A Man, A Plan, A Cabal: Panama

first_imgEvolutionists duke it out on the age of the Isthmus of Panama. Every solution breeds new problems.No fewer than 35 scientists, from nearly that many institutions, signed on to a paper in Science Advances in August that strongly argued for a date for the Isthmus of Panama at 2.8 million years. Their confidence is palpable:The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.Their view got good press on Science Daily. Too bad Nature didn’t take this challenge sitting down. Carrie Arnold writes about new evidence supporting a much earlier date:Colliding tectonic plates pushed up a strip of land from the watery abyss that once divided North and South America, forming the isthmus of Panama. But a study now hints that this happened millions of years earlier than scientists had thought.Evolutionary and population-genetics data from Eciton army ants, which can only travel on dry ground, suggest that the isthmus formed 4–8 million years ago. The research, published on 25 October in Molecular Ecology, challenges the long-held idea that the link between continents emerged no more than 3 million years ago.Why is the first of these papers so concerned about the date? “Populations of marine organisms divided by the rising land forged separate evolutionary paths in response to new and contrasting environments, and the timing of their divergence is now used to calibrate rates of molecular evolution.” So there it is. They don’t want to reset their clock to Darwin Savings Time.In the Science Advances paper, the gang of 35 tries to avoid charges of circular reasoning. They know they can’t calibrate the molecular clock by the Panama isthmus which separated sea creatures from the two sides, then turn around and use it to date the formation of the land bridge.To estimate dates from molecular divergence, it is necessary to (i) determine the phylogeny of a group, to identify sister species in either ocean, and (ii) estimate the rate at which their molecules have evolved. Most calibrated phylogenies of marine organisms have assumed that the Isthmus closed at 4 to 3 Ma, but it would be circular to use these estimated rates of molecular evolution to date Isthmus formation. In an attempt to remove this circularity, Bacon et al. assumed a “universal” rate of mitochondrial DNA (mtDNA) divergence of 2% per million years. However, this is inappropriate, because rates of molecular evolution vary substantially between clades; two approaches are taken here.First, they used molecular divergence dates for sister species on the Atlantic and Pacific sides of the land bridge that they claim are independent of the date for the isthmus formation, because they are supposedly calibrated by fossils. Second, they did the same for phylogenies that are not calibrated by fossils, and asked if the results are reasonable for old or young dates. It’s hard to see how they escape the charge of circularity here, though, since both approaches assume molecular evolution rates. Dating fossils also assumes evolutionary divergence rates. It’s circularity folding back on itself.One consequence of the late date (2.8 million years) is how to get animals across without the land bridge. In the case of North and South America, this includes rodents, monkeys and sloths found on both hemispheres. The gang of 35 argues that it’s not a problem, because evolution did it before with the aid of rafts:Some researchers have argued that dispersals of animals and plants between North and South America before 3 Ma indicate an early completion of a land bridge. Bacon et al., for example, estimated that Copernicia and Pritchardia palms dispersed between North and South America from 31 to 9 Ma, and thus concluded that the Isthmus of Panama had emerged in the Oligocene or Miocene. However, many kinds of plants and animals are known to have dispersed over ocean barriers with surprising frequency, often crossing gaps much wider than the widest plausible strait separating North and South America in the Miocene. Primates and rodents crossed the Atlantic in the Eocene. All the post-Eocene mammal lineages of Madagascar, including hippopotamuses, arrived from Africa. Most of the (endemic) modern fauna of the West Indies is derived from overwater colonists. Salt water–intolerant amphibians crossed sea barriers numerous times.Readers may be unaware of rafting monkeys and hippos, having never seen any on cruise ships (see “Improbable Sailors,” 4/27/15). Is this a case of fudging data to rescue a popular theory? (3/12/15). The gang of 35 stretches credibility to make their case:Rafting is the most likely method of dispersal of terrestrial organisms over water. Natural rafts of soil and vegetation that form when floods wash away parts of river banks are frequently observed far out at sea, especially in areas receiving tropical storms, and these rafts might harbor enough food and fresh water to maintain animals for a journey of weeks or months. Rafts frequently reach very large sizes, and prevailing winds, currents, or storms can carry them relatively rapidly over long distances. The Atlantic crossings by monkeys and rodents, which almost certainly occurred by rafting, indicate the great potential of this mechanism for long-distance dispersal.That’s where the Nature paper comes in. Army ants only travel on dry ground, says Carrie Moreau of the Field Museum in Chicago. “She picked the perfect species for this kind of study,” a California colleague remarks, since the paper in Molecular Ecology claims they are “unable to disperse across water”. Since the ants are found on both sides of the land bridge, and molecular evolution demands that they diverged 4-7 million years ago, the bridge must be older as well. Nature then adduces additional evidence to support the old age:Other recent biological and geological evidence also suggests that the isthmus emerged earlier, through a more complex process, than previously thought. That evidence includes fossil teeth from ancient monkeys, which suggest that the primates crossed the gap between the continents 18 million years ago. And 20-million-year-old plant fossils found during the excavation of the new Panama Canal in 2012 also challenge the accepted age of the isthmus.One member of the gang of 35 is not convinced. He waves his sword:Anthony Coates, a geologist at the Smithsonian Tropical Research Institute in Panama City, is sceptical of any evidence contradicting the isthmus’s 3-million-year age. “There are still a series of investigations in different disciplines that all converge on the same number of around three million years ago. This is extremely rigorous evidence,” Coates says.The geological battle lines, it seems, have been drawn. Whether they can form an ideological land bridge similar to the physical one that researchers are studying remains to be seen.Much of this depends on the molecular clock dating method, which assumes Darwinian evolution. But another conundrum arises independent of that. If rafting is so easy and common, why don’t we see it going on today? And why wouldn’t frequent rafting of everything from ants to hippos dilute the evidence for evolution from biogeography? Typically, evolutionists date divergence patterns from natural barriers, including oceans, such as in the familiar case of the Wallace Line. But then there are animals that can’t swim, raft, or travel long distances, yet are found on every continent (see the case of worm lizards, 4/27/15). Nick Longrich was so dumbfounded by the rafting hypothesis as an explanation for the spread of worm lizards and “nearly every other living thing as well,” he remarked, “Once you eliminate the impossible, whatever you’re left with, no matter how improbable, must be the truth.”I’m Popeye the sailor ant,I’m Popeye the sailor ant;Can’t Darwin diminish though ain’t good at swimmishI’m Popeye the sailor ant.Ever heard of the “Popeye Theory of Evolution”? 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