Until last year Hallucigenia was one of the strangest animals that ever lived. This sausage-shaped sea creature, which died out half a billion years ago, early in the Cambrian Period, was said to have walked on seven pairs of spikes and to have sported a row of wavy tentacles along its back. But last year, in the Yunnan province of China, paleontologists dug up some new specimens closely related to Hallucigenia. Those fossils made clear that the Hallucigenia researchers had known was a figment of their imagination: they had been looking at it upside down.
With the spikes now protecting its back and the tentacles converted into two rows of legs, Hallucigenia has joined the class of onychophoran-like animals, whose modern representatives look a little like centipedes--homely, perhaps, but not exotic at all. Yes, a bit embarrassing, concedes Simon Conway Morris, the British paleontologist who described and named Hallucigenia back in 1977. I always suspected we might be looking at it the wrong way, but until the Chinese fossils came along we couldn’t be sure.
If that were all there was to the Hallucigenia story, it would be worth a scholarly paper or two and no more. But the confusion surrounding Hallucigenia is emblematic of a much larger debate now going on in paleontological circles, one that opposes two radically different--indeed, inverted--views of the history of life on Earth. At the center of the debate is an event known as the Cambrian explosion, of which Hallucigenia was a part. During this event, which took place between 600 million and 500 million years ago, nature crossed an important threshold: for the first time multicellular organisms evolved, and they did so in a welter of basic body plans, or phyla, from which all modern organisms are descended.
The key question is, How many body plans arose during the Cambrian explosion? explains Conway Morris. Were there roughly the same number as exist today? Or were there many more, most of which quickly became extinct? Until just a few years ago, most paleontologists would have argued for the first answer--that the 30 or so phyla of today (most of them obscure worms, mollusks, and arthropods) are essentially the same as the Cambrian phyla, and that since then there has been a gradual increase in the diversity of species within each phylum. But in his 1989 book Wonderful Life, Harvard biologist Stephen Jay Gould stood this cone of increasing diversity on its head. In describing the fauna of the Burgess Shale, a paradigmatic Cambrian outcrop in the Canadian Rockies, Gould argued forcefully for the second answer: that the initial explosion of forms in the Cambrian was promptly followed by an implosion.
There may have been as many as a hundred body forms back in the Cambrian, says Gould. The majority became extinct; who knows why, just chance. The result is that the shape of life today was determined by which body plans happened to survive. Run history through again and a whole different set of phyla would have survived, and life today would be very different. Almost certainly, according to Gould, human beings (which belong to the phylum of chordates) would not be around.
Very quickly, history according to Gould became the new conventional wisdom among evolutionary biologists. It makes a wonderful story: evolution, in its first wild experimentation with multicellular creatures, produced a myriad of bizarre forms, most of which quickly became extinct; life today, in all its brilliant diversity, is the result of repeated variations on the themes of the few survivors of that early experiment, a meager representation of what might have been. In Gould’s view, many of the fossils from half a billion years ago look bizarre for the very good reason that, their body plans having become extinct, nothing like them exists today. They are like creatures from outer space-- Hallucigenia being a dramatic example.
The discovery that Hallucigenia can be accommodated in a modern phylum and is not so odd after all seems to put a dent in Gould’s theory. What’s more, the moral of the Hallucigenia story is being repeated by other fossil discoveries, notably those of Conway Morris--who, ironically enough, did a lot of the work on the Burgess Shale. With J. S. Peel of the Geological Survey of Greenland, Conway Morris has for the first time found complete specimens of halkieriids, armored sluglike creatures of the Cambrian. The new fossils, which the researchers dug up in northern Greenland, showed that what had been thought to be a series of mysterious creatures was in fact different body parts of just one. This further reduces the supposedly bizarre character of the Cambrian fossils, says Conway Morris.
