A new genetic study (Miller et al. 2012, now in print) suggests that Polar bears arose between 4-5 million years ago and thus survived the more than 50 glacial/interglacial cycles of the 2.5 million year Pleistocene epoch. Here I’ll begin to explore some of the claims of that paper on the emergence of Arctic bears, suggesting why we might want to take them with a grain of salt.
See my introduction to this topic, which contains information on the so-called ‘ABC’ bears of Southeast Alaska (as well as some notes on my adventures in ABC bear territory a few years ago). It’s worth taking a look at Doug Hoffman’s excellent Aug. 5 summary of the original paper here. The NIPCC also has a summary here.
Here are some of the points regarding this new paper by Miller and colleagues that I think deserve a critical look. In this post, I’ll elaborate on Claim #1.
Claim #1 Polar bears and Brown bears (aka grizzlies) arose 4-5 million years old. See below for critique.
Claim #2 Hybridization in both directions occurred repeatedly throughout the evolutionary history of Polar bears and Brown bears.
Claim #3 Svalbard may have been an important refugium for Polar bears during warm interglacial periods.
Claim #1 Polar bears and Brown bears (aka grizzlies) arose 4-5 million years old
From their abstract:
We demonstrate that brown bears and PB [polar bears] have had sufficiently independent evolutionary histories over the last 4-5 million years to leave imprints in the PB nuclear genome that likely are associated with ecological adaptation to the Arctic environment.
The problem with this claim is that the date is in significant conflict with the fossil record for bears in Europe and Asia (see Fig. 1 below).
Björn Kurtén (1968) points out that the fossil record of bears is better than virtually any other mammal (except for Polar bears, which have a decidedly lousy fossil record – there are only six specimens of Polar bear that pre-date the Last Glacial Maximum (LGM), which lasted from about 26,500-19,000 years before present, see Lindqvist et al. 2010).
Figure 1. Summary of fossil evidence for the various bear species in the genus Ursus. Note the arrangement here does not depict evolutionary relationships, but age of fossils. It is apparent that the oldest Brown bear fossils are much less than 1 million years old and even the oldest Asian Black bear fossils are no older than about 2 million years. The oldest Polar bear fossil is only about 120,000 years old. Click to enlarge.
Here are the details on the Ursus fossil record (skip to the end if you’re not interested)
There was a small primitive bear that lived from ca. 4-5 million years ago (ma) until the early Pleistocene (ca. 2 ma or so), called the Auvergne bear Ursus minimus (ancestor of all Ursus). Its fossils are common in Europe and Asia. The Etruscan bear Ursus etruscus evolved from U. minimus and became widespread across Europe and Asia in the Late Pliocene/Early Pleistocene, ca. 3-1 ma (Kurtén 1968; Kurtén and Anderson 1980). See Fig.1.
Various ‘cave bears’ (Savin’s bear Ursus savini (not shown in fig. 1), Deninger’s bear Ursus deningeri , and ultimately, the Cave bear Ursus spelaeus) emerged in Europe (from U. etruscus) in the early-mid Pleistocene ca. 1 ma (Kurtén 1968) but eventually move into Asia (Knapp et al. 2009). The last of these forms, the Cave bear, became extinct during the LGM (Bon et al. 2008:17450).
The earliest Brown bear Ursus arctos fossils are found in Asia only (apparently evolving from Asian U. etruscus) and date to about 659 thousand years ago (ka), the time of the MIS 12 glacial. There is a continuous fossil record of this species from that time to the present.
The first European Brown bear fossils date to ca. 427-334 ka (during the MIS 11 interglacial), overlapping in distribution with the Cave bear in Europe (Kurtén 1968). The Cave bear dies out during the LGM but the Brown bear continues.
A primitive Black bear, Ursus abstrusus (which appears to be a North American form of Ursus minimus, suggesting that the earliest form of Ursus had a global distribution) has been found in North America just after 5 ma (Bell et al. 2004; Kurtén and Anderson 1980).
