Wonderful Life: The Burgess Shale and the Nature of History (53 page)

Run the tape again, and let the tiny twig of
Homo sapiens
expire in Africa. Other hominids may have stood on the threshold of what we know as human possibilities, but many sensible scenarios would never generate our level of mentality. Run the tape again, and this time Neanderthal perishes in Europe and
Homo erectus
in Asia (as they did in our world). The sole surviving human stock,
Homo erectus
in Africa, stumbles along for a while, even prospers, but does not speciate and therefore remains stable. A mutated virus then wipes
Homo erectus
out, or a change in climate reconverts Africa into inhospitable forest. One little twig on the mammalian branch, a lineage with interesting possibilities that were never realized, joins the vast majority of species in extinction. So what? Most possibilities are never realized, and who will ever know the difference?

Arguments of this form lead me to the conclusion that biology’s most profound insight into human nature, status, and potential lies in the simple phrase, the embodiment of contingency:
Homo sapiens
is an entity, not a tendency.

By taking this form of argument across all scales of time and extent, and right to the heart of our own evolution, I hope I have convinced you that contingency matters where it counts most. Otherwise, you may view this projected replaying of life’s tape as merely a game about alien creatures. You may ask if all my reveries really make any difference. Who cares, in the old spirit of America at its pragmatic best? It is fun to imagine oneself as a sort of divine disk jockey, sitting before the tape machine of time with a library of cassettes labeled “priapulids,” “polychaetes,” and “primates.” But would it really matter if all the replays of the Burgess Shale produced their unrealized opposite—and we inhabited a world of wiwaxiids, a sea floor littered with little penis worms, and forests full of phororhacids? We might be shucking sclerites instead of opening shells for our clambakes. Our trophy rooms might vie for the longest
Diatryma
beak, not the richest lion mane. But what would be fundamentally different?

Everything, I suggest. The divine tape player holds a million scenarios, each perfectly sensible. Little quirks at the outset, occurring for no particular reason, unleash cascades of consequences that make a particular future seem inevitable in retrospect. But the slightest early nudge contacts a different groove, and history veers into another plausible channel, diverging continually from its original pathway. The end results are so different, the initial perturbation so apparently trivial. If little penis worms ruled the sea, I have no confidence that
Australopithecus
would ever have walked erect on the savannas of Africa. And so, for ourselves, I think we can only exclaim, O brave—and improbable—new world, that has such people in it!

I must end this book with a confession. I pulled a small, and I trust harmless, pedagogical trick on you. In my long discussion of Burgess Shale organisms, I purposely left one creature out. I might offer the flimsy excuse that Simon Conway Morris has not yet published his monograph on this genu—for he has been saving the best for last. But that claim would be disingenuous. I forbore because I also wanted to save the best for last.

In his 1911 paper on supposed Burgess annelids, Walcott described an attractive species, a laterally compressed ribbon-shaped creature some two inches in length (figure 5.8). He named it
Pikaia gracilens
, to honor nearby Mount Pika, and to indicate a certain elegance of form. Walcott confidently placed
Pikaia
among the polychaete worms. He based this classification on the obvious and regular segmentation of the body.

Simon Conway Morris therefore received
Pikaia
along with his general thesis assignment of the Burgess “worms.” As he studied the thirty or so specimens of
Pikaia
then known, he reached a firm conclusion that others had suspected, and that had circulated around the paleontological rumor mills for some time.
Pikaia
is not an annelid worm. It is a chordate, a member of our own phylum—in fact, the first recorded member of our immediate ancestry. (Realizing the importance of this insight, Simon wisely saved
Pikaia
for the last of his Burgess studies. When you have something rare and significant, you must be patient and wait until your thoughts are settled and your techniques honed to their highest craft; for this is the one, above all, that you must get right.)

