The Method of the Multiple Working Hypotheses: Why Disagreement Between Scientists is a Good and Necessary Thing.

Young Earth Creationists don't understand how science works - and they do their best to communicate that misunderstanding to their audience.

The Great American Biotic Interchange

The latest issue of Journal of Creation arrived today. In it is an article by Michael J. Oard that immediately grabbed my attention: "The Great American Biotic Interchange pushed back over 10 million years". The GABI, as it is called for short, is the period of time when large numbers of species went from North America into South America, while other species came from South America into North America. This took place once the Panamanian land bridge was established, some time between 3 and 5 million years ago. We have long known that a very few species made the trip prior to the bridge being established, notably two lineages of giant ground sloths coming from south to north, and one carnivore, a relative of the raccoon, going from north to south.

The claim that the GABI has been pushed back 10 million years is based on an extensive fossil fauna from the Rio Acre region of western Amazonia State, in Brazil. Of the thousands of fossils collected, representing 41 species of mammals, only one single species, represented by a partial skeleton of an elephant, was discovered in 1991, and named Amahuacatherium peruvium by Pittman-Romero in 1996. Amahuacatherium was until recently, the only mammal with North American affinities from the Rio Acre fauna.  Campbell and his colleagues determined its age to be late Miocene, between 9 and 10 million years ago (See Figure 1).

Figure 1, adapted from Campbell, K. E., Jr., Frailey, C. D., Romero-Pittman, L. (2009): In defense of Amahuacatherium (Proboscidea:Gomphotheridae. N. Jb. Geol. Palaont. Abh. Vol.252(1): Figure 4.

The close resemblance between Amahuacatherium (A) and Haplomastodon (C) is readily apparent. 

Subsequently, most paleontologists have disagreed with this on two bases: first, that the fossil is identical to Pleistocene specimens of the well known gomphothere Haplomastodon, and second, that the fossil was recovered from deposits which have not been tied to the dated stratigraphy of the region, which contains many small areas of Pleistocene deposits in addition to the Miocene deposits. 

Recently, these same authors, perhaps influenced by their desire to support a Late Miocene date for a "first pulse" of the GABI, were led to describe what I and other paleontologists believe to be a Pleistocene cervid as the first known South American dromomerycid, a horned ruminant well known from North America. They had at first claimed an associated fragmentary horn, which I and others pointed out was actually a tortoise scapula.

Figure 2.  Mandible of Sudameryx acrensis (type) from Prothero, D. R., Campbell, K.E. Jr., Beatty, B. L and Frailey, C. D., (2014):New late Miocene dromomerycine artiodactyl from the Amazon Basin: Implications for interchange dynamics, Journal of Paleontology: Figure 2.

A number of paleontologists, including me, believe this to be a modern cervid of at most Pleistocene age.  Some pretty sharp paleontologists believe it to be a dromomerycid.  The collecting of further material representing this taxon, and better dating of the sediments, will help resolve this question.

So it is on somewhat shaky grounds, contested by many paleontologists, that this claim rests, and on which Oard bases his comments.

Of course, Oard's conclusion is the predictable Creationist accusation that if there is this sort of controversy among evolutionists, then none of their claims can be relied upon. Oard throws in, at the very end, that recently zircons have been described from basins and rivers in the northern Andes, some of which may have had their origin in Panama about 13 -15 million years ago.

What Oard misses, of course, is that the overall concept of the GABI is still valid - even if such early dates for some Interchange animals were confirmed. The majority of the migrants from South America show up in central Mexico around 3.5 million years ago, and in the southern United States around 2.5 million years ago. Those going into South America show up mostly in the very latest Pliocene and earliest Pleistocene at about the same time. That some made it across earlier does not invalidate that scenario.

Note also that Oard does not quote the scientific paper on the zircons, but rather the teaser from the Prospectives section of that issue of Science. The actual article is worded rather less certainly than Oard would like:

"The Central American Seaway, which once separated the Panama Arc from South America, may have closed 10 million years earlier than is believed. Montes et al. report that certain minerals of Panamanian provenance began to appear in South America during the Middle Miocene, 15 to 13 million years ago (see the Perspective by Hoorn and Flantua). The presence of the minerals indicates that rivers were flowing from the Panama Arc into the shallow marine basins of northern South America. One interpretation of this finding is that large-scale ocean flow between the Atlantic and Pacific had ended by then. If this is true, then many models of paleo-ocean circulation and biotic exchange between the Americas need to be reconsidered."

