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.

Voyage of the HMS Beagle, 1831-1836.  Creative Commons - Share Alide
Voyage of the HMS Beagle, 1831-1836. Creative Commons – Share Alike

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.

Family of ratite birds
Family of ratite birds

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 20th 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.

Tectonic Plates - USGS
Tectonic Plates – USGS

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

Olive baboon, Old World monkey by Mohammad Mahdi Karim
Olive baboon, Old World Monkey by Mohammad Mahdi Karim

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.


Brown spider monkey, New World monkey.  Creative Commons - Share Alike
Brown spider monkey, New World monkey. Creative Commons – Share Alike

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.

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

Pangaea: The first but not the last globalization of ecology

The continents have been sliding about on the Earth since it was “created”(1) approximately 4.5 billion years ago.  Although geologists tell us that the continents came together and broke apart several times prior to the formation of the supercontinent geologists call Pangaea, this is the geologic period of most interest to us because life forms were sufficiently complex by that period that we can recognize their modern counterparts.

The supercontinent Pangaea

Pangaea is said to have been assembled about 237 million years ago, during the Early Triassic Period, shortly after the great Permian extinction, the period of the most extensive extinctions of plant and animal species in the history of the Earth.   Pangaea began to break apart about 50 million years later, but the African and South American continents remained fused–into a continent dubbed Gondwana–until about 100 million years ago. (1)

During that period of nearly 160 million years, many new life forms emerged and others died out.  Cone-bearing plants replaced some spore-bearing plants before Pangaea formed and dominated the Earth during much of Pangaea’s existence.  The first true mammals, flowering plants, birds, lizards, and salamanders appeared before the break up of Pangaea was complete.

What are the implications of the development of new species of life on Earth at a time when there was a single, unified continent?  That is the question we are considering today.  Obviously, the transport of plant and animal species into new territories is facilitated by their proximity.  Seeds are more easily transported by wind and animals if they need not cross barriers such as oceans, as they must today.   As a result there was greater homogeneity of species during the geologic periods of Pangaea.  And species diversified rapidly when Pangaea broke up into the 7 continents of today. (2)  These diversified species have common ancestors. 

Even after Pangaea began to break up into separate continents, there were land bridges between some of the continents during periods of glaciations when water was locked into ice, draining the oceans.  Animals could travel over these land bridges from one continent to another, often bringing plant species with them, usually unwittingly.  That’s how the first humans in North America and ultimately South America traveled from Asia about 13,000 years ago at the time of the last ice age.

The common ancestry of many plants and animals is one of many reasons why the concept of “native” is ambiguous and is often debated.  We will consider a few examples in which the designation of a particular plant as native or non-native seems debatable.

Is the Dawn Redwood native to California?

Dawn redwood (Metasequoia glyptostroboides) is closely related to our redwood trees, Coast Redwood and Giant Sequoia.  Dawn redwood is unique in being a conifer that is also deciduous (loses its foliage in winter), unlike our redwood trees which are evergreen.  Dawn redwoods were until recently considered native to remote regions of China where they are considered “critically endangered.”

Dawn redwood in spring. Wikimedia Commons

However, scientists at the Museum of Paleontology at UC Berkeley tell us that there is fossil evidence that dawn redwoods grew in California about 40 million years ago.  Dawn redwoods now grow successfully in the Bay Area.  There is a famous specimen in front of McLaren Lodge in Golden Gate Park, headquarters of San Francisco’s Recreation and Park Department.  Every autumn, when the tree turns red, park staff receives calls from the public expressing their concern that the beautiful tree is dying.

Dawn redwoods died out in California during the last ice age because the climate was cooler than dawn redwoods could tolerate.  So, now that the climate has warmed again, and dawn redwoods are back, why not welcome them as a “return of the natives?”  That’s the kind of flexibility that makes sense to us, particularly in a time of rapidly changing climate.

Dawn redwood in autumn. Wikimedia Commons

Unfortunately, we don’t find such flexibility in the native plant ideology.  Dawn redwoods are rare both in California and in China from which it was reintroduced, and it is therefore not one of the trees that native plant advocates demand be eradicated.  Monterey pine and Monterey cypress are not so fortunate.  These are also trees for which fossil evidence suggests that they lived in San Francisco in the distant past and their native range is less than 150 miles down the coast in Monterey.  Both tree species are also considered threatened in their native range.  Yet, native plant advocates demand their eradication in San Francisco.

This is an example of the rigidity of the native plant ideology that has earned them the reputation of fanatics.

Does Rhododendron ponticum “belong” in Britain?

We told our readers in a recent post that Rhododendron ponticum is one of only about a dozen plants in Britain that are considered “invasive.”  It is a stunningly beautiful plant which is being aggressively eradicated in Britain.  Richard Mabey in Weeds:  In Defense of Nature’s Most Unloved Plants offers this explanation for why this particular plant is “invasive” in Britain:

“The next most serious weed is probably rhododendron which, unusually, has the ability to invade existing ancient woodland, especially in the west of Britain.  This may be because, if one employs a very long time scale, it is not strictly an alien.  The species that forms impenetrable thickets in western Britain is Rhododendron ponticum, whose pollen remains have been found in deposits in Ireland dating back to the last interglacial.  The species was plainly accustomed to growing in Atlantic woodland and may have retained a genetic “memory” of how to cope with this habitat and its competing species.  But it didn’t grow spontaneously in Britain for the next 30,000 years, and all the current feral colonies are regarded as originating from garden escapes.”(3)

Rhododendron ponticum. Wikimedia Commons

Once again, we wonder if “welcome home” isn’t a more appropriate response to this beautiful plant.  We find the definition of “native” as arbitrary as the definition of “invasive.”  Both seem to be terms used by people who abhor change.  And in a rapidly changing world, does such resistance to change make any sense?  We don’t think so. 

(1) The use of the word “created” implies no particular origin of the earth, merely its beginning.

(2) Crosby,Alfred, Ecological Imperialism, 2nd Edition, Cambridge, University Press, 2004

(3) Mabey, Richard, Weeds:  In Defense of Nature’s Most Unloved Plants, Harper-Collins, 2010