The structures that Walcott had identified as annelid segments exhibit the characteristic zigzag bend of chordate myotomes, or bands of muscle. Furthermore,
Pikaia
has a notochord, the stiffened dorsal rod that gives our phylum, Chordata, its name. In many respects
Pikaia
resembles, at least in general level of organization, the living
Amphioxus
—long used in laboratories and lecture rooms as a model for the “primitive” organization of prevertebrate chordates. Conway Morris and Whittington declare:

5.8.
Pikaia
, the world’s first known chordate, from the Burgess Shale. Note the features of our phylum: the notochord or stiffened rod along the back that evolved into our spinal column, and the zigzag muscle bands. Drawn by Marianne Collins.

Fossils of true vertebrates, initially represented by agnathan, or jawless, fishes, first appear in the Middle Ordovician, with fragmentary material of uncertain affinity from the Lower Ordovician and even the Upper Cambrian—all considerably later than the Burgess
Pikaia
(see Gagnier, Blieck, and Rodrigo, 1986).

I do not, of course, claim that
Pikaia
itself is the actual ancestor of vertebrates, nor would I be foolish enough to state that all opportunity for a chordate future resided with
Pikaia
in the Middle Cambrian; other chordates, as yet undiscovered, must have inhabited Cambrian seas. But I suspect, from the rarity of
Pikaia
in the Burgess and the absence of chordates in other Lower Paleozoic
Lagerstätten
, that our phylum did not rank among the great Cambrian success stories, and that chordates faced a tenuous future in Burgess times.

Pikaia
is the missing and final link in our story of contingency—the direct connection between Burgess decimation and eventual human evolution. We need no longer talk of subjects peripheral to our parochial concern—of alternative worlds crowded with little penis worms, of marrelliform arthropods and no mosquitoes, of fearsome anomalocarids gobbling fishes. Wind the tape of life back to Burgess times, and let it play again. If
Pikaia
does not survive in the replay, we are wiped out of future history—all of us, from shark to robin to orangutan. And I don’t think that any handicapper, given Burgess evidence as known today, would have granted very favorable odds for the persistence of
Pikaia
.

And so, if you wish to ask the question of the age—why do humans exist?—a major part of the answer, touching those aspects of the issue that science can treat at all, must be: because
Pikaia
survived the Burgess decimation. This response does not cite a single law of nature; it embodies no statement about predictable evolutionary pathways, no calculation of probabilities based on general rules of anatomy or ecology. The survival of
Pikaia
was a contingency of “just history.” I do not think that any “higher” answer can be given, and I cannot imagine that any resolution could be more fascinating. We are the offspring of history, and must establish our own paths in this most diverse and interesting of conceivable universe—one indifferent to our suffering, and therefore offering us maximal freedom to thrive, or to fail, in our own chosen way.

Aitken, J. D., and I. A. McIlreath. 1984. The Cathedral Reef escarpment, a Cambrian great wall with humble origins.
Geos: Energy Mines and Resources, Canada
13(1):17–19.

Allison, P. A. 1988. The role of anoxia in the decay and mineralization of proteinaceous macro-fossils.
Paleobiology
14:139–54.

Anonymous. 1987. Yoho’s fossils have world significance.
Yoho National Park Highline
.

Bengtson, S. 1977. Early Cambrian button-shaped phosphatic microfossils from the Siberian platform.
Palaeontology
20:751–62.

Bengtson, S., and T. P. Fletcher. 1983. The oldest sequence of skeletal fossils in the Lower Cambrian of southwestern Newfoundland.
Canadian Journal of Earth Sciences
20: 525–36.

Bethell, T. 1976. Darwin’s mistake.
Harper’s
, February.

Briggs, D. E. G. 1976. The arthropod
Branchiocaris
n. gen., Middle Cambrian, Burgess Shale, British Columbia.
Geological Survey of Canada Bulletin
264:1–29.

Briggs, D. E. G. 1977. Bivalved arthropods from the Cambrian Burgess Shale of British Columbia.
Palaeontology
20:595–621.

Briggs, D. E. G. 1978. The morphology, mode of life, and affinities of
Canadaspis perfecta
(Crustacea: Phyllocarida), Middle Cambrian, Burgess Shale, British Columbia.
Philosophical Transactions of the Royal Society, London
B 281:439–87.

