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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.
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 zoƶ-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.

Posted at July 13, 2006 01:03