Sunday, October 17, 2010

Ghosts of Evolution Past

When the clock stops on a life, all things emanating from it become
precious, finite, and cordoned off for preservation.
                                ~ Jennifer Egan, Dealing With The Dead
Fall is an apt time to reflect on the past.  It’s a territory that should be labeled “Here be ghosts.”

In her short essay Dealing With The Dead (The New Yorker, October 8, 2010), novelist Jennifer Egan reveals her penchant for gathering and wearing “loans from the dead” – her grandmother’s fake pearl necklace, sweaters from her father and her stepfather.  This wearing in her daily life of mementos from departed loved ones, she calls “borrowing from the dead,” and considers it “a way of keeping them engaged in life’s daily transactions – in other words, alive.”  To me, it’s a way of calling the ghosts.

This loss of an individual person, the loss of that yin to your yang, is wrapped up in expectation.  The expectation that the person will, any minute now, walk into the room and resume the life recently ended.  I suspect Egan’s tokens keep alive those expectations, those ghosts, much longer than is true for most people.

And now for ghosts with still longer lives.

These ghosts are called by a certain autumnal smell.  Yes, there’s the usual constellation of scents that signals autumn for me, including the sweet scent of wood burning and the musty one of decaying leaves.  But, this is also the time of year when a remarkably different fall fragrance comes to mind.   For many years, during my commute to and from work, I walked past the southwest corner of the block that the U.S. Supreme Court Building occupies.  It was there, particularly on late afternoons in Indian Summers, that I’d frequently catch the whiff of that villain of autumn aromas, the one emitted by the malodorous fruit-like product of the female ginkgo (Ginkgo biloba).  Nearly everyone describes that odor by invoking either excrement or vomit (or more colorful synonyms).  A most vile seed and covering.

Last week, I retraced that commute I used to make, looking for ginkgos, sniffing for that fetid smell.  I found ginkgos, but not the ones that would have generated the odor at the corner of the Supreme Court building.  The ginkgos that I visited this time stand across the street on the lawn of the main building of the Library of Congress.  I assume these are male trees (pictured below) because there were no seeds on the trees or festering on the ground, although, it takes a very, very mature female ginkgo tree to produce the seeds.  (The small twig with leaves was lying at the foot of one of the trees.)

Here is a picture of the seeds that would have appeared were these females of the right age.  (The picture was obtained via Wikimedia Commons and is identified as the “own work” of Love Krittaya and in the public domain.) 

Has the female ginkgo that pungently punctuated my fall commute for so many years been removed?  Did she fall victim to the odor police or to all of the new security fences and other barriers that have taken over Capitol Hill?  Still, as I went over familiar ground, I found it easy to revive the memory of that smell.

The noxious odor is only one of this seed’s several offenses.  The pulp is laced with urushiol which can cause serious skin rashes, just as this chemical does when you encounter it in poison ivy.  The nut itself, though edible in small quantities, is well-armed with toxins – cyanogenic glycosides (eating uncooked nuts releases hydrogen cyanide, a perfectly nasty chemical suitable for a plot crafted by Agatha Christie) and 4-methoxypyridoxine (highly toxic for children, depriving them of vitamin B6 – not surprisingly, the top of the Google hit list for this compound is a piece entitled Ginkgo Seed Poisoning appearing in the journal Pediatrics)  The various ills of the ginkgo seed are nicely reviewed in the Washington Post’s Urban Jungle column for October 12, 2010 (on the Urban Jungle page, look for #2 (how appropriate) in October). 

When a resident of San Jose, California, recently posted a notice on Craigslist asking (well, almost begging) people to come and harvest the produce from the female ginkgo tree outside his house, he ended his posting with this desperate stipulation – “All I ask is that you take the entire fruit away with you, and not just the nuts.”  There’s no point getting technical when the smell of vomit pervades your home, but, the ginkgo is a gymnosperm, which means, I think, that technically it produces a seed with a covering, not a fruit, though, I doubt that I will be consistent with my terminology in this posting

The ginkgo tree is often referred to as a living fossil, having a lineage that, according to the University of California’s Museum of Paleontology (UCMP), traces back to the Lower Jurassic, some 190 million years ago.  During the Cretaceous Period (which ended some 65 million years ago), there were possibly a half dozen different ginkgo species.  But, then, in short order (in paleontological time), there was but one, Ginkgo adiantoides, its fossilized leaves “virtually indistinguishable from modern-day Ginkgo biloba.”  Some 9 million years ago the ginkgo slipped out of the North American fossil record, and, finally, completely disappeared from all fossils records, according to the UCMP, by the Pleistocene Epoch (beginning some 2.6 million years ago).  The many ginkgos populating our urban areas owe their existence to Buddhist monks in mountainous areas of China who cultivated the trees and may have saved the species.