If the beasts of half a billion years ago weren’t as weird as they’ve been made out to be, maybe there’s no need to postulate a massive extinction after the Cambrian explosion. Maybe all the Cambrian animals, like Hallucigenia, can be fitted into modern phyla--30 phyla back then, 30 phyla now, and half a billion years of evolution in between. Yes, these kinds of discoveries are negative for my view of things, concedes Gould, but for every one you lose like this, there are others to be gained-- creatures that, according to Gould, have been shoehorned into known groups but will one day be seen to be different.
But another Burgess Shale expert, paleontologist Derek Briggs of the University of Bristol, is also skeptical of the Wonderful Life theory. A couple of years ago he and Richard Fortey, of the Natural History Museum in London, made a close comparative study of Cambrian arthropods, the group that in the modern world includes crabs, spiders, insects, millipedes, and so on. If Gould is right that the number of phyla declined precipitously after the Cambrian, then it stands to reason that diversity should have declined within each surviving phylum as well; that is, Cambrian arthropods should be much more diverse morphologically than their modern descendants. To measure the arthropods’ morphological differences, Briggs and Fortey chose a technique known as cladistics.
In a cladistic analysis, one tries to determine how a number of organisms are related and arrange them on the most plausible evolutionary tree--or cladogram, as the cladists call it--by focusing on certain key characteristics. For instance, all primates have nails on their fingers, not claws, and all apes have a bony ridge above their eyes, but no other primates do. Accordingly, gorillas and chimpanzees, both of which are apes, are more closely related to each other than either is to, say, lemurs, which are primates but not apes. In other words, gorillas and chimpanzees branched off from each other after apes as a group branched off from lemurs.
After compiling data on Cambrian and modern arthropods for a whole range of anatomical characteristics--including the shape of their head shields, the position of their eyes, and the structure of their antennae--Briggs and Fortey drew up a cladogram. They concluded that the Cambrian arthropods were no less closely related than the living ones--and therefore no more morphologically diverse, either. Contrary to Gould’s hypothesis, they said, there was no evidence that the Cambrian produced particularly weird forms. Steve made an interesting argument, and it persuaded a lot of people, but our analysis shows it to be an oversimplification, says Briggs.
Gould, however, is not impressed with Briggs and Fortey’s cladistic analysis. They commit a logical error, he wrote in a recent issue of the journal Paleobiology. Cladistic analysis gives information on branching order, not on morphological disparity. That is, Briggs and Fortey may have correctly discerned which arthropod branched off from which other arthropod, but the cladogram doesn’t tell them in a quantitative way how different the two organisms really are--which is what the debate over the Cambrian explosion is all about.
In response to Gould’s criticism, Briggs and Fortey have since tried a quantitative approach. They have measured the arthropods’ size and shape and so on, and then done a statistical analysis of those measurements--a technique called morphometrics. The results, they say, are the same as those of the cladistic analysis: the Cambrian arthropods were not more structurally diverse than their modern counterparts; they were not all exotic beasts out in morphological left field.
We found that most Cambrian animals were within the central morphology, says Fortey. The outliers are the living things, like barnacles. If Gould’s picture were correct, you’d find the Cambrian forms all over the morphological spectrum, especially at the peripheries, but you don’t see that.
Have the British pair handed Gould his answer, as well as defeat? No, not at all, responds Gould. They’ve made an honest and serious attempt to find an answer here, but their method of morphometrics isn’t up to the challenge. Even once you have a quantitative measure of morphological differences, says Gould, you have to have a way of gauging the significance of those differences--of determining whether the differences are enough to place two organisms in separate species, families, or even phyla. That type of judgment is not easy to make. (Is a coral more different from an oak tree than a cow? Gould asks rhetorically.) So far, according to Gould, no one has found a reliable way of making it regarding fossils.
What is at stake in this methodological debate, for all its apparent arcaneness, is nothing less than the basic structure of the history of life. Briggs and Fortey and Conway Morris contend that evolution since the Cambrian has, if anything, gradually expanded diversity; Gould says it has acted as a grim and arbitrary reaper, ridding the world of diversity rather than adding it. We are left with a standoff, with no neat resolution in sight. But if there is one thing everyone can agree on, it is that the fossil record is rarely neat.