By: Priscilla Hayes
In December, I had knee replacement surgery. Since I am a school garden educator, I timed it with the “off-garden” season. The recuperation involves lying with my leg elevated, heating and icing my knee, so I am catching up on my reading. I’ve chosen a book called The Ghosts of Evolution, by science writer Connie Barlow, for my first book review. I learned about the book from my Aunt Dency, who is a member of the Webster Groves Nature Study Society’s book club (http://www.wgnss.org/about-wgnss.html). The society was founded in 1920 with the intent of helping its members to better experience nature.
The “ghosts of evolution” that the title refers to are “evolutionary anachronisms.” This term is mostly used to describe plants which “designed” their seeds over millennia of evolution to be dispersed by a specific animal or type of animals through co-evolution. Some plants have encased their seeds in fruit, and used that fruit to attract “seed dispersers.” Some plants encase the seeds in burs, which attach to some passing creature, and fall off in a new location. Evolutionary anachronisms are plants whose seed dispersers, or at least the primary ones, have disappeared from the face of the earth. Now the plant may be in danger of dying out, since it cannot get its seeds moved, and they may just pile up under the few remaining trees or plants.
A modern, not-anachronistic example of co-evolved seed dispersal would be the relationship between acorns and squirrels. Over the course of evolution, the oak tree evolved a fruit—the acorn—that was attractive to squirrels. Each squirrel would stash a number of acorns in various locations by burying them, and most, not right under the original tree. Since the squirrel would normally not go back for all the stashed acorns, voila! Some acorns are planted in new locations to hatch out new little tree seedlings.
So, oak trees are in no danger of going extinct, since there are plenty of squirrels in the world (more on that in a future post). Picture instead, an ancient forest, an avocado tree and ancient gomphotheres. No, I could not visualize gomphotheres either, but this link will help you get a better idea (http://www.ucmp.berkeley.edu/mammal/mesaxonia/gomphotheriidae.php)! Suffice it to say, each is large enough to take an entire avocado into his/her mouth, sort of gum the fruit, eventually digest it (presumably with the skin) and poop out the pit. As Barlow says: “A parent [avocado] tree could wish for no more desirable fate for its offspring than to have its seeds plopped into the world within steaming heaps of dung.”
Why do I say “gum” the avocado? Why doesn’t our lovely gomphothere also chew up the nice big pit (which the avocado tree has thoughtfully packed with a year’s worth of nourishment for the little embryo hiding inside)? Why, because that brilliant avocado tree, through eons of evolution, has packed the pit’s food layer with enough bad-tasting toxins to make the first time the gomphothere bites into the pit its last time. The toxins aren’t going to kill the creature, but he/she will surely spit out the nasty seed, and recall not to do that again.
Doesn’t this all sound just like those howler monkeys that ate fruit whole and pooped out the seeds, along with lovely poop for dung beetles to work with? But, tell me, when was the last time you saw a gomphothere—or even the first time? This is because they have disappeared and have not been replaced in wild avocado forests with any other animals capable of eating the fruits whole and dispersing the seeds in those piles of poop. There are mostly only us humans, and we have adopted only one or two kinds of avocados to grow. Unlike our friends the gomphotheres, we remove the seeds and skins before we eat the fruits. I know they plant seeds at my schoolroom, but those seeds don’t make it back to the forest. And because we are fixated on only a couple varieties, other varieties of avocado tree are in trouble. Fruit drops to the ground and either rots or gets ransacked by critters that aren’t good at moving the seeds—animals that eat the pulpy fruit part, and leave the seed, where it may start growing right in the shade of its parent.
The original pairing of avocado and gomphothere was worked out through co-evolution over time, so why can’t the avocado either re-evolve to match a new seed disperser, or get on with it, and just die out? Turns out that plants, especially perennial plants like the avocado tree, take way longer to evolve than animals. Animals, on the other hand, may evolve their way out of existence before that tree has a chance to figure out they are gone. Avocados may be evolving, but that process may be too slow to save most of them.
But, here’s the kicker. Early humans may be the ones responsible for the disappearance of the gomphotheres, and of other avocado seed dispersers. And, we are surely the ones driving any number of other creature extinctions since: what Barlow quotes scientist Paul Martin as calling “much more familiar extinctions of historic time, the dodo of Mauritius, the solitaire of Labrador duck, Carolina parakeet, and Tasmanian wolf, to begin a litany of the names from the Doomsday Book.” As Barlow’s book warns, we are on the verge of a cascade of extinctions, with each creature or plant we extinguish impacting any co-evolved species, and cascading down.
The book ends with a possible “outrageous proposal:” bring ‘em back! Bring back the ancient creatures, or their nearest surviving analogs, to the forests that depend on them for seed dispersal. Or, bring back the plant based habitats that will encourage some of our endangered species to prosper. Bring them back, both for them, and for us.