The Monkey’s Voyage: How plants and animals are dispersed throughout our planet

The Monkey’s Voyage (1) is as much a history of the science of evolution and ecology as it is a report of the prevailing scientific opinion of the means by which plants and animals were dispersed around the world.  Just as life has evolved, so too has the science that studies it.

In the beginning….

The story begins with Charles Darwin, the author of the first publications that identified natural selection as the mechanism that drives the evolution of life on the Earth.  These ideas came to him as the result of a five-year voyage around the world in 1831-1836:  down the coast of Africa, across the Atlantic, down the coast of South America, around the horn, to the Pacific Ocean to many islands—most famously the Galapagos—to New Zealand, Australia, islands in the Indian Ocean, round the horn of Africa to home.

He spent 3-1/2 of the 5 years on land, collecting plant and animal specimens, including many fossils.  The fossils suggested to him the existence of animals no longer occupying the land.  He also observed many similar plants and animals with slightly different forms around the world.  The classic example of closely related, but widely dispersed animals is a family of large, flightless birds:  the ratite family.

These similarities suggested a common ancestry to Darwin.  Yet, their dispersal across oceans was puzzling to him because at that time the continents were considered fixed in place both going back in time and going forward into the future.  Nothing was known at the time about the constant movement of continents, known as continental drift, because the movement was too slow to be observed by humans.

Darwin’s theory about the similarities he found in widely dispersed plants and animals was consistent with his perception of the fixed nature of the geography in which they were found.  He theorized that the common ancestors of the similar plants and animals had been dispersed by wind, ocean currents, carried by birds, or other means of transportation. 

He conducted experiments to determine how long seeds could survive in sea water to test his theory and he examined migrating birds for evidence of seeds and small animals in their feet and feathers.  What he found supported his theory that it was physically possible for plants and animals to be dispersed across oceans to new ranges where subsequent evolution in a different environment would eventually result in alterations of form.  When plants and animals are moved from their home ranges and are physically isolated, their genetic compositions diverge.  Over time they are sufficiently genetically and morphologically distinct to be considered different species. 

Continental Drift

Around the turn of the 20^th century, scientists began to theorize that Africa and South America may have been merged at one time because maps revealed that they fit together like pieces of a puzzle.  Alfred Wegener is best known for his pursuit of this theory.  He visited both sides of the Atlantic and observed that seams of rock and sediments lined up on the two shores, suggesting their past connections.  Although Wegener’s theory gained considerable traction, he did not propose an equally compelling theory about the physical mechanism that would be capable of moving the continents apart.

The mechanism that moves the continents was identified about 50 years later when the ocean floor was studied as a result of developments in radar and sonar.  These analytical tools eventually identified seams running the length of the oceans that separate the tectonic plates on which continents ride.  Beneath the crust of the earth magma of molten material moves in a current, emerging through the seams of the Earth’s crust as volcanic activity.  As molten material emerges from this seam between the tectonic plates, it cools on the ocean floor to form new sea floor.  The expansion of the sea floor moves the plates away from the seams, which moves the continents.   This is the engine that drives continental drift.

By the late 1960s there was scientific consensus about plate tectonics and consequent continental drift. That knowledge led to an understanding of the history of the continental configuations.  About 300 million years ago, all continents were fused into one, called Pangaea.  Pangaea began to break up about 100 million years later.  However, South America, Africa, Madagascar, Australia, New Zealand, and Antarctica remained fused in a continent called Gondwana until about 100 million years ago.

 

“The history of life is the history of the earth.”

This new understanding of the history of the earth’s geology resulted in a paradigm shift in scientific theories regarding dispersal of life forms.  Very quickly, scientific consensus formed around the theory that life moved as a result of movements in the continents.  This theory was succinctly expressed as “The history of life is the history of the earth.”  That is, where life is found depends upon changes in the geology of the earth.  For example, scientists assumed that life found on Madagascar originated in Africa before Madagascar separated from the African continent.  Similarly, scientists assumed that life found in New Zealand originated in Australia before New Zealand separated from the Australian continent.  In other words, life migrated from the continent along with the land, like Noah’s ark carrying the animal kingdom.   Previous theories about trans-oceanic voyages of plants and animals were quickly abandoned in favor of this new, elegant theory which seemed so much more plausible than its predecessor.

