Old fruit: "Ghosts of Evolution"
By DAVID GREEN
A biologist named Dan Janzen came up with a wild and revolutionary idea in 1977.
It was October and the fruit was falling from trees. It fell to the ground and then it just lay there, rotting on the ground.
Janzen wondered why a plant would go to the trouble to create a fruit that was never eaten—and therefore its seed was never dispersed.
Maple and ash trees produce winged fruit that travels with the wind. The same for dandelion and lots of other plants. Raspberries, strawberries, apples—so many fruits are eaten and dispersed via the rear end of the consumer.
Janzen knew that over the eons, every plant has developed a method to ensure the growth of another generation.
And so, he wondered, why did the seeds of the giant cassia, where he was working in Costa Rica, have no means of getting around?
Eventually he hit upon what he first called his “screwy idea.” The fruit was adapted for an animal that no longer lived here. It was once eaten by a creature that had been extinct for 13,000 years.
The sweet pulp of most fruit is merely a lure to attract an animal to come and eat it. Nothing much was coming for the cassia.
The fossil record tells the tale of what once lived in a region. Europe and on into temperate Asia lost all of its great mammals, including elephants and rhinos. There were once six species that weighed in at more than 2,200 pounds.
Australia was once inhabited by giant kangaroos, enormous wombats and rhino-sized marsupials. North America featured mastodons and mammoths, giant ground sloths and beavers as big as bears.
Janzen worked as an ecological biologist. He needed help with his theory and turned scientist Paul Martin. Martin specialized in life from the Pleistocene era, a span of Earth’s history covering more than a million years and ending with the last of the ice age glaciers about 11,000 years ago. Martin would be able let Janzen know if his idea had credence.
Martin provided a list of animals dominating the tropics around 100,000 years ago, including:
• giant ground sloths 18 feet tall with teeth like pruning shears;
• the rhino sized Toxodon with a mouth full of big buck teeth, perhaps good for shredding bark;
• glyptodonts as large as a Honda Civic;
• peccaries twice the size of the modern species.
Martin decided there were certainly candidates for cassia fruit. The two scientists developed their ideas until the theory was ready for publication. They referred to certain plants as anachronisms—something that belongs to another time. These were plants that developed a system of fruiting 30 to 40 million years ago, with the intent of being attractive to large mammals.
The big animals are gone, but the fruit remains. Thirteen thousand years just isn’t long enough for the plants to respond.
Fruit simply rots on the ground or is eaten by small animals interested only in the pulp. In some cases the seeds are eaten by insects, piece by piece, although they were designed to be swallowed whole.
There’s no need to travel to the tropics to find biological anachronisms. A walk along Bean Creek reveals many of what science writer Connie Barlow calls “the ghosts of evolution.”
- Jan. 7, 2004
A few local "ghosts"
Honey locust seeds are hidden inside long, twisted pods often growing more than a foot long. A sweet pulp surrounds the seeds.
Connie Barlow writes that there’s no reason to produce a seed pod bigger than the mouth of the intender disperer. Actually, there is no disperser in the North American woods today.
Seed pods might travel a small distance in the wind, but seeds are often eaten by mice, birds and insects.
In order to germinate, the tough seeds need to be scarified—slitting the hard seed coat—or better yet, passed through the digestive system of something like a mastodon.
The seed pods hang on the trees and drop to the ground day by day, offering a sources of food as grassy sources dry up.
The honey locust tree is covered with clusters of spines designed to keep something away, perhaps a bark-stripping mammal or a leaf browsing animal. Now, the armor only keeps children from climbing.
Overbuilt and underutilized, Barlow says.
If there’s one native species that falls into the classification of extreme anachronism, it has to be the osage orange. Nothing eats the large, orange-size clusters of seeds.
Squirrels have been seen eating seeds, but the fruit just doesn’t fit into the scheme of life today, nor is there any reason for heavy armor of spines.
Fossil records suggest that like many plants we know today, this species developed during the Cenozoic era (dating back to 65 million years) and seeds could have been dispersed by the now extinct brontotheres—a large, hoofed mammal that once grew to elephant size. Later in the Cenozoic, the large rhinos that inhabited North America could have done the job.
The pawpaw produces the largest edible fleshy fruit of any native North American tree. Lewis and Clark’s party survived on little more than pawpaw for three days in Missouri, on their way back from the West.
The large seeds inside the pawpaw fruit are bitter tasting and contain harmful alkaloids. It’s clearly a fruit that’s meant to swallow whole with little chewing. It takes a big mouth to get the job done right.
Although small mammals such as raccoons can help with dispersal, pawpaws also reproduce by suckers and tend to be found in patches. Remember the song? “Way down yonder in the pawpaw patch.”
The Kentucky coffeetree is a rare one and Morenci is fortunate to have a pair growing at Wakefield Park.
The seedpods are extremely tough, and like the honey locust, they’re surrounded by a sweet pulp.
Scientists often look for similar species of North American plants across the ocean. A tree with a familiar characteristics in Africa produces pods that are eaten by elephants. The spherical seeds—much like the coffeetree seeds—readily roll between the beasts’ molars, promoting scaring rather than breaking them apart.
The seed coat is very resistant, and cutting into it promotes germination. A bath in gastric juices might help the process along.
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