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Thursday, July 13, 2006
Reading List: The Origins of Larvae
- Williamson, Donald I. The Origins of Larvae. Dordrecht, The Netherlands: Kluwer Academic, 2003. ISBN 1-4020-1514-3.
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I am increasingly beginning to suspect that we are living
through an era which, in retrospect, will be seen, like the early
years of the twentieth century, as the final days preceding
revolutions in a variety of scientific fields. Precision
experiments and the opening of new channels of information about
the universe as diverse as the sequencing of genomes, the imminent
detection of gravitational waves, and detailed measurement
of the cosmic background radiation are amassing more and more
discrepant data which causes scientific journeymen to further
complicate their already messy “standard models”, and
the more imaginative among them to think that maybe there are simple, fundamental
things which we're totally missing. Certainly, when the
scientific consensus is that everything we see and know about comprises
less than 5% of the universe, and a majority of the last generation
of theorists in high energy physics have been working on a theory
which only makes sense in a universe with ten, or maybe eleven, or
maybe twenty-six dimensions, there would seem to be a lot of room
for an Einstein-like conceptual leap which would make everybody
slap their foreheads and exclaim, “How could we have missed
that!
But still we have Darwin, don't we? If the stargazers and particle
smashers are puzzled by what they see, certainly the more down-to-earth
folk who look at creatures that inhabit our planet still stand on a
firm foundation, don't they? Well…maybe not. Perhaps, as this
book argues, not only is the conventional view of the “tree of
life” deeply flawed, the very concept of a tree,
where progenitor species always fork into descendants, but there is
never any interaction between the ramified branches, is incorrect.
(Just to clarify in advance: the author does not question
the fundamental mechanism of Darwinian evolution by natural selection
of inherited random variations, nor argue for some other explanation
for the origin of the diversity in species on Earth. His argument is
that this mechanism may not be the sole explanation for the
characteristics of the many species with larval forms or discordant
embryonic morphology, and that the assumption made by Darwin and his
successors that evolution is a pure process of diversification [or
forking of species from a common ancestor, as if companies only
developed by spin-offs, and never did mergers and acquisitions] may be
a simplification that, while it makes the taxonomist's job easier, is
not warranted by the evidence.)
Many forms of life on Earth are not born from the egg as small
versions of their adult form. Instead, they are born as larvae,
which are often radically different in form from the adult. The best
known example is moths and butterflies, which hatch as caterpillars,
and subsequently reassemble themselves into the winged insects which
mate and produce eggs that hatch into the next generation of
caterpillars. Larvae are not restricted to arthropoda and other
icky phyla: frogs and toads are born as tadpoles and live in one
body form, then transform into quite different adults. Even species,
humans included, which are born as little adults, go through intermediate
stages as developing embryos which have the characteristics of other,
quite different species.
Now, when you look closely at this, (and many will be deterred because
a great deal of larvae and the species they mature into are
rather dreadful), you'll find a long list of curious things which
have puzzled naturalists all the way back to Darwin and before. There
are numerous examples of species which closely resemble one another
and are classified by taxonomists in the same genus which have
larvae which are entirely different from one another—so much
so that if the larvae were classified by themselves, they would probably
be put into different classes or phyla. There are almost identical
larvae which develop into species only distantly related. Closely
related species include those with one or more larval forms, and others
which develop directly: hatching as small individuals already with
the adult form. And there are animals which, in their adult form,
closely resemble the larvae of other species.
What a mess—but then biology is usually messy! The
author, an expert on marine invertebrates (from which the vast
majority of examples in this book are drawn), argues that there is a
simple explanation for all of these discrepancies and anomalies, one
which, if you aren't a biologist yourself, may have already occurred
to you—that larvae (and embryonic forms) are the result of a
hybridisation or merger of two unrelated species,
with the result being a composite which hatches in one form and then
subsequently transforms into the other. The principle of natural
selection would continue to operate on these inter-specific mergers,
of course: complicating or extending the development process of an
animal before it could reproduce would probably be selected out, but,
on the other hand, adding a free-floating or swimming larval form to
an animal whose adult crawls on the ocean bottom or remains fixed to a
given location like a clam or barnacle could confer a huge selective
advantage on the hybrid, and equip it to ride out mass extinction
events because the larval form permitted the species to spread to
marginal habitats where it could survive the extinction event.
