Gould, The Reversal of Hallucigenia

with plates and short spines, the Greenland form with longer spines, and finally, inverted Hallucigenia with greatly elongated spines.

The reversal of Hallucigenia has capped and sealed the tale. The larger conclusion seems inescapable: in the great period of maximal anatomical variety and experimentation that followed right after the Cambrian explosion first populated the earth with multicellular animals of modern design, the Onychophora represented a substantial and independent group of diverse and successful marine organisms. The modern terrestrial species are a tiny and peripheral remnant, a bare clinging to life for a lineage that once ranked among the major players. The shoehorn and straightening rod have served us poorly as strategies of interpretation. Groups with few species may be highly distinct in genealogy.

Onychophorans, moreover, are not the only small cluster of straggling survivors within groups that were once major branches of life's tree. The distinct phylum of priapulid worms, for example, contains fewer than 20 species worldwide, compared with some 8,000 for marine polychaete worms, members of the dominant phylum Annelida. Yet, in the Cambrian period at the beginning of multicellular history, priapulids and polychaetes were equally common and similarly endowed (or so it seemed) with prospects for long-term success. Moreover, just as onychophorans have held on by surviving in the peripheral habitat of terrestrial life (for a formerly marine group), modern priapulids all live in harsh and marginal environments--mostly in cold or deep waters and often with low levels of oxygen.

In recognizing the Onychophora as a distinct group with an ancient legacy of much greater breadth, we may regret the loss of tidiness provided by the shoehorn and straightening rod, but we should rejoice in the interesting conceptual gains. For by our latest reckoning of life's early history, "uncomfortable" groups like the Onychophora should exist today. We once thought that the history of life moved upward and outward from simple beginnings in a few primitive, ancestral lines to ever more and ever better--the conventional notion that I have called the cone of increasing diversity. On this model, an ancient and distinctive genealogical status for several small groups (like the Onychophora) makes no sense--for life's early history, at the point of the cone, shouldn't have featured many distinct anatomies at all. The large living groups of mollusks arthropods, annelids, vertebrates, and so on--all of which have fossil records extending back to this early time--provide quite enough material for legacies from these early times of limited simplicity. But the reinterpretation of the Burgess Shale, and our burgeoning interest in the early history of multicellular life in general, have indicated that the cone model is not only wrong but also backward. Life may have reached a maximal spread of anatomical experimentation in these early days--and later history may be epitomized as a diminution of these initial possibilities by decimation, rather than a continual expansion.

In this reversed model of a grass field, with most blades clipped off and just a few proliferating wildly thereafter, we should expect to find a fair number of blades that survived the mower but never flowered extensively again--whereas, in the cone model, the forest of blades never existed, and the early history of life provides insufficient raw material for many distinct modern groups such as the Onychophora.

However much I may regret the loss of a wonderful weirdo in the reversal of Hallucigenia, and in its consequent change in status from oddball to onychophoran, I am more than compensated by fascinating insight into the history of the ancestors of my favorite name bearer, Peripatus. I revel in the knowledge that these marginal and neglected animals belong to a once-mighty group that included armored members with plates and long spines. And I rejoice in the further knowledge thus provided about the strange and potent times of life's early multicellular history. (My regret, in any case, could not possibly be more irrelevant to nature's constitution, either now or 500 million years ago. Hallucigenia was what it was: my hopes, and those of any scientist, are only worth considering as potential biases that can block our understanding of nature's factuality.)

Peripatus may walk prouder in the pleasures of pedigree. We humans, as intellectual descendants of Aristotle, the original peripatetic, might consider a favorite motto from "the master of them that know"--well begun is half done (from the Politics, book 5, chapter 4). Apply it first to the onychophorans themselves--for in a tough world dominated by contingent good fortune in surviving extinction, a good beginning of high diversity affords maximal prospect for some legacy long down the hard road. But apply it also to us, the paleontologists who strive to understand this complex history of life. By turning Hallucigenia upside down, we have probably taken a large step toward getting the history of life right side up.

Stephen Jay Gould teaches biology, geology, and the history of science at Harvard University.