Download Abiogenesis – Students should know basic problems a successful

Abiogenesis – Students should know basic problems a successful hypothesis must overcome. Arguably the most important problem area in the development of life is the origin of the first biological cell from
nonliving chemicals. That is because a living and reproducing cell is required before evolution can get started.
This origin of the first cell is often referred to as abiogenesis.
Abiogenesis is a fascinating problem that has been awaiting a solution for decades. Most textbooks and these
supplements pass lightly over the problem and simply leave the students ignorant of why the problem exists.
Over time a few solution attempts have been advanced and one or another of these is often found in
textbooks. But it is universally recognized that these attempts are woefully inadequate. For example New
Scientist magazine quotes the famed professor Paul Davies saying, “Nobody knows how a mixture of lifeless
chemicals spontaneously organized themselves into the first living cell.”(1) Our students should be given a
basic understanding of why this is and an acquaintance with the most important issues that must be dealt with
by a successful hypothesis explaining the origin of the first cells from naturally occurring nonliving chemicals.
The first cell requires a complex system of coordinated molecules that work together like the parts of a
machine to metabolize the surrounding sources of energy and regularly reproduce the system before any form
of evolution can start taking place. In all known life forms the complex of machines are largely composed of
systems of protein machines with a control system that enables orderly operation of the machines and the
system’s self-replication. The control system is embodied in the cell’s DNA. A few of the more important
elements of this abiogenesis problem are discussed briefly below.
Origin of proteins and systems: Before the first cells could arise from nonliving chemicals a system of
proteins must be brought together to form the components of a machine. These proteins cannot be some
randomly occurring set; rather, they must be highly specific in order to work together like the parts of a complex
machine to perform many necessary functions. Each of these proteins is made up of a long chain of amino
acids ordered in a very specific sequence. A typical chain length is 400 specific amino acids. To have a
specific sequence of amino acids occur by random processes is difficult to justify so let’s first consider a very
favorable idealized case. Assume we have a beaker or vat filled with nothing but the 20 amino acid molecules
needed for these proteins. If only random events are at work there will be one chance in 20400, or about 10520,
of obtaining the specific protein sequence needed. This number is so large it is difficult to comprehend. For
comparison one might note that astronomers estimate that there are about 1080 atoms in the observable
universe. But even that is an extremely tiny number compared to 10520 ! So by random unguided processes
there is no useful chance of obtaining any one specific protein. What is worse is that conservative estimates of
the number of coordinated proteins needed to form the system of molecular machines in the simplest cell are
at least several hundred. A 2006 estimate by Hamilton Smith at the J. Craig Venter Institute came up with a
minimum size of 387 proteins.(2) At present there is no reasonable way known to overcome the immense
improbability of a random unguided process producing a protein system like this even under ideal conditions.
This is a major problem and a fascinating opportunity for young scientists.
Folding and Chirality: The problem is actually much larger than just obtaining the necessary sequence of
amino acids found in the protein chains because these chains must be folded into the correct three
dimensional shape in order to function as parts of molecular machines. The necessary shape for functionality,
once we have the required sequence, is most often not one of the naturally occurring shapes. In living cells
there are molecular machines called chaperones that enable this folding in the correct manner and conduct the
new protein to the place where it is needed. But how is this done before the first cell was complete? Further,
each of the 20 amino acids naturally occurs in two different three dimensional forms or stereoisomers with a
symmetry like our left and right hand. This left and right handed symmetry is referred to as chirality. However,
the proteins in living systems are made exclusively of left handed amino acids! If a right handed amino acid
gets into the protein chain it usually cannot fold into the correct functional shape. So this folding and
stereoisomers add two additional dimensions of complexity to obtaining a system of proteins. But here is
another problem: all naturally occurring chemical processes produce a fifty-fifty mixture of the two
stereoisomers of each amino acid. So how did the first cells exclude the right handed amino acids molecules –
fully one-half of the naturally available molecules?
1 Destructive chemical processes: Any naturally occurring environment will be far from ideal and there will be
a number of processes working against the assembly of a system of proteins with the correct sequences of
amino acids of exclusively left-handed chirality all folded in the three dimensional shape that will make them
functional parts of a machine. Let’s briefly consider four of the most basic natural chemical processes that are
working to take protein chains apart. First if there is a large proportion of water present, like a pond, lake or
ocean, then the water itself will react with the amino acid chains and break the bonds by a process called
hydrolysis. Living cells have elaborate mechanisms to protect their proteins from hydrolysis but how would
this work before the first complete cell? Second, if there is sunlight or lightning present then there will be
substantial amounts of ultraviolet light present. The photons of UV light have enough energy that they will
break down the amino acid bonds. Third, if there is any free oxygen present then the oxygen will vigorously
react with the protein chains destroying them. The process of photo-dissociation due to UV light from the sun
hitting water vapor molecules in the atmosphere ensures that there has always been some free oxygen
present in the atmosphere. No solid consensus has been reached on the amount of oxygen present in the
early atmosphere, but there is sufficient evidence in the geologic record to make some geologists conclude
that there has always been some significant amount of oxygen in the Earth's atmosphere. Fourth, in any
natural environment there will be a great variety of chemicals contaminates present that will react with the
amino acids as the proteins form and that will destroy the needed sequence and three dimensional shapes and
eliminate any functionality.
DNA and information: Of course all living cells have the information necessary for assembling proteins and a
control system that regulates the operation of the cell’s molecular machines stored in DNA. So some biologists
have conjectured that DNA and RNA came first before the cell. But that would entail all of the same kinds of
problems with assembly by natural processes as with proteins. In any case a living cell would require a
complete system of information and a control right from the start! This obviously raises the question, where did
the information come from? Systems of information are never observed to arise from random processes. Some
biologists insist that it must happen because here we are! But they can offer no physical cause and effect
hypothesis that is testable. On top of that all of the same general kinds of destructive processes discussed
above also operate to destroy DNA and RNA as well as proteins. The bottom line is that DNA requires the
protection and operating machinery of a cell before it can do anything or even survive. And the information
content of the first cell must be accounted for in a rigorous and rational way.
These are some of the basic problems that must be overcome to develop a potentially successful hypothesis
explaining the origin of the first cells from nonliving chemical in a natural environment. No hypothesis has come
anywhere close to overcoming these problems yet. Some have tried to avoid UV light and free oxygen by
supposing the first cells developed deep in the oceans; but even so all the rest of the problems above are still a
huge difficulty.
It is interesting that the history of science shows with surprising frequency that the most intractable problems
are often solved by the innovation of fresh young minds. Perhaps knowledge of these issues will prompt a
young Texas student to pursue science and eventually provide the needed innovation. Without a doubt anyone
who makes a major step forward on the abiogenesis problem will win a Nobel Prize. In any event all biology
students need to have a basic knowledge of the basic difficulties involved in the abiogenesis problem in order
to be considered reasonably educated in biology.
All of these issues relate strongly to the requirements of TEKS section (c)(9)(D).
(1) Davies, P., Australian Centre for Astrobiology, Macquarie University, Sydney, New Scientist 179:32, 12 July,
2003.
(2) Smith, H. O. Essential genes of a minimal bacterium, Proc. Natl. Acad. Sci. 103:425–430, 2006
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