What was the earth
like, back before life appeared?
During the very earliest stages of its existence,
the Earth was still ‘under construction’ and simply too
hot and too chaotic for the formation of life. In fact, current theory
suggests that a Mars-sized planet collided with the Earth about 4.5
billion years ago , blasting away a large amount of crust material
which then fused into the Moon. A collision that huge in scale would
have also propelled most of the Earth’s oceans and atmosphere
into space. Needless to say, any life that might have been present
before such a giant impact event was probably eradicated.
Sometime between 4.4 and 3.8 billion years
ago, the early Earth began to grow relatively calmer and cooler,
and we can start thinking about possible life-friendly conditions.
Current theory suggests that the early Earth
was quite a bit like today-- with continents and oceans composed
of about the same materials as we have now.
The biggest difference was the atmosphere,
which probably did not contain any oxygen . Most likely it consisted
mainly of carbon dioxide, possibly mixed with methane, ammonia and
other ‘reducing’ gasses.
There was no life, but almost certainly there
were many simple organic compounds in the oceans, including hydrogen
cyanide, formaldehyde, methanol, methane and ammonia . All of those
compounds are found in interstellar space and on other bodies in
our Solar System-- so it seems very likely they were present here
The famous Miller and Urey experiments of
the 1950’s showed that fairly complex organic compounds are
easily synthesized when simple molecules such as methane and ammonia
are exposed to an energy source.
Recent research has shown that many complex
organic compounds can also form in directly in space, perhaps via
catalytic activity present in interstellar dust particles . Meteorites
have been found that include amino acids , nucleic acids , complex
hydrocarbons and sugars . The early earth was barraged with comets
and meteorites at a much greater rate than today, and many of those
compounds would have survived impact and ended up in the seas.
So, one way or another, we can imagine an
ocean 4 billion years ago that contained a wide variety of simple
organic compounds— including many of the key components of
modern life such as amino acids, purines, pyrimidines, fatty acids
and simple sugars.
This is the primordial soup.
What was in the Soup?
There is some argument among exobiologists
about the contents of the primordial soup. Of course, all of the
original soup contents have long since been eaten by later life forms,
and there are no surviving rocks from that period— so about
the best anyone can do is draw conjectures from chemical experiments
using simulated conditions, or look at spectroscopic data from other
planets and their moons.
It seems likely the soup contained a variety
of amino acids— they are created readily in most prebiotic
simulations (the original Miller/Urey experimental setup converted
about 2% of its carbon into amino acids ).
Adenine (a purine used in DNA and RNA) is
also produced readily in prebiotic simulations . It’s a particularly
important compound for life, especially when it is combined with
ribose sugar and phosphate to become AMP (adenosine monophosphate),
ADP (adenosine diphosphate) or ATP (adenosine triphosphate).
It is less certain whether other important
life compounds were found in the primordial soup. For example, cytidine
(a pyrimidine used in DNA and RNA) is produced only rarely in ‘soup’ experiments
. Ribose (part of the external skeleton of DNA and RNA) can be synthesized
easily from 5 molecules of formaldehyde, but it is questionable whether
that would happen under prebiotic conditions, and whether the ribose
would be stable .
As we’ll see later, the exact contents of the early soup is
not very critical for the origins of life, so we won’t worry
too much right now about what it contained.
How Thick was the Soup?
There is no direct evidence that helps us
determine the density of the primordial soup, but we can take some
known figures and make some fairly wild guesses about it.
The Earth today contains about 1.2 x 1021
kilograms of surface water. It seems reasonable to guess that there
was approximately the same amount of water 4 billion years ago .
The Earth today also contains about 1.8 x
1015 kilograms of biomass . It also seems a good guess that a similar
mass of organic matter was present prebiotically (though in the form
of simpler molecules, rather than living organisms).
Dissolve those organic materials in the oceans,
and the result is about 1 milligram of organic material per kilogram
of water. That is a very dilute solution (about .000001 molar, or
the equivalent of dissolving one lima bean in a bathtub full of water).
In the past few billion years, vast quantities
of carbon and hydrocarbons have been deposited underground in the
form of coal, petroleum and kerogen. Our current atmosphere also
contains a high percentage of diatomic nitrogen, some of it created
by bacterial action on ammonia, nitrates and other organic materials.
If that ‘lost’ carbon and nitrogen were in organic form
in the early Earth, it’s possible that the amount of organic
molecules in the primordial soup would have been significantly larger.
That means the soup may have been thicker by a couple of orders of
However, as we’ll see later, the exact concentration of the
early soup is not very critical for the origins of life, so we won’t
speculate any further about it.