The earliest fossils attributed to the American Black bear Ursus americanus date to ca. 1.8 ma (Kurtén and Anderson 1980). This Black bear is often missing premolar (pm) 2 and pm 3 may be reduced in size link http://wildpro.twycrosszoo.org/S/0MCarnivor/ursidae/ursus/Ursus_americanus/02Ursus_americanusAMHead.htm
Asiatic Black bear Ursus thibetanus fossils date to about early-mid Pleistocene, ca. 2 ma (probably evolved from U. minimus, but perhaps from U. etruscus)(Herrero 1972). This species appears to be the most primitive living species of the genus Ursus, as it usually retains a full complement of premolars. ( See Fig. 2 below).
The earliest Polar bear Ursus maritumus fossil dates to ca. 130,000-115,000 ka, during the MIS 5e interglacial (Svalbard)(Lindqvist et al. 2010).
Figure 2. Primitive bear dentition of Asiatic black bear, modified from wikipedia image. Adult Brown bears and Polar bears usually lack premolar (pm) 2 and 3, while American Black bears are usually missing pm 2.
[end of fossil record details]
In summary, as far as the fossil evidence goes, the only bears that lived between 4-5 million years ago (when Miller et al. propose that Brown bears and Polar bears arose) were Ursus minimus (in Europe and Asia) and Ursus abstrusus (in North America). Both were primitive forms with a full complement of premolars and in this regard, were morphologically distinct from Brown bears, Polar bears, and American Black bears. See figure 3 below.
The earliest fossils of Brown bears are less than 1 million years old and the early forms are found in Asia only.
Figure 3. Summary of fossil evidence for the various bear species in the genus Ursus, with the origin of Brown and Polar bears proposed by Miller et al. 2012 noted. Click to enlarge.
If the dates for the split of Brown bear and Polar bear, based on the Miller et al. genetic study, are correct, it means that both must have co-existed, over several million years, with both primitive ancestral forms of bears (U. miniumus and U. etruscus) as well as the early cave bears and early Asiatic bears, without leaving a fossil record.
Plausible? Not to me.
It seems more probable that at least some of the common genetic haplotypes Miller et al. found in Brown and Polar bears (and to a lesser degree, in American Black bears) derive from one or both of the ancestral Ursus species, the Auvergne bear U. minimus and/or the Etruscan bear U. etruscus.
If so, it seems that these retained ancestral genes and/or haplotypes have been misinterpreted as evidence of greater-than-expected antiquity for these species and as evidence of ancient hybridization events. There may be another explanation but this seems the most likely.
A similar issue arose in the early days of dog genetic studies: a widely hyped paper in Science in 1997 by Carlos Vilà and colleagues suggested that dogs might have been domesticated 135,000 years ago! This result was in conflict with all archaeological evidence (it still is) and is no longer considered credible (e.g. Dobney and Larson 2006; Koop et al. 2000 and my book, Crockford 2006).
In addition, it’s worth mentioning that molecular clock estimates of divergence times are known to have some serious problems (as discussed in depth by Pulquério and Nichols 2007). As Dobney and Larson (2006:266) point out, “…molecular clocks often do not have the resolution to date very recent events (such as domestication)…”.
In other words, trying to determine the date of speciation events from genetic data is not foolproof and such dates should always be viewed with caution.
However, in my opinion, the conflict of Miller et al.’s 2012 genetic results with the known fossil record for bears is the strongest reason for questioning the validity of their conclusions.
I plan to take the 4-5 million year old date for the origin of Polar bears and Brown bears with a healthy dose of salt. Expect to see another paper showing different results within a few years.
