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Science Forum Index » Bio Evolution Forum » Book review: Cancer: the evolutionary legacy
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| Anthony Campbell |
 Posted: Sat May 03, 2008 7:44 am |
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Mel Greaves
CANCER
The evolutionary legacy
__________________________________________________________________
Book review by Anthony Campbell. The review is licensed under a
Creative Commons License.
__________________________________________________________________
The subtitle of this book tells you the approach the author takes,
which is a profoundly evolutionary one. Darwinism relates to cancer in
two main ways -- the why and the how. It explains why cancer exists as
a consequence of evolution and how it arises and develops within the
body as an evolutionary process.
Greaves is concerned to counter simplistic views about cancer. There
are many different kinds of the disease and many things determine
whether it will occur. "The risk of getting cancer is a complicated
mosaic of inherited genetics, exposure patterns, other modifying
activities (diet, for example), and, inevitably, chance." There is
therefore no single "cause" and no prospect of a single cure.
We are often told that cancer is a consequence of the "unnatural" way
we live, and certainly cultural factors such as smoking and diet, for
example, are important in its production. Some industries have been
responsible for inducing cancer in those who work in them. But, as
Greaves emphasizes, cancer is not a new disease; it is even detectable
in fossils. And it is best understood as the inevitable consequence of
the fact that we are multicellular organisms with a complex structure
and lengthy developmental history.
The starting point of cancer is mutation. Cell reproduction is not, and
cannot be, error-free. If cells reproduced themselves with perfect
fidelity there would be no evolution. But because DNA copying is not
error-free it can lead to cancer. This is why tissues containing cells
which reproduce particularly rapidly, such as those in the bone marrow,
the skin, and the intestines often give rise to cancer. Slowly growing
cancers, however, are often more lethal than those that develop more
rapidly.
Cancer usually begins with a single cell that goes off the rails and
gives rise to a clone. There is nothing abnormal about cloning,
however. It goes on all the time; all the cells in our body are members
of a single clone that derived from just one cell (the fertilised egg).
Numerous subclones develop as the embryo matures, leading to the
various tissues that are found in the mature individual.
In fact, our immune system would not work were it not for cloning
coupled with natural selection, because lymphocytes are generated
continuously in the bone marrow using a process of random gene
shuffling to generate millions of antibodies, ready to match the
antigens of almost any bacterial invader. Natural selection then allows
the appropriate lymphocytes to multiply and produce more antibodies. So
cloning is absolutely essential for our survival, even though it is
also potentially lethal. Cancer is a clone that becomes autonomous and
wild. A cancer clone can give rise to subclones just like any other
clone in the body.
Cancer can occur at any time in our lives, but it tends to do so mainly
either in childhood or in old age. The two manifestations have
different roots. Childhood cancer is due to a developmental error and
is therefore similar to other kinds of developmental abnormality.
Cancers in old age are often due to insults that were received many
years earlier; cancer typically takes a long time to develop although
once it does it can accelerate rapidly.
As others have done, Greaves sees a similarity between cancer cells and
exogenous parasites such as bacteria and viruses. This is more than an
analogy -- it is an exact description. In fact, cancer could be thought
of a form of endogenous parasitism. And just as we have evolved
mechanisms to resist exogenous parasites, so also we have mechanisms to
prevent multiplication of rogue clones in our bodies. Usually these
work -- most cancers do not survive or never progress very far.
To succeed, a cancer clone has to pass through several evolutionary
bottlenecks. There are intrinsic restraints that normally prevent the
uncontrolled proliferation of cells. If it bypasses these successfully
the tumour needs to acquire a blood supply in order to grow; there are
constraints on this too, and interfering with the generation of such
vessels may be potentially one of the best weapons we have for treating
cancer. Even if the tumour becomes established it may be restricted by
the size of its environment, so it has to find a way of disseminating
itself more widely; tumours do this by spreading via the blood stream
or the lymphatics, for example. Finally, cells generally have a finite
life span, so cancer cells need to make themselves immortal by ignoring
or turning off the internal "clock" which normally limits the number of
times a cell can reproduce. Some cancer cell lines in the laboratory
are potentially immortal.
Our cancer treatments are still fairly crude, although this is
beginning to change. There is a close parallel between the ways that
bacteria evolve resistance to antibiotics and cancer cells evolve
resistance to anti-cancer drugs. In both cases the usual mechanism is
that a subclone appears that, by chance, is resistant, and this then
survives by means of natural selection. This is evolution in miniature,
so to speak, but there is also a parallel between cancer evolution and
species evolution. Cancer progression nearly always proceeds in a jerky
manner, with starts and stops, offering "a clear evolutionary parallel
to both species diversification and parasite adaptation".
Although Greaves does discuss some of the historical and social aspects
of his subject, his main message is that cancer can only be understood
in the evolutionary context. Because it is so deeply rooted in our
nature we cannot expect to be free from it, though we may certainly
hope to control it better. "There is a sense in which all our ailments
and particularly our 'modern' chronic disorders are reflections of
design limitations, delayed trade-offs, and nature--nurture mismatches.
They are part of the natural scheme of things even if we would like to
believe that we have been sculpted to perfection."
Although this book is about cancer it has implications for our
understanding of disease as a whole. It deals with what has been called
Darwinian medicine, something which many doctors, even today, are
unfamiliar with. Reading it deepens one's understanding of disease and
indeed of evolution.
2 May 2008
--
Anthony Campbell - ac@acampbell.org.uk
Microsoft-free zone - Using Debian GNU/Linux
http://www.acampbell.org.uk (blog, book reviews,
on-line books and sceptical articles) |
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