The acquisition of a larva by successful hybridisation could spread
among the original species with no larval form not purely by
differential selection but like a sexually transmitted
disease—in other words, like wildfire. Note that many marine
invertebrates reproduce simply by releasing their eggs and sperm into
the sea and letting nature sort it out; consequently, the entire ocean
is a kind of of promiscuous pan-specific singles bar where every
pelagic and benthic creature is trying to mate, utterly
indiscriminately, with every other at the whim of the wave and
current. Most times, as in singles bars, it doesn't work out, but
suppose sometimes it does?
You have to assume a lot of improbable things for this to make
sense, the most difficult of which is that you can combine the
sperm and egg of vastly different creatures and (on extremely
rare occasions) end up with a hybrid which is born in the form
of one and then, at some point, spontaneously transforms into the
other. But ruling this out (or deciding it's plausible) requires
understanding the “meta-program” of embryonic
development—until we do, there's always the possibility we'll
slap our foreheads when we realise how straightforward the mechanism
is which makes this work.
One thing is clear: this is real science; the author makes unambiguous
predictions about biology which can be tested in a variety of ways:
laboratory experiments in hybridisation (on p. 213–214 he advises those
interested in how to persuade various species to release their eggs
and sperm), analysis of genomes (which ought to show evidence of
hybridisation in the past), and detailed comparison of adult species which are
possible progenitors of larval forms with larvae of those with which they
may have hybridised.
If you're insufficiently immersed in the utter weirdness of life forms
on this little sphere we inhabit, there is plenty here to astound you.
Did you know, for example, about Owenia
fusiformis (p. 72), which undergoes “cataclysmic
metamorphosis”, which puts
the chest-burster of Alien
to shame: the larva develops an emerging juvenile worm which,
in less than thirty seconds, turns itself inside-out
and swallows the larva, which it devours in fifteen minutes. The
larva does not “develop into” the juvenile, as is
often said; it is like the first stage of a rocket which is discarded
after it has done its job. How could this have evolved smoothly by
small, continuous changes?
For sheer brrrr factor, it's hard to beat the nemertean worms, which
develop from tiny larvae into adults some of which exceed thirty
metres in length (p. 87).
The author is an expert, and writes for his peers. There are many
paragraphs like the following (p. 189), which will send you to the glossary
at the end of the text (don't overlook it—otherwise you'll spend
lots of time looking up things on the Web).
Adult mantis shrimp (Stomatapoda) live in burrows. The five anterior thoracic appendages are subchelate maxillipeds, and the abdomen bears pleopods and uropods. Some hatch as antizoeas: planktonic larvae that swim with five pairs of biramous thoracic appendages. These larvae gradually change into pseudozoeas, with subchelate maxillipeds and with four or five pairs of natatory pleopods. Other stomatopods hatch as pseudozoeas. There are no uropods in the larval stages. The lack of uropods and the form of the other appendages contrasts with the condition in decapod larvae. It seems improbable that stomatopod larvae could have evolved from ancestral forms corresponding to zoeas and megalopas, and I suggest that the Decapoda and the Stomatopoda acquired their larvae from different foreign sources.
In addition to the zoö-jargon, another deterrent to reading this book is the cost: a list price of USD 109, quoted at Amazon.com at this writing at USD 85, which is a lot of money for a 260 page monograph, however superbly produced and notwithstanding its small potential audience; so fascinating and potentially significant is the content that one would happily part with USD 15 to read a PDF, but at prices like this one's curiosity becomes constrained by the countervailing virtue of parsimony. Still, if Williamson is right, some of the fundamental assumptions underlying our understanding of life on Earth for the last century and a half may be dead wrong, and if his conjecture stands the test of experiment, we may have at hand an understanding of mysteries such as the Cambrian explosion of animal body forms and the apparent “punctuated equilibria” in the fossil record. There is a Nobel Prize here for somebody who confirms that this supposition is correct. Lynn Margulis, whose own theory of the origin of eukaryotic cells by the incorporation of previously free-living organisms as endosymbionts, which is now becoming the consensus view, co-authors a foreword which endorses Williamson's somewhat similar view of larvae.
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