Was there oil in the soup? Maybe. There are
three possible sources for hydrocarbons on the early Earth— UV-initiated
synthesis from atmospheric methane and ethane , input from incoming
comets and carbonaceous meteorites , and welling up of hydrocarbons
created deep under the surface .
It is possible that the early earth contained
many hydrocarbons at the time of its birth, similar to present-day
conditions on Titan and some other bodies in the Solar System.
Depending on conditions, oil in water will
form a surface film, droplets suspended in the water itself, or a
coating on solid surfaces. Those oil/water interfaces tend to gather
non-polar organic compounds, including some of the amino acids. Under
the right conditions, phospholipids and other amphiphilic hydrocarbons
can even form natural membranes .
Hydrocarbons may have played an important
role in the development of life, and we’ll mention them in
a few places, later on. However they were not critical to biogenesis,
and it is possible that they were not formed in quantity until the
earliest life forms started to create them enzymatically.
There are three basic chemical properties
of the ‘primordial soup’ that would have made biogenesis
much more difficult.
First of all, the soup was probably very
dilute. Because it was so thin, there would have been strong thermodynamic
forces in favor of any hydrolyzing reactions (adding water). That
means, for example, that any long-chain proteins that might happen
to form would quickly break down into individual amino acids.
Likewise, RNA or DNA would not form chains,
and probably would break down into individual components (nucleic
acids, ribose and phosphate ions). In fact ribose is not particularly
stable in a dilute solution, so it probably didn’t exist in
Secondly, the primordial soup was very random--
it probably contained a huge range of different organic compounds,
just whatever happened to come in on comets or synthesize naturally.
There were probably hundreds of different amino acids present, not
just the 20 found in modern life.
Likewise, at least some of the nucleic acids
and other organic compounds used in modern life were probably present,
but many small variations and unrelated compounds would also have
been in the soup with them.
That means there would have been many ‘poisons’ in the
soup— similar compounds that would interfere with the formation
of early biochemistry.
For example, even if by some rare circumstances
the soup managed to randomly assemble a few complete DNA nucleotides
(nucleic acid, deoxy-ribose and phosphate) and then link them into
chains, it could easily have run into a ‘terminator’ nucleotide
that included a dideoxy-ribose component. After that, the chain would
be unable to link to another nucleotide, and chain formation would
Modern life is based on precise, enzyme-controlled
synthesis of a narrow range of compounds, but the soup would have
been more random, and far less friendly for life processes.
Finally, the soup was racemic. That means
that any compounds which can exist in mirror image forms (called
enantiomers) would have contained equal quantities of both forms
(usually expressed as levo-rotary and dextro-rotary, depending on
the way they rotate a beam of polarized light). That is very difference
from current life, which is built almost exclusively from levo-rotary
amino acids, and dextro-rotary sugars.
Even if the soup contained the correct chemicals,
half of them would have been completely wrong for any emerging life
Chaos and Catastrophe
One other detail that affects theories of
biogenesis is the frequent number of large impacts that occurred
on the earth, particularly during the Late Heavy Bombardment era
about 3.85 billion years ago. The massive collisions during that
period may have caused enough damage to obliterate any life near
There is some evidence in early rock deposits
that life had already started to develop before that era. That may
indicate that life began in a more protected environment that was
less affected by large collisions (for example, underground or deep
in the ocean).
It also may indicate that life arose quickly,
during a gap between major impacts, and then spread rapidly to a
large number of locations and environments. In that case, some organisms
could have survived each impact event and then repopulated the world
during the aftermath.
Even the simplest of modern life is based
on hundreds or thousands of proteins, along with long chains of DNA
that carry the genetic code to create them.
The chances of an organism like that arising
from the soup’s raw materials in one step are infinitesimally
small— about akin to the chances of an automobile arising spontaneously
from a random pile of scrap metal.
In fact, even something much simpler-- say,
a single DNA gene capable of creating a gene-duplicating protein--
could never have happened in one step from pure random combinations
of molecules in the soup.
Any movement towards life in the prebiotic
days must have been based on very simple chemistry. It would have
taken many baby steps to move from organic chemistry to biochemistry,
and every step would have required a reasonable chance of happening
unassisted, even in a chaotic world of many random chemicals.
Fortunately for us, there were some physical
and chemical processes in the early Earth that would have helped
with the first steps— specifically the dehydration reactions
which would have taken place in a few places, and the capacity of
fairly simple molecules to act as catalysts, enabling reactions that
can produce more complex compounds.
We will take a closer look at those factors
in the next two chapters.