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Science Forum Index » Bio Evolution Forum » Book review: Riddled with Life (Marlene Zuk)...
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| Anthony Campbell... |
 Posted: Fri Aug 01, 2008 7:22 am |
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Marlene Zuk
RIDDLED WITH LIFE
Friendly worms, ladybug sex, and the parasites that make us who
we are
━━━━━━━━━━━━━━━━━━━━━━━━━━ Book review by Anthony Campbell. The review is licensed under a
Creative Commons License.
━━━━━━━━━━━━━━━━━━━━━━━━━━
This is a book about parasites in the light of Darwinian
medicine. It starts with an execellent chapter on "Why doctors
need Darwin". Here Zuk makes it clear that, as evolved
organisms, our bodies work 'well enough' but not perfectly.
Contrary to what a widespread but sentimental view of our nature
assumes, we are not primarily designed to be healthy; natural
selection maximises reproduction, not survival or health. This
basic truth underlies much of the book.
Many of us today seem to think that to live free from parasites
is an achievable goal, but Marlene Zuk's view is that this is an
unrealizable ambition. Parasites have evolved along with us and
are an inescapable feature of our world, and the best we can
hope for is to live with them. Zuk dislikes the militaristic
language that many writers use to describe our 'battle' with
parasites.
In fact, a parasite-free existence is perhaps not even
desirable. As Zuk points out, there is some evidence to show
that exposure to parasites early in life protects us against the
subsequent development of allergies. It seems possible that the
remarkable increase in allergies that has occurred in many
industrialized countries in recent times is partly due to
excessive hygiene. 'Parasites' here includes viruses and
bacteria; young children who encounter a lot of viruses as a
result of contact with other children are less likely to develop
allergies later.
Zuk discusses the theory that sex has arisen in evolution as an
adaptation to parasites and thinks it is correct. But that does
not mean that sex is wholly a good thing. Sexual intercourse
provides many parasites with the opportunity to infect a new
host. And in many species males seem to incur particular risks
and costs. They may fight with one another for females,
sometimes being injured or killed in the process, and high
levels of testosterone, though they often make a male more
attractive to females, can also have deleterious effects. As a
rule females tend to live longer than males.
There is an optimistic view of parasites which holds that they
will always evolve to become less virulent because it is not in
the parasite's interest to kill its host. As Zuk makes clear,
this happy scenario is by no means universal. All we can say is
that the parasite will evolve in a way that favours its own
ability to pass on its genes. Sometimes this will result in a
decline in virulence, but by no means always; in some cases it
is advantageous to kill the host.
Perhaps the most interesting part of the book is Zuk's account
of how parasites sometimes modify a host's behaviour. As she
remarks, this is an illustration of Richard Dawkins's 'extended
phenotype'. She provides many examples of this, but of course
the really interesting question is whether the same thing
happens in humans. It is difficult to be certain, but the
possibility definitely exists. The most likely candidate is
toxoplasmosis.
Toxoplasmosis primarily occurs in cats, the intermediate host
being rodents. In rats, infection with toxoplasma makes them
indifferent to the smell of cats and hence more likely to be
eaten, thus infecting the cats. Humans who have been infected
from cats or cat faeces seem to have subtle personality changes.
They are more accident-prone. Men who are infected are more
reserved, less trusting, and more likely to break rules. Women,
in contrast, are more out-going, trusting, and self-assured.
It is still not clear whether these differences are due to the
toxoplasmosis or are personality features that make certain
people more liable to infection. If toxoplamosis is indeed
responsible it suggests that humans, thought not the 'intended'
target, can show the same personality changes as do infected
rats. As Zuk points out, such ideas have profound implications
for how we think of ourselves.
Again, few would argue that the pathogen alone dictates our
essence, but then few would argue that we are solely the
product of any one factor, whether it be our genes, our
hormones, our birth order, or our early experiences with
toilet training. Personality is the sum of all these things,
and it seems artificial to disregard parasites' influence.
The chapters are fully referenced to allow readers to pursue the
ideas discussed.
30 July 2008
━━━━━━━━━━━━━━━━━━━━━━━━━━ %T Riddled with LIfe
%S Friendly worms, ladybug sex, and the parasites that make us
who we are
%A Marlene Zuk
%I Harcourt
%C Orlando, Austin, Nerw York, San Diego, London
%D 2007
%G ISBN 978-0-15-603468-5
%P 328pp
%K biology
%O paperback edition
━━━━━━━━━━━━━━━━━━━━━━━━━━
--
Anthony Campbell - ac at (no spam) acampbell.org.uk
Microsoft-free zone - Using Debian GNU/Linux
http://www.acampbell.org.uk (blog, book reviews,
and sceptical articles) |
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