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A Ghost of Evolution: The Curious Case of the Avocado, Which Should Be Extinct But Still Exists

“Even after thirteen thousand years, avocado is clueless that the great mammals are gone.”

In any market economy, it’s common sense that as soon as the consumer for a certain product ceases to exist, the product itself becomes moot and soon vanishes from stores. In nature, however — or market ecology, if you will — that need not necessarily be the case. In the altogether fascinating The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms (public library), popular science writer and evolutionary biology champion Connie Barlow builds on the work of renowned ecologists Dan Janzen and Paul Martin, who in 1982 published a provocative paper arguing that many of the fruits and nuts found in Central American forests today evolved to be eaten by animals that have been extinct for thousands of years. Barlow explores the curious anachronistic existence of these species, ranging from papayas to persimmons to ginkgo biloba, and even coffee.

But as an avid aficionado of the avocado, I was especially taken with its particular story: Since fruits propagate by seeds, their progeny doesn’t grow far from the tree, as the proverb goes; their only chance of spreading their seeds across the land, then, are the animals who eat the fruit, along with its seeds, then “plant” those elsewhere when they poop. The avocado’s abnormally giant seed presents anything from a severe digestive hazard to a death sentence for contemporary earthly species but, apparently, avocados coevolved with ground sloths and were originally eaten by gomphothere — elephant-like creatures that lived during the Miocene and Pliocene, between 12 million and 1.6 million years ago, who happily reaped the fruit with their hefty trunks, crunched them with their massive teeth, and passed the seeds comfortably through their oversized digestive tract.

Relic of a ghost: An avocado fruit and seed paired with the tooth of its missing partner in evolution, the gomothophere Cuvieronius.

The problem, of course, is that gomphothere no longer roam the Earth — and yet avocados still exist. Barlow writes:

Avocado’s strategy for propagation made a great deal of sense throughout the long life of its lineage — until the present moment. Even after thirteen thousand years, avocado is clueless that the great mammals are gone. For the avocado, gomphothere and ground sloths are still real possibilities. Pulp thieves like us reap the benefits. Homo Sapiens will continue to mold the traits of the few species of genus Persea it prefers. Ultimately, however, wild breeds will devolve less grandiose fruits, or else follow their animal partners into extinction.

In this fascinating short video for PBS, Joe Hanson of It’s Okay To Be Smart explains the avocado’s curious fate, along with those of its brethren of ecological anachronism:

Read more about this time warp of evolutionary biology in Barlow’s The Ghosts of Evolution.

HT The Dish

Published December 4, 2013




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