IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
What is the earliest evidence we have of life on
Our understanding of when and how life ﬁrst appeared on Earth continues to improve.
Evidence today points to life appearing almost as soon as the Earth’s crust solidiﬁed
enough to support it. But how can we be so sure?
Coral-like structures called stromatolites provide some of the earliest evidence of life and date back 3.5
billion years. But ﬁguring out the age of a stromatolite is anything but straightforward ….
O the coasts of certain shallow seas and in some
lakes, you can ﬁnd curiously shaped mineral depos-
its. Not quite coral, not quite rock, nevertheless it
seems obvious to look at them that they were made
by some kind of life.
Called stromatolite (from a Greek word for
“bed-like rock”) these odd formations are made
by microorganisms such as blue-green algae.
They range in form from towering cones to round
pillow-shaped structures, or uninteresting vaguely
rounded collections of tiny grains all cement-
ed together. They’re made by tiny, single-celled
creatures that put out a mucus, which then picks
up grains of silt. As the microorganisms build their
calcium carbonate bodies (like modern coral),
the silt gets glued into the structure. Over
time—lots and lots of time—layers of silt build
up into distinctive domes, columns, and other
These are pretty basic life forms. It’s not a
sophisticated colony of complex creatures, but
rather a biological “mat”—a layer of scum that
slowly grows over the remains of the previous
layer of scum. Hardly exciting … unless you’re a
Paleontologists use radiometric dating to
ﬁgure out how old a rock sample is. The prob-
lem with fossils is that they’re usually made of
types of rock that don’t contain the necessary
radioactive particles for dating.
In this case, scientists compare the fossil to
the rocks around it. If the fossil is between two
layers of rock that can be dated, it seems com-
mon sense to assume the fossil is aged some-
where between the two rocks. This is why you
often see descriptions of dinosaurs like “This
species lived 80 to 95 million years ago.”
Stromatolites are extremely common in the fossil
record, and they exist at many dierent layers. They are
an excellent constant in the story of evolution.
Around 3.5 billion years ago the stromatolites
absolutely dominated the biosphere. Life, it seemed,
was all about stromatolites.
By examining how modern stromatolites live and grow,
our theory of evolution now suggests they were respon-
sible for producing a lot of the oxygen in our atmosphere.
Even today, phytoplankton and cyanobacteria (the
scientiﬁc name for blue-green algae) pump out billions of
gallons of oxygen.
That’s right, stromatolites still exist today—you can
see living examples in Western Australia, the Bahamas,
British Columbia (Canada), and a few other places. But
it’s their presence in the fossil record that gets scientists
But our theories about the origin of life aren’t entirely
tied to this one type of ancient microbe. We can look at the
DNA of modern life forms and make assumptions about
how long it’s been since any two species were closely
By “reverse evolving” modern life, scientists can see
that all life on the planet had a common ancestor that,
given the apparent rate of evolutionary change, must have
lived at least 3.5 billion years ago.
What’s interesting about this number is it suggests
Earth is such a perfect place for life to grow that living
things appeared as early as possible—only a billion years
or so after the planet formed. The crust may not even
have been entirely solid. Seas of lava could have jostled
for space with warm, shallow seas. And these seas were
already full of chemicals just itching to combine and even-
tually form the amazing biodiversity we see today.
After death and burial, wood and
bones lose C-14 as it changes to
N-14 by beta decay
When a neutron collides
with a nitrogen atom, a
nitrogen 14 atom becomes
a carbon 14 atom
The Sun’s rays enter the Earth’s
atmosphere and collide with atoms
creating energetic neutrons
and store carbon 14
and take in