Even stranger than Intelligent Design

“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…