Why do viruses make us sick … but only some viruses? 66 – Science Mysteries Explained

Q:
A:
life science
IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
58
What is the earliest evidence we have of life on
Earth?
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 Earths 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
shapes.
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
paleontologist!
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.
LIFE SCIENCE
59
Stromatolites are extremely common in the fossil
record, and they exist at many dierent 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
excited.
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
related.
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
Nitrogen 14
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
Nitrogen 14
Neutron
Carbon 14
Plants absorb
carbon dioxide
and store carbon 14
by photosynthesis
Proton
Neutron
capture
Animals
and people
eat plants
and take in
carbon 14
Carbon 14
Beta
decay