The Wisdom of the Burrowers

Anthony J. Martin, from the author's website:

Burrows are a refuge from predators, a safe home for raising young, or a tool to ambush prey. Burrows also protect animals against all types of natural disasters: fires, droughts, storms, meteorites, global warmings―and coolings. On a grander scale, the first animal burrows transformed the chemistry of the planet itself many millions of years earlier, altering whole ecosystems. Many animal lineages alive now―including our own―only survived a cataclysmic meteorite strike 65 million years ago because they went underground.

The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet

Arthur Crudup:

World of Wonders

I read this book by the late Stephen Jay Gould shortly after it was published in 1989, and more or less randomly grabbed it off the shelf and re-read it recently. I'm not a scientist, let alone an invertebrate paleontologist, so the impressions that follow are strictly those of a layperson.

To be as brief as possible (and with no pretense of doing justice to a rich and intricately argued book), Gould's immediate subject was the rare assemblage of soft-bodied fauna discovered, a century ago, by Charles Doolittle Walcott in the geological formation known as the Burgess Shale in the Canadian Rockies. Walcott, a geologist who was the Secretary of the Smithsonian Institution at the time, classified (Gould's word is "shoehorned") his new discoveries into existing taxa (mostly of arthropods), but subsequent work by Harry Whittington, Simon Conway Morris, Derek Briggs, and others revealed that many of the creatures properly belonged in previously unknown classes or even phylla that later become extinct. These Cambrian fossils, Gould reasoned, provided evidence that in the aftermath of the Cambrian explosion that produced the ancestors of the metazoan animals with which we are familiar (e.g. crustaceans, molluscs, vertebrates — and of course us) there was a wide diversity or disparity of body plans, many of which ultimately proved to be evolutionary dead ends. Instead of a steady progress of expanding diversity from a small number of original phylla, the analysis of the Burgess Shale fauna suggested a winnowing over time of possible avenues for evolutionary development. There may be more species now, but the roster of higher-level taxa was greater during the Cambrian than now. (Or so it seemed.)

Gould's broader argument, which he used the Burgess Shale reclassification to support, centered on the importance of contingency to the evolutionary history of life on earth. The lineages that became extinct were not necessarily essentially inferior to the lineages that survived; they perished because of circumstances that were equivalent, at least in part, to a roll of the dice. Climates changed, continents moved around, asteroids impacted the earth, etc. The fact that these lineages no longer exist (while others do) proves, in a certain sense, that they were less fitted to the challenges that followed, but since the challenges themselves were contingent and not predictable we can't say that a different roll of the dice might not have produced an entirely different outcome. A minor shift might well have led to the premature extinction of the chordates and hence precluded our own existence. (Gould's title alludes explicitly to Frank Capra's film It's a Wonderful Life, in which one small change holds the possibility of triggering a cascade of altered consequences.)

In the past thirty years continuing analysis of the Burgess fauna has, inevitably, complicated the picture considerably. It's no longer as clear as it seemed in 1989 that all of the re-classified species are true "weird wonders" that can't be contained within the familiar major phyla and classes. That process is highly technical and well beyond my ken, but although Gould, had he lived, might have been disappointed by the implications of more recent developments for his thesis, he was well aware of (and specifically notes) the fact that science proceeds by a grinding process of error and rectification. ("One cannot hope to do anything significant or original in science unless one accepts the inevitability of substantial error along the way.") His larger point, reading the role of contingency, remains open.

Ironically, one of Gould's main sources (fully credited and repeatedly praised in the book), Simon Conway Morris, has been one of Wonderful Life's most vigorous detractors. For Conway Morris, who has made a particular study of convergent evolution, the logic of evolution imposes such constraints that certain outcomes can be said to be predictable or even inevitable; one of them, in his opinion, is higher intelligence of the kind that we possess. Even if the chain of contingent events had been modified, for instance, so as to extinguish the primate lineage, "humanoid" intelligence would have been highly likely to evolve (or would eventually evolve) in another. Underlying Conway Morris's argument are his Christian beliefs and his philosophical antimaterialism (although criticism of Gould's book is not limited to those who hold such beliefs).

Though both scientists — Gould and Conway Morris — have used specific paleontological evidence to buttress their positions, the broader logical and philosophical arguments are fairly resistant to easy resolution by mere fact, and one suspects that if and when the classification of the Burgess fauna is finally settled, the debate will simply be continued in another venue. Regardless, Wonderful Life seems to me to hold up as a landmark of intelligent science writing for a general audience.

Travelers

Evolutionary biology and biogeography are basically outside of my bailiwick here, not because I'm not interested in those fields (I am) but because, as is generally the case with all the sciences, a smattering of layperson's knowledge really doesn't qualify one to give an informed evaluation of advances in the discipline undertaken by people who have both years of scientific training and a thorough knowledge of the relevant literature. (Which doesn't stop any number of crackpots and pseudoscientists from jumping in with both feet, of course.) So I will only say of Alan de Queiroz's The Monkey's Voyage: How Improbable Journeys Shaped the History of Life that it is enjoyably written in a way that is comprehensible to a lay reader without being unnecessarily dumbed down, and that it puts forward what appears to be a plausible, if certain to be disputed, argument that may shed new light on how the world's flora and fauna came to be distributed where they are.

To sum up the book's argument very briefly, one of the early problems that Darwin and his contemporaries faced was how species that were clearly related to each other came to be distributed in places oceans apart; the example alluded to in de Queiroz's title is the monkey lineage, which is found in both Old and New worlds. The discovery of plate tectonics seemingly provided an answer: the ancestral homeland of these species had drifted apart, and the descendants went their separate evolutionary ways thereafter, in a process known as vicariance. The problem is that recent DNA studies suggest that the timing is all wrong.

If the opening of the South Atlantic caused the separation between platyrrhines [New World monkeys] and catarrhines [Old World monkeys], then that split in the evolutionary tree should have occurred on the order of 100 million years ago. To put this in some perspective, such an old date would imply that the New World and Old World monkey lineages, which we know are not early branches in the primate tree, are actually about 50 million years older than the earliest known primate fossils of any kind. In fact, these monkey lineages would have to be some 35 million years older than the first known fossils of any placental mammal.

The DNA evidence, which gets a bit complicated, suggests that the split in fact took place within the last 51 million years, and possibly as late as 33 million years ago. But if those numbers are right, then why are there monkeys in the Americas at all?

De Queiroz, a biologist at the University of Nevada, suggests that they got there the same way that a surprising number of seemingly out-of-place species came to be where they are: by accidental ocean crossings long after the continents had drifted apart. This may seem far-fetched, and in fact de Queiroz recognizes that the monkey example, of all the cases of potential oceanic dispersal that he examines, requires the greatest suspension of one's initial disbelief. But by building up careful evidence for other, less extreme dispersals, he makes a plausible case for even the ancestor of the platyrrhines as an accidental transoceanic migrant. The scenario — monkeys clinging to driftwood or to "islands" of vegetation swept out to sea by drainage from Africa's rivers (or, he might have added, by tsunamis) — would have extremely long odds against it, but one should bear in mind the vast quantity of time available; a one-in-10 million-years freak event might actually stand a fair chance of happening, given the tens of millions of years during which it could potentially have taken place. (The Atlantic would have been substantially narrower than it is now, and de Queiroz suggests that the monkeys — or maybe one pregnant monkey — might only have had to cling on for a week or so.)

There are many other well-documented case studies in the book — New Zealand, Hawaii, the Falkland Islands — and de Queiroz, whether or not he is ultimately proved to be right, appears to have done his homework. It seems likely that we'll be hearing more about his book in the coming years.