Briggs, D. E. G. 1979.
Anomalocaris
, the largest known Cambrian arthropod.
Palaeontology
22:631–64.

Briggs, D. E. G. 1981a. The arthropod
Odaraia alata
Walcott, Middle Cambrian, Burgess Shale, British Columbia.
Philosophical Transactions of the Royal Society, London
B 291:541–85.

Briggs, D. E. G. 1981b. Relationships of arthropods from the Burgess Shale and other Cambrian sequences. Open File Report 81–743, U.S. Geological Survey, pp. 38–41.

Briggs, D. E. G. 1983. Affinities and early evolution of the Crustacea: The evidence of the Cambrian fossils. In F. R. Schram (ed.),
Crustacean Phylogeny
, pp. 1–22. Rotterdam: A. A. Balkema.

Briggs, D. E. G. 1985. Les premiers arthropodes.
La Recherche
16:340–49.

Briggs, D. E. G., E. N. K. Clarkson, and R. J. Aldridge. 1983. The conodont animal.
Lethaia
16:1–14.

Briggs, D. E. G., and D. Collins. 1988. A Middle Cambrian chelicerate from Mount Stephen, British Columbia.
Palaeontology
31:779–98.

Briggs, D. E. G., and S. Conway Morris. 1986. Problematica from the Middle Cambrian Burgess Shale of British Columbia. In A. Hoffman and M. H. Nitecki (eds.),
Problematic fossil taxa
, pp. 167–83. New York: Oxford University Press.

Briggs, D. E. G., and R. A. Robison. 1984. Exceptionally preserved nontrilobite arthropods and
Anomalocaris
from the Middle Cambrian of Utah.
University of Kansas Paleontological Contributions
, Paper 111.

Briggs, D. E. G., and H. B. Whittington. 1985. Modes of life of arthropods from the Burgess Shale, British Columbia.
Transactions of the Royal Society of Edinburgh
76:149–60.

Bruton, D. L. 1981. The arthropod
Sidneyia inexpectans
, Middle Cambrian, Burgess Shale, British Columbia.
Philosophical Transactions of the Royal Society, London
B 295:619–56.

Bruton, D. L., and H. B. Whittington. 1983.
Emeraldella
and
Leanchoilia
, two arthropods from the Burgess Shale, British Columbia.
Philosophical Transactions of the Royal Society, London
B 300:553–85.

Cann, R. L., M. Stoneking, and A. C. Wilson. 1987. Mitochondrial DNA and human evolution.
Nature
325:31–36.

Collins, D. H. 1985. A new Burgess Shale type fauna in the Middle Cambrian Stephen Formation on Mount Stephen, British Columbia. In
Annual Meeting, Geological Society of America
, p. 550.

Collins, D. H., D. E. G. Briggs, and S. Conway Morris. 1983. New Burgess Shale fossil sites reveal Middle Cambrian faunal complex.
Science
222:163–67.

Conway Morris, S. 1976a.
Nectocaris pteryx
, a new organism from the Middle Cambrian Burgess Shale of British Columbia.
Neues Jahrbuch für Geologie und Paläontologie
, 12:705–13.

Conway Morris, S. 1976b. A new Cambrian lophophorate from the Burgess Shale of British Columbia.
Palaeontology
19:199–222.

Conway Morris, S. 1977a. A new entoproct-like organism from the Burgess Shale of British Columbia.
Palaeontology
20:833–45.

Conway Morris, S. 1977b. A redescription of the Middle Cambrian worm
Amiskwia sagittiformis
Walcott from the Burgess Shale of British Columbia.
Paläontologische Zeitschrift
51:271–87.

Conway Morris, S. 1977c. A new metazoan from the Cambrian Burgess Shale, British Columbia.
Palaeontology
20:623–40.

Conway Morris, S. 1977d. Fossil priapulid worms. In
Special papers in Palaeontology
, vol. 20. London: Palaeontological Association.