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We've seen that the conditions on the
early
Earth became clement for life rather early
on.
But what is the evidencee for early life
on Earth?
The only direct evidence we have is from
fossils and
chemical signatures preserved in the
geological record, and this evidence may
be proof that life was established on the
Earth by 3.5
billion years ago, possibly as early as
3.8 billion years ago.
It's thought that the evidence does
suggest life
by 3.8 billion years, then it's likely to
have
arisen some time before that, because that
early
evidence is probably not preserved in the
fossil record.
This would suggest that life was present
soon
after the formation of the crust and
oceans.
And at the end of that period of late
heavy bombardment, that
period of intense asteroid and comet
impacts on the surface of early Earth.
Now, before we look
at some of that evidence, it's worth
reminding ourselves
that the evidence is subject to intense
scientific controversy.
And to base, in fact, the evidence for
early life on
Earth is one of the most hotly debated
areas of astrobiology.
There are several lines of evidence that
have been
developed to suggest the presence of life
on early Earth.
One line of evidence of early life on
Earth, is features called stromatolites.
Stromatolites is laminated mounds
which formed today in shallow marine
water.
You can find them in Shark Bay in
Australia, for example.
They're built up by successive
accumulation of sediments
and microbes, and also rocks such as
calcium carbonate.
So as the microbes form, sediments collect
on top of them and new layers of
microbes form, and eventually you end up
with these macroscopically visible mounds
of microbial activity.
The oldest stramphrolites have
been found in 3.46 billion year old apex
church silica rocks in Western Australia's
Warraweena group.
One of the reasons why they're so subject
to controversy is because these
mounds, as you can understand, look like
features that can form by non-biological
processes.
For example, if sediments are laid down in
shallow marine environments or rivers for
example, they can also form these wavy
textures that look at least, on the large
scale, a little bit like stromatolites.
So, further lines of evidence have been
the search for microfossils.
Fossils of individual microorganisms in
these stromatolites or other
types of rocks that can be evidence for
life.
This 3.46 billion year old stromatholites
in Western Australia contain
these filamentous structures which are
thought to resemble modern cynobacteria.
That composed of kerogen,
which is the alteration product of heated
and pressurized organic matter.
And you can see some examples here of
proported filaments in these apex charts.
These micro fossils have also been subject
to intense debate.
Particularly because non-biological
processes
can also form filamentous structures.
We don't really know the context of these
ancient rocks,
so this is another line of evidence,
though, that astrobiologists
seek in order to demonstrate early
life on Earth, fossils of individual
microorganisms.
There are also indirect ways to show the
presence of life.
And one way is through chemical fossils.
For example, carbon that we met earlier in
this course and is the
backbone of most molecules used by life,
comes in different forms or isotopes.
For example, there's Carbon-12, which is
the most common type of carbon in the
environment, and Carbon-13.
Carbon-13 has seven neutrons, Carbon-12
has six neutrons.
In other words, they have a slightly
different atomic mass.
Life will preferentially use this lighter
isotope, or Carbon-12.
So wherever there has been life, the
carbon in those molecules within that life
will tend to be made up of the Carbon-12
rather than the much rarer Carbon-13.
And this
preferential uptake of this isotope
Carbon-12 can be
used as evidence for the presence of life.
And you can see some calcium carbonate
shells
here, in which these early signatures can
be preserved.
These are much more recent fossils.
But similar sorts of organisms taking out
carbon through photosynthesis, depositing
that carbon either
in minerals or in organic matter, can also
be used to find chemical signatures on
early Earth.
This so called isotope fractionation, this
preferential uptake of
light, lighter isotopes compared to
heavier isotopes by life, is
seen in carbonate rocks from as far back
as 3.5
billion years ago and slightly more
controversially earlier as well.
So what is the problem with all of this
evidence?
These three lines of evidence, macroscopic
features such as stromatolites,
microfossils of individual microorganisms,
and
indirect evidence such as chemical
signatures.
Well, first of all, these are rocks very
heavily metamorphosed.
In other words, they've been altered by
heat and
pressure since their formation those many
billions of years ago.
This makes it much more difficult to
interpret the evidence.
The geological settings are usually
uncertain while heavily disputed.
We don't know exactly
what the conditions were like in the
regions where those early
rocks were formed, and so were they really
conducive for life?
That's very important to determine to find
out whether the fossils
or chemical signatures that we see are
really plausible evidence for life.
And many of the features that we
observed can be produced by nonbiological
processes.
The wavy textures in stromatolites can be
produced in sediments without biology.
Microfossils can be formed with similar
types of structures by non biological
processes.
Filamenters, non biological structures
that just look like biology.
Chemical signatures, isotopic
fractionation can also
be caused by non biological processes.
But scientist think that when this
evidence is taken together, it
does provide some compelling evidence for
life on the early earth.
So what have we learned?
We've learned that it's thought that life
is established
on the Earth by at least 3.5 billion years
ago.
The main evidence of this life
comes from macroscopic features such as
stromatolites,
microfossils of creatures, and independent
chemical alterations,
such as the fractionation of carbon
isotopes.
Many people dispute this evidence on the
grounds that
similar features could be a result of
non-biological processes.
But it does show that the search for life
on early Earth and
the evidence for life on early Earth is an
ongoing challenge of astrobiology.
And this work will reveal whether the
early life on Earth was established 3.5
billion years ago or earlier, and what the
nature of this life might have been.
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