Aerin

During the later Pleistocene, as humans were inexorably invading nearly every landmass on the planet, a gradual extermination was simultaneously taking place.  This mass extinction, the latest in a long line throughout the history of the biosphere, primarily targeted megafauna - from the mastodons and giant sloths of North America, to the woolly mammoths and woolly rhinoceroses of Europe, to the giant lizards and flightless birds of Australia.  In most cases, their annihilation shortly followed the arrival of humans, and it is hypothesized that some combination of overkill by human hunters and climate change (that had, in many cases, allowed humans to colonize the area in the first place) led to their demise.  (Interestingly, the reason that wild megafauna have survived in Africa, parts of Asia, and virtually nowhere else, is thought to be that humans have been in those areas the longest and actually co-evolved with large animals there.)

In North America, humans began to arrive some 40,000 years ago, and it wasn't long before the native relatives of horses, camels, elephants, and rhinos began to die off.  Most native American megafauna have been long gone now for some 10-15,000 years, but their legacy remains in odd places.  The botany of the Americas, particularly, bears witness to what Connie Barlow calls "the ghosts of evolution".

It's well known that many plant and animal species have evolved mutually beneficial partnerships. Often plants will attract animals with nutrient-rich fruit, which animals will devour, defecating the seeds intact elsewhere.  The animal gets a tasty meal and the plant gets a new start in life for its offspring, far from the malignant shade of the parent.  But there are many plant species whose fruits now go uneaten, rotting away on the forest floor, their seeds undispersed.  Barlow argues that no plant would expend the energy to evolve a fruit that is destined to rot en masse.  No, there are animals who eat these fruits - they just don't exist anymore.

Take the avocado, for instance.  This is a medium-sized fruit with a relatively large seed, and it was clearly designed to be swallowed whole.  After all, animals with smaller maws (like us) will just eat around the seed, which doesn't help the plant at all.  But what animal in the Americas has a mouth and a throat big enough to effortlessly swallow an avocado seed?  Well, many do - glyptodonts, toxodons, gomphotheres, ground sloths... but they are all extinct.

Plants take longer to evolve than do animals, and in any case 10,000 years is an eyeblink in evolutionary time.  In essence, these plants that evolved in tandem with megafauna have not had time to "notice" that their partners have gone extinct.  In some cases, these species have started to adapt to the loss of their primary dispersers; in others, they make do with less effective means of seed dispersal (flood waters, domesticated horses and cows); and in others, they are slowly diminishing and dying away.  It's important to recognize these evolutionary "ghosts" if we want to understand, and to save, many of these species.

In this book, Barlow sets out to identify potential evolutionary anachronisms, from extreme examples like the osage orange (no living species is known to eat its toxic fruit) to milder cases that just seem somehow overbuilt for the purposes of any extant partnerships.

Overall, I found the book's premise really interesting, but its execution was fairly dry.  Still, it's a topic I hadn't read much about and there were all kinds of little tidbits that made slogging through the boring parts worthwhile.  For instance, did you know that ginkgo fruit - which smells like rotting flesh - may have evolved to attract carrion-eating dinosaurs?  How cool is that?

(2014 #4)