get big and complex.
And it produced the ozone layer,
which protects modern life
from the harmful effects
of the sun’s UVB radiation.
So in an ironic twist, microbial life
made way for complex life,
and in essence, relinquished
its three-billion-year reign
over the planet.
Today, we humans dig up
fossilized complex life
and burn it for fuel.
This practice pumps vast amounts
of carbon dioxide into the atmosphere,
and like our microbial predecessors,
we have begun to make
substantial changes to our planet.
And the effects of those
are encompassed by global warming.
Unfortunately, the ironic twist here
could see the demise of humanity.
And so maybe the reason
we aren’t connecting with life elsewhere,
intelligent life elsewhere,
is that once it evolves,
it extinguishes itself quickly.
If the rocks could talk,
I suspect they might say this:
life on Earth is precious.
It is the product of
four or so billion years
of a delicate and complex co-evolution
between life and Earth,
of which humans only represent
the very last speck of time.
You can use this information
as a guide or a forecast –
or an explanation as to why it seems
so lonely in this part of the galaxy.
But use it to gain some perspective
about the legacy that you
want to leave behind
on the planet that you call home.
Thank you.
(Applause)