“Metamorphosis” is a book by Frank
Ryan, a biologist who occasionally stints as a popular science writer. This
seems to be the original edition. A later edition comes with forewords by Lynn
Margulis and Dorion Sagan. There may be some other differences between the
editions, too, but unfortunately I haven't seen the later one.
“Metamorphosis” is presumably directed at the “educated public”, and often
comes across as rather boring. It contains chapters on various scientists who
unraveled parts of the mystery of metamorphosis among amphibians, insects or
marine invertebrates. What makes the book really interesting is that Ryan
supports various “heretical” notions about the evolution of metamorphosis and
the origins of species in general. This explains Lynn Margulis' interest in his
work. The late Margulis was most famous for her proposal that symbiosis is a
major factor in the emergence of new species. Her idea that the eukaryotic cell
emerged through symbiosis between different kinds of bacteria have been
accepted by science, but not her wider (and wilder) claims about
“symbiogenesis” as a new evolutionary mechanism overall. Ryan does Margulis one
better, introducing the ideas of marine biologist Donald Williamson to a wider
audience.
I admit that I had never heard of Williamson before reading Ryan's tome,
despite being an unofficial connoisseur of alternative theories about this and
that. Like all marine biologists, Williamson was struck by the enormous
differences between larvae and adults in many species of marine organisms.
Juveniles and adults can look like two different species. In some starfish, the
larva (which is “bilaterally symmetric”) and the adult (which is “radially
symmetric”) develop simultaneously (!) from the same embryo, which raises all
kinds of awkward questions about how the same set of genes can produce two
entirely different body-plans. Other starfish have adults that develop from the
larva as off-shots, rather than the larva as a whole metamorphosing. Other
anomalies include starfish without a larval stage and an embryonic development
suggesting that they are “protostomes” rather than “deuterostomes”. Since these
categories are seen as evolutionary basic, it's a problem why some creatures
end up on the wrong side of the fence (imagine if some humans procreated through
laying eggs – it's that weird). Finally, there are strong similarities between
the larvae of completely unrelated organisms, so similar that convergent
evolution seems something of a stretch.
Williamson proposed a radical solution to these conundrums. The complex life
cycles of the contentious creatures didn't evolve gradually through the usual
Neo-Darwinist processes. Rather, we are dealing with a saltationist event. The
reason why echinodermates (the phylum to which starfish belong) have
chimera-like life cycles is because the larvae and the adults really were
different species originally. The primordial starfish didn't have a larval
stage at all, and hence no metamorphosis. The larvae were “captured” from an
entirely different line of evolution through chance hybridization. The sperm of
a larva-like creature fertilized the eggs of a non-metamorphosing starfish (or
perhaps the other way around), and the result was a bizarre combination of both
creatures, confounding marine biologists ever since…
To most biologists, Williamson's thesis sounds crazy. It's the kind of
“solution” pre-scientific medieval monks might come up with in their leisure
time. One obvious counter-argument is that completely different evolutionary
lines can't hybridize. Hybrids, even sterile hybrids, always take place between
closely related species. Dogs and wolves are a good example, horses and donkeys
another. Yet, Williamson was proposing that organisms from entirely different
phyla could mate and beget viable and fertile offspring. A “phylum” is a very
basic category, animals belonging to different phyla having virtually nothing
in common, since most phyla emerged (and diverged!) about 500 million years ago
during the so-called Cambrian explosion. Humans belong to the phylum previously
known as vertebrates, while insects belong to the phylum Arthropoda (except in
Ryan's book, where they have gotten a phylum all their own). Can one hybridize,
say, sperm from a fly and a human egg? Obviously not (except in horror flicks).
Yet, Williamson was suggesting that such genetic crosses were possible between
at least some phyla of marine organisms.
Sensationally, Williamson did have some experimental backing for his radical
idea. He had single-handedly carried out hybridization experiments in his
laboratory at the Isle of Man in the Irish Sea, and later convinced an entire
team of scientists to carry on with the work. Some of the hybrid embryos,
usually crosses between sea urchins (another echinodermate) and sea squirts (a
tunicate), developed into larvae. Others developed into a kind of spheroids,
creatures nobody had observed before. The spheroids reproduced by budding, a
process unknown among both echinodermates and tunicates. Unfortunately, DNA
tests on the putative hybrids have so far been non-existent, inconclusive or
negative, making it possible that the larvae are really echinodermates that
have suppressed the tunicate sperm (some echinodermates can reproduce by a kind
of parthenogenesis). The spheroids are harder to explain away, but they could
be a previously unknown stage of the echinodermate life cycle (hence another
bizarre anomaly to be explained, rather than the sought for explanation).
Undeterred, Williamson has gone even further, suggesting that all or most
larvae are “transferred” from other phyla, and that all creatures undergoing
complete metamorphosis are hence a kind of chimeras, combining different
species into one. This novel evolutionary mechanism is known as
“hybridogenesis”. In a famous paper, Williamson proposed that caterpillars are
descended from velvet worms, while adult butterflies are real insects, the two
combining through hybridization. Margulis was supportive, but most other
scientists have so far rejected the idea. Both Williamson and Ryan believes
that the Cambrian explosion, when a host of new body plans emerged during a
geologically relatively short period, can be explained by hybridogenesis, with
enormous mats of eggs and sperm cross-fertilizing in the primordial oceans. The
fact that many extinct Cambrian animals look pretty weird, combining traits of
several different animal groups, could mean that nature was experimenting
wildly back then, and that the basic body plans didn't become fixed until
later. The fact that genetic studies reveal genes showing up in all the wrong
places on the tree of life could also suggest massive gene transfer in the
evolutionary past, either through Margulis' proposed symbiogenesis or through
Williamson's hybridogenesis. In Ryan's opinion, while the jury is still out,
Williamson's hypothesis is at least plausible, and it's perfectly possible to
test. The crucial ingredient lacking is more advanced DNA testing on any
“spheroids” or other strange critters that may show up during hybridization
experiments.
My prediction is that such test will be made, sooner or later, as Neo-Darwinism
quietly leaves the stage, replaced by the Extended Synthesis, making exotic
speculations of this kind more legitimate. If the tests will succeed, is
(perhaps) another matter. Part of me hope they will fail. I mean, if completely
unrelated creatures can hybridize, think of the staggering consequences if Big
Business or Big Military-Industrial Complex (or the local dog breeder) gets
hold of the research... Nor do I fancy the notion that H G Wells or H P Lovecraft
might have been right after all! On the other hand, I do consider
hybridogenesis a fascinating idea. Nature is filled with “anomalies” (or things
we consider anomalous, due to our narrow theorizing), so whatever the answer
will turn out to be, it certainly won't be conventional…