DNA analysis trumps elegant theory

Although scientists were comfortable with their new theory of how life was dispersed, the inexorable forward movement of human knowledge intervened to disrupt their complacency.  The new analytical tool that overturned this theory was DNA analysis which enabled scientists to study the genetic composition of life forms. 

When there are two morphologically similar species in physically isolated locations, their common ancestry can now be determined by DNA analysis.  And the genetic distance between the species can help scientists determine when those species became physically separated.  When populations become separated their genetic pools become progressively more distant from one generation to another.  This rate of genetic change is called the “molecular clock” and it can be used to determine when the physical separation occurred if the rate of change is known.  Unfortunately, the molecular clock varies from one lineage to another, so first scientists must calibrate the clock and when they do they can estimate the arrival of a specific plant or animal in a new territory that is physically isolated from its former range and therefore its ancestors.

Genetic analysis has overturned former theories of how life was dispersed on the earth.  In most cases, plants and animals arrived in their present locations long after the continents separated into their present configuration. Plants are more likely to have been dispersed by wind and ocean currents than animals.  New ranges of plants are often on the receiving end of ocean currents and plumes from big rivers.

Also, new understanding (1980s) of the most recent mass extinction approximately 65 million years ago—when dinosaurs disappeared from the earth—would predict the same result.  The mass extinction at the end of the Cretaceous period occurred after the separation of the continents.  Therefore, most life forms that moved along with the separating continents were wiped out by the mass extinction about 65 million years ago.  Life forms found now are more likely to have arrived after present continental configurations formed and therefore are more likely to have arrived by long-distance dispersal. 

Evolutionary science comes full circle

There are some die-hard scientists that have not made the transition from the “life-moves-with-the earth” theory.  However, the molecular evidence that life has dispersed across vast expanses of ocean is mounting and most scientists have accepted the reality of the evidence.  Science has come full circle, to return to Darwin’s original theory.  As improbable as it may seem, monkeys made the voyage from Africa to South America, across the Atlantic Ocean.

 

But is that voyage really so improbable?  Within the past decade, we have witnessed two massive earthquakes that caused massive tsunamis.  In December 2004, a tsunami following an earthquake in Asia killed approximately 200,000 people.  A few survivors tell harrowing stories of clinging to rafts of debris at sea to arrive many days later on a foreign shore.  And less than 10 years later, in March 2011, an earthquake and tsunami in Japan killed tens of thousands of people.  Over a year later, huge rafts of debris washed ashore on the West Coast of America, encrusted with sea life that accumulated on that long trip.  They were called “invasive species” when they arrived.  But were they really?  After all they arrived as the result of a natural occurrence with no assistance from humans.

These may seem rare events to us because of our short time perspective.  Multiply those two catastrophic disasters by the millions of years of life on earth to arrive at the conclusion that these events are routine when put into the context of the lifespan of the earth rather than the lifespan of humans. 

Bringing it home

What we learn from The Monkey’s Voyage is relevant to the concerns of Million Trees:

• Life is constantly in motion whether we are capable of perceiving it or not.  To choose some specific landscape that existed in the distant past as an ideal to be re-created is to deny the reality of nature.  The concept of “native plants” is meaningless.  Native to where?  Native to when?
• Change in nature is random and therefore unpredictable.  Cataclysmic events render humans impotent to manipulate complex ecosystems.  Human attempts to “manage” nature are arrogant at best and harmful at worst.  For example, when we kill one animal based on a belief that it will benefit another animal, we haven’t sufficient knowledge to predict the outcome with certainty.
• Science is constantly evolving, just as nature is evolving.  Invasion biology is stuck in a cul-de-sac that is contradicted by the reality of the dynamism and complexity of nature.  There is little scientific evidence that supports the assumptions of invasion biology.

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(1)    Alan de Queiroz, The Monkey’s Voyage:  The improbable journeys that shared the history of life, Basic Books, New York, 2014