References
Bell, C.J., Lundelius, E.L., Barnosky, A.D., Graham, R.W., Lindsay, E.H., Ruez Jr., D.R., Semken Jr., H.A., Webb, S.D., and Zakrzewski, R.J. 2004. The Blancan, Irvingtonian, and Rancholabrean Mammal Ages. pg. 232-314 in Late Cretaceous and Cenozoic Mammals of North American: Biostratigraphy and Geochronology, M. O. Woodburne, ed. Columbia University Press, New York. http://www.amazon.com/Cretaceous-Cenozoic-Mammals-North-America/dp/0231130406
Bon, C., Caudy, N., de Dieuleveult, M., Fosse, P., Philippe, M., Maksud, F., Beraud-Colomb, E., Bouzaid, E., Kefi, R., Laugier, C., Rousseau, B., Casane, D., van der Plicht, J., and Elalouf, J.-M. 2008. Deciphering the complete mitochondrial genome and phylogeny of the extinct cave bear in the Paleolithic painted cave of Chauvet. Proceedings of the National Academy of Science USA. 105:17447-17452. http://www.pnas.org/content/105/45/17447.full.pdf+html
Crockford S.J. 2006. Rhythms of Life: Thyroid Hormone and the Origin of Species. Victoria: Trafford. available here
Dobney K. and Larson, G. 2006. Genetics and animal domestication: new windows on an elusive process. Journal of Zoology 269:261–271. open access
Herrero, S. 1972. Aspects of evolution and adaptation in American black bears (Ursus americanus Pallas) and brown and grizzly bears (U. arctos Linné) of North America. In. S. Herrero, ed., Bears – their biology and management. Morges, IUCN New Series 23: 221-231. http://www.bearbiology.com/fileadmin/tpl/Downloads/URSUS/Vol_2/Herrero.pdf
Knapp, M., Rohland, N., Weinstock, J., Baryshnikov, G., Sher, A., Nagel, D., Rabeder, G., Pinhasi, R., Schmidt, H.A., and Hofreiter, M. 2009. First DNA sequences from Asian cave bear fossils reveal deep divergences and complex phylogeographic patterns. Molecular Ecology 18: 1225-1238. http://www.zin.ru/labs/theriology/staff/baryshnikov/references/knapp_et_al_2009.pdf
Koop B.F., Burbidge M., Byun A., Rink U. and Crockford, S.J. 2000. Ancient DNA evidence of a separate origin for North American indigenous dogs. In: Dogs Through Time: An Archaeological Perspective, S.J. Crockford (ed). Archaeopress, B.A.R. S889 Oxford, 2000: 271-286.
Kurtén, B. 1968. Pleistocene Mammals of Europe. Weidenfeld and Nicolson, London.
Kurtén, B. and Anderson, E. 1980. Pleistocene Mammals of North America. Columbia University Press, New York.
Lindqvist, C., Schuster, S.C., Sun, Y., Talbot, S.L., Qi, J., Ratan, A., Tomsho, L., Kasson, L., Zeyl, E., Aars, J., Miller, W., Ingólfsson, Ó., Bachmann, L. and Wiig, Ø. 2010. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceedings of the National Academy of Sciences USA 107:5053-5057. http://www.pnas.org/content/107/11/5053.full.pdf+html
Miller, W., Schuster, S.C., Welch, A.J., Ratan, A., Bedoya-Reina, O.C., Zhao, F., Kim, H.L., Burhans, R.C., Drautz, D.I., Wittekindt, N. E., Tomsho, L. P., Ibarra-Laclette, E., Herrera-Estrella, L., Peacock, E., Farley, S., Sage, G.K., Rode, K., Obbard, M., Montiel, R., Bachmann, L., Ingolfsson, O., Aars, J., Mailund, T., Wiig, O., Talbot, S.L., and Lindqvist, C. 2012. Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences USA 109:E2382-E2390. doi: 10.1073/pnas.1210506109. see also: paper Paper plus supplemental data
Pulquério, M. J. F. and R. A. Nichols. 2007. Dates from the molecular clock: how wrong can we be? Trends in Ecology and Evolution 22:180-184.
http://www.bio-nica.info/Biblioteca/Pulquerio2006MolecularClock.pdf
Vilà, C., Savolainen, P., Maldonado, J.E., Amorim, I.R., Rice, J.E., Honeycutt, R.L., Crandall, K.A., Lundeberg, J. and Wayne, R.K. 1997. Multiple and ancient origins of the domestic dog. Science 276:1687-1689. http://www.sciencemag.org/content/276/5319/1687.abstract
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