But, there are startling ghosts summoned by the ginkgo seed.

In their 1982 paper in Science entitled Neotropical Anachronisms:  The Fruits the Gomphotheres Ate, Daniel Janzen and Paul Martin hypothesize that the seed dispersal systems of many Central American plant species evolved so as to give herbivore megafauna a key role.  Among these megafauna was the gomphothere, a four-tusked, large (at least some 2.5 tons) mammal that went extinct, according to the authors, along with many other megafaunal species in Central America about 10,000 years ago.  Under this hypothesis, the disappearance of these megafauna left their flora partners dependent upon less efficient alternate dispersal agents.  A prime piece of evidence is the vast overproduction of edible fruit by some tree species with the result that much of the fruit rots on the ground.  Other evidence includes the production of fruits rejected by extant dispersal agents.  Janzen and Martin argue that such fruiting traits are anachronisms, developed for a faunal partner that is now gone.

Martin and others like science writer Connie Barlow are more than happy to label the extinct faunal partners of these flora as ghosts.  Indeed, Barlow’s marvelous book exploring the Janzen/Martin hypothesis is entitled The Ghosts of Evolution:  Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms (2000).  Fruits that are too big, too tough, too toxic for contemporary dispersing agents are prime candidates for the anachronism label and Barlow writes joyfully of her exploration of these flora and their ghosts.  How can one resist this? – “Grocery stores are excellent places to encounter ghosts.  They lurk in the fruit section, feasting on anachronisms.”  (p. 7)  (Papaya and avocado are among the anachronisms being devoured.)

The concluding sentence of Janzen and Martin’s article is part of their effort to expand the potential reach of their hypothesis.  This sentence has nothing to do with the Pleistocene megafauna, but it brings us to the ginkgo and the ginkgo’s ghosts:

The vesicatory [i.e., blister causing] ripe fruits and weak-walled nuts of Ginkgo biloba might even have been evolved in association with a tough-mouthed herbivorous dinosaur that not chew its food well.  (p. 27)

Ah, the ginkgo’s ghosts – dinosaurs – which, except for the avian variety, disappeared at the end of the Cretaceous.  Barlow entitles her book’s section on the ginkgo, The Tree Who Remembers the Dinosaurs.  But, Janzen and Martin may have had the wrong ghost.  Barlow describes how Peter Del Tredici, currently Senior Research Scientist at Harvard University’s Arnold Arboretum, wrestled with this ghost and ultimately reached a slightly different conclusion.  In Barlow’s words,

The first set of target dispersers could have been small scavenging dinosaurs that lived during the Jurassic or Cretaceous.  As Del Tredici sees it, the odor of a ginkgo seed, after it has lain on the ground for a few days, may mimic rotting flesh well enough to attract scavengers of all stripes. . . . Pegging partnership on carrion-feeding rather than plant-eating dinosaurs solves one very big problem.  The seed is physically protected by a shell too thin to withstand contact with grinding machinery. . . . Herbivorous dinosaurs of the Mesozoic had two kinds of machinery that might have crushed a ginkgo seed.  Teeth would have been an obstacle in some, and for the rest, a stone-filled gizzard would have threatened even more injury.  (p. 142-143)

Barlow adds a cautionary note,

Peter Del Tredici is the first to admit the weakness of the evidence on which the dinosaur hypothesis is based.  Still, nobody has offered a better supported explanation for the mystery of the ginkgo.  (p.146)

So, there it is.  The expectant ginkgo tree, waiting for those scavenging carnivorous dinosaurs to gather up her stinking seeds and start them on their productive journey.  But now only ghosts heed the call and gather beneath her.

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