"Abstract

Uranium-lead geochronology in detrital zircons and provenance analyses in eight boreholes and two surface stratigraphic sections in the northern Andes provide insight into the time of closure of the Central American Seaway. The timing of this closure has been correlated with Plio-Pleistocene global oceanographic, atmospheric, and biotic events. We found that a uniquely Panamanian Eocene detrital zircon fingerprint is pronounced in middle Miocene fluvial and shallow marine strata cropping out in the northern Andes but is absent in underlying lower Miocene and Oligocene strata. We contend that this fingerprint demonstrates a fluvial connection, and therefore the absence of an intervening seaway, between the Panama arc and South America in middle Miocene times; the Central American Seaway had vanished by that time.

Oard asks (following Hoorn and Flantua), if a land bridge existed 13-15 million years ago, "...why did many organisms wait until migrating around 3 million years ago?" Aside from the distances involved, and the very different ecological zones which had to be crossed, I suppose the real answer is that the GABI critters did not have the convenience of the Noachian Ark to ferry them up along the coast in less than a year.

Now, what is it that so upsets Creationist Michael J. Oard?  Even a cursory reading of his short article quickly reveals the source of his discomfort:  Uncertainty.  For someone used to the absolute certainty he invests in the Biblical account of Genesis (and, indeed, all of the Bible) at least according to his interpretation of the texts, and their infallibility, the uncertainty that is an integral part of science seems to make Oard very uneasy, indeed.  His comments reveal this:  "Since a presumed 'factual' tie point has been challenged......" and "The controversy over this date is at least showing how arbitrary this tie point is and how delicate is uniformitarian chronology."   His conclusion:  "It is best that creation scientists not take these 'events'....seriously, even in a relative timescale."

And don't forget - if science can't agree on a date, and it keeps changing, then nothing science says is true, and Young Earth Creationism wins by default. According to them, of course.

Oard, and other Creationists would do well to re-read an old but very appropriate description of how science works written by T. C. Chamberlain in Science in 1890, entitled "The Method of the Multiple Working Hypotheses".  Chamberlain tells us that the method of the multiple working hypotheses differs from the simple working hypothesis in that it distributes the effort and divides the affections [for any one particular explanation]...... The effort is to bring up into review every rational explanation of the phenomenon in hand and to develop every tenable hypothesis relative to its nature..."

Thus, we have several levels of this at work in therms of the GABI.  We have differing ideas on the identity of the gomphothere from the Rio Acre deposits; we have differing ideas concerning its age. These will gradually be decided when further collecting and dating gives us sufficient information to favor one interpretation over the other.  That the Great American Biotic Interchange was a process that took place over several million years is really not in question. The date when the Panamanian Land Bridge became established remains to be decided.

References:

Campbell, K. E., Jr., Frailey, C. D. & Romero-Pittman,
L. (2000): The late Miocene gomphothere Amahuacatherium
peruvium (Proboscidea: Gomphotheriidae) from
Amazonian Peru: Implications for the Great American
Faunal Interchange. – Instituto Geológico Minero y
Metalúrgico, Serie D: Estudios Regionales, Boletín, 23:
1-152.

Campbell, K. E., Jr., Heizler, M., Frailey C. D., Romero-Pittman,L.
& Prothero, D. R. (2001): Upper Cenozoic
chronostratigraphy of the southwestern Amazon
Basin. – Geology, 29: 595-598

Campbell, K. E., Jr., Frailey, C. D., Romero-Pittman, L. (2009): In defense of Amahuacatherium (Proboscidea:Gomphotheridae. N. Jb. Geol. Palaont. Abh. Vol.252(1):113-128.

Hoorn, C and Flatuna, S. (2015): An early start for the Panamanian land bridge. Science 348:186-187.

Montes, C et al (2015): Middle Miocene closure of the Central American Seaway, Science:Vol 348, Issue 6231:226-229.

Ord, Michael J. 2016, The Great American Biotic Interchange pushed back over 10 million years, Journal of Creation, Volume 30(3): 14-15.

Prothero, D. R., Campbell, K.E. Jr., Beatty, B. L and Frailey, C. D., (2014):New late Miocene dromomerycine artiodactyl from the Amazon Basin: Implications for interchange dynamics, Journal of Paleontology.

Romero-Pittman L. (1996): Paleontología de Vertebrados.
– Instituto Geológico Minero y Metalúrgico, Carta
Geológica Nacional, Boletín, Serie A, No. 81, 171-178