Posted by: Rob Viens | October 18, 2012

Bridging the Continents

Darwin’s focus for the last couple days was on wrapping up work in Bahia Blanca.  Today, the Beagle started its return trip to Montevideo and the two schooners began their solo survey of the coast south of Bahia Blanca.  He writes:

“The Beagle & the two Schooners, forming a little fleet sailed together & anchored at night in the entrance of the Bay.” (Oct 17)

“We continued to sound.— At noon the Schooners made sail to the South; we gave them three hearty & true cheers for a farewell.” (Oct 18)

In the last post I mentioned that the “camel” originated in North America and migrated to South America and Asia via “land bridges”.  Interestingly, being the master of observation that he was, Darwin recognized the connections between the Old World, North America and South America back in the 1830’s when he was writing Voyage of the Beagle.

To be fair, he did not have the story entirely correct, suggesting that the three regions had once been connected and were now isolated by the Bering Strait (between the Old World and North America) and the mountains of Central America and the Caribbean Sea (between North and South America). He posed the idea that these barriers developed recently and isolated the three regions. THis is partially correct, but there is a lot more to the story.

The truth is that the Bering Strait did probably connect the Old World with North America on and off for some time.  The sea there is relatively shallow – underlain by shallow continental crust.  So if you lower sea level a little bit, say about 35 meters (~100 feet), you create a land bridge between the two regions. This probably happened frequently and explains the major similarities between North America and the old world (we both have wolves, squirrels, deer, to name just a few examples from fairy tales).

Isthmus of Panama (from NASA)
Isthmus of Panama

On the other hand the Isthmus of Panama that connects North and South America is a relatively recent phenomena, having formed roughly 3 million years ago.  Prior to that South America had been completely isolated for the better part of 100 million years. When the isthmus formed, animals such as pumas and camels migrated south from North America, and armadillos and ground sloths migrated north from South America.  This represents one of the largest biological exchanges in recent geological times (not including those now caused by humans). Many of Darwin’s fossils came from younger (<3 million year old) rocks that formed after the connection was formed.

So although he didn’t get it entirely right, the fact that he made the connection and realized that things had changed regarding the links between these three regions of the world is a brilliant piece of deduction.  Is there anything Darwin didn’t figure out?

Below is the full section from Voyage of the Beagle explaining Darwin’s idea.  It amazed me to read this – knowing that he wrote this after one (long) field season and a lot of analyzing others accounts of the animals of the world. I have more respect for Darwin every day (and I already started with a high bar). I’d recommend reading his words/thoughts below to experience your own set of awe:

“The existence in South America of a fossil horse, of the mastodon, possibly of an elephant,† and of a hollow-horned ruminant, discovered by MM. Lund and Clausen in the caves of Brazil, are highly interesting facts with respect to the geographical distribution of animals. At the present time, if we divide America, not by the Isthmus of Panama, but by the southern part of Mexico in lat. 20°, where the great table-land presents an obstacle to the migration of species, by affecting the climate, and by forming, with the exception of some valleys and of a fringe of low land on the coast, a broad barrier; we shall then have the two zoological provinces of North and South America strongly contrasted with each other. Some few species alone have passed the barrier, and may be considered as wanderers from the south, such as the puma, opossum, kinkajou, and peccari. South America is characterized by possessing many peculiar gnawers, a family of monkeys, the llama, peccari, tapir, opossums, and, especially, several genera of Edentata, the order which includes the sloths, ant-eaters, and armadillos. North America, on the other hand, is characterized (putting on one side a few wandering species) by numerous peculiar gnawers, and by four genera (the ox, sheep, goat, and antelope) of hollow-horned ruminants, of which great division South America is not known to possess a single species. Formerly, but within the period when most of the now existing shells were living, North America possessed, besides hollow-horned ruminants, the elephant, mastodon, horse, and three genera of Edentata, namely, the Megatherium, Megalonyx, and Mylodon. Within nearly this same periods (as proved by the shells at Bahia Blanca) South America possessed, as we have just seen, a mastodon, horse, hollow-horned ruminant, and the same three genera (as well as several others) of the Edentata. Hence it is evident that North and South America, in having within a late geological period these several genera in common, were much more closely related in the character of their terrestrial inhabitants than they now are.

The more I reflect on this case, the more interesting it appears: I know of no other instance where we can almost mark the period and manner of the splitting up of one great region into two well-characterized zoological provinces. The geologist, who is fully impressed with the vast oscillations of level which have affected the earth’s crust within late periods, will not fear to speculate on the recent elevation of the Mexican platform, or, more probably, on the recent submergence of land in the West Indian Archipelago, as the cause of the present zoological separation of North and South America. The South American character of the West Indian mammals seems to indicate that this archipelago was formerly united to the southern continent, and that it has subsequently been an area of subsidence.

When America, and especially North America, possessed its elephants, mastodons, horse, and hollow-horned ruminants, it was much more closely related in its zoological characters to the temperate parts of Europe and Asia than it now is. As the remains of these genera are found on both sides of Behring’s Straits and on the plains of Siberia, we are led to look to the north-western side of North America as the former point of communication between the Old and so-called New World. And as so many species, both living and extinct, of these same genera inhabit and have inhabited the Old World, it seems most probable that the North American elephants, mastodons, horse, and hollow-horned ruminants migrated, on land since submerged near Behring’s Straits, from Siberia into North America, and thence, on land since submerged in the West Indies, into South America, where for a time they mingled with the forms characteristic of that southern continent, and have since become extinct.” (Voyage of the Beagle)

Great American Interchange of species – the image shoes species that migrated from the “other” continent (from American Scientist)
Isthmus of Panama

(RJV)

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Responses

  1. […] and were isolated there until a few million years ago when North and South America were joined (see Bridging the Continents to see Darwin’s speculations about this event). They consist of three distinct groups of […]

  2. […] To continue using the ship analogy, about 3 million years ago the two continents were ultimately bridged by a “gangplank” – the Isthmus of Panama. Suddenly it was possible for species from both hemispheres to “jump ship” and walk north or south to a new continent. This massive migration was known as the Great American Interchange, and led to a major mixing of species in the Americas. (For more on Darwin’s thoughts about the interchange, see Bridging the Continents). […]

  3. […] the litopterns that survived the “Great American Interchange” (see Bridging the Continents) ended up in direct competition with a very similar group of animals that evolved in North America […]


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