Fruits of Millennia Past

You’ve most likely seen the insides of a pumpkin one way or another in the past month.  It’s a stringy, gooey mess full of seeds surrounded by thick fleshy walls.  Have you ever stopped to consider why it’s structured that way?  How are those seeds going to get out of that gourd and into a place where they can germinate into baby pumpkin vines?  These days humans cultivate and eat pumpkins, planting their seeds year after year for the past several thousand years.   However, the ancestors of today’s pumpkins were growing wild 30 million years ago in a landscape devoid of humans in what is now North and Central America.

Two ways humans use pumpkins.

  Ancestral pumpkins were smaller than modern ones, but still softball sized, and were full of bitter, toxic compounds to deter potential seed predators.  These gourds were adapted to be eaten whole, with minimal chewing, so as to avoid crushing the seeds.  What animal has the appropriately large teeth and esophagus to handle such a task and the body mass to tolerate toxic compounds?  Did you guess a five ton ground sloth? How about a Gomphothere or their more familiar cousin the Mastodon? Maybe a 12 foot tall camel? If you guessed any of the above you are probably a Paleontologist, and most likely correct.  There were dozens of species of megafauna populating the American continent for millions of years and they co-evolved with many of the plant species (or their ancestors) that are still here today.

 Plants have been growing on land for nearly half a billion years.  In that time countless growth and reproductive strategies have been attempted.  Most people are familiar with the pollination process: male flower parts produce pollen grains that need to be transferred to the receptive female ovary, and the resulting genetic combination is encapsulated in a seed.  Pollination as a means of reproduction wasn’t always an option.  Spores were the dominant means of reproduction in plants for the first several hundred million years after they colonized land.  This strategy is so successful that many plants, such as ferns and mosses, still use spores to reproduce to this day.  A major limiting factor of this strategy is that it requires a moist or wet environment for the spores to combine their genes and sprout a new individual. 

The development of pollen and seed about 380 million years ago allowed plants to move into drier areas and expand their potential range immensely.  Another 250 million years later saw the rise of flowering plants that encased the seed(s) in a fruit.  Fruits are a major part of why flowering plants (Angiosperms) are now the most diverse and wide-ranging branch of the plant family tree.  By protecting their seeds and exploiting animals’ need for calories plants gained the ability to move long distances to new areas and free fertilizer. Fruits and animals have been in a co-evolutionary tango ever since that has shaped the habits of both.

Hawthorn (Crataegus uniflora) fruits on thorny branches.

 In the past 100 million years that flowering plants have been producing fruits lots of animal species have evolved, thrived for millions of years, and then gone extinct.  What happens when the agent of dispersal that loves to eat fruits and unwittingly transport and deposit seeds in far off places suddenly (geologically speaking) disappears?  Fruits will rot on the vine or on the ground and seeds will fall victim to fungal attack, rodent seed eaters, or be forced to try and grow in the shade of their parent.  A new dispersal mechanism must be found or plant populations will become isolated, inbred, and slowly wink out of existence.  This is the situation a number of North American native plants found themselves in recently.  The Pleistocene megafauna that roamed the continent for millions of years went extinct between 15,000 and 8000 years ago.  Fruits that were meant to appeal to these giant mammals still hang on trees today waiting for a ride that will never arrive.

Honey Locust (Gleditsia triacanthos) seed pod that was still on the tree from last year. (6 in. scale)

Plants that appeal to and/or defend against extinct animals are said to be ‘anachronisms,’ that is “belonging to a period other than that in which it exists.”  A few examples of plants with anachronistic fruits in the Southeast are Honey Locust (Gleditsia triacanthos), Paw Paw (Asimina triloba), Persimmon (Diospyros virginiana), and Kentucky Coffee Tree (Gymnocladus dioicus).  Beyond the Southeast, other familiar new world fruits such as Papaya, Avocado, Guava, Passionfruit, and Mesquite pods were originally adapted to appeal to beasts much larger than humans that could swallow the rind, pulp and seeds together in one or a few bites.  One can also see anachronisms in the thorny defenses on plants such as Hawthorns (Crataegus spp.), Honey Locust (Gleditsia triacanthos), Devil’s Walking Stick (Aralia spinosa), and Yucca (Yucca spp.).  These thorns are meant to discourage leaf browsing, but are larger than necessary, and continue up the stems well above the height needed to discourage any living herbivores. 

Serious defense on the Honey Locust (Gleditsia triacanthos).

 

The arrival of humans in the Americas (14,000 to 40,000 years ago, depending on who you ask) coincides with the extinction of the majority of the large seed dispersers, and may have been a contributing factor.  So it goes. It just so happened that many of the fruits and seeds here also appealed to the humans’ mammal palette, or they found inventive ways to neutralize toxins and make them edible and nutritious.  This new avenue of dispersal allowed many plants to continue to spread and thrive even as they produced fruits that were originally meant for someone else.   

The next time you see an avocado, butternut squash, papaya, or if you’re lucky enough to get a pawpaw, think about who it originally appealed to in a world without ovens or knives.  Millions of years ago when humans and chimpanzees were just diverging from a common ancestor in Africa, giant camels, sloths, rhinos, mastodons, and other beasts roamed the North American continent plucking persimmons, pawpaws and locust pods from the trees.   These giants feasted on prickly pear fruits and desert gourds during their seasonal migrations and they consistently deposited the seeds in a pile of fertilizer further down the trail.  Plants are still using their tried and true methods that worked for millions of years, so the brief absence of their partners for 15,000 years has not yet sunk into permanence.  In the meantime they’ve found us human types that are all too happy to pamper their descendants and spread them far and wide if they happen to also appeal to our senses.  Bon Appetit!

Immature Pawpaw fruit in July

 

Credits, References:

The inspiration for this post, and from which I drew heavily for information, is the book “The Ghosts of Evolution: Nonsensical Fruit, Missing Partners and Other Ecological Anachronisms” by Connie Barlow (2001). It’s a great read, and very accessible even if you know nothing of Paleontology, Botany, or Ecology.  She weaves the story of two of the progenitors of the concept of Evolutionary Anachronisms, Dan Jantzen and Paul Martin, with her own investigations into the edibility of fruits, the natural history of plant species, and interviews with botanists, paleontologists, and ranchers. She thoroughly examines past hypotheses of seed dispersal syndromes and shows how many plants have found alternative vehicles (humans and otherwise) to survive.

The wikipedia page for “Pleistocene megafauna” is also a great summary and jumping off point if you click through the references at the bottom to learn more about the giants that took the place of dinosaurs.