On the Origins of Polar bears

From PNAS  March 16, 2010   vol. 107  no. 11  5053-5057 "Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear" Lindqvist et al. 2010.

The polar bear has become the flagship species in the climate-change discussion. However, little is known about how past climate impacted its evolution and persistence, given an extremely poor fossil record. Although it is undisputed from analyses of mitochondrial (mt) DNA that polar bears constitute a lineage within the genetic diversity of brown bears, timing estimates of their divergence have differed considerably. Using next-generation sequencing technology, we have generated a complete, high-quality mt genome from a stratigraphically validated 130,000- to 110,000-year-old polar bear jawbone. In addition, six mt genomes were generated of extant polar bears from Alaska and brown bears from the Admiralty and Baranof islands of the Alexander Archipelago of southeastern Alaska and Kodiak Island. We show that the phylogenetic position of the ancient polar bear lies almost directly at the branching point between polar bears and brown bears, elucidating a unique morphologically and molecularly documented fossil link between living mammal species. Molecular dating and stable isotope analyses also show that by very early in their evolutionary history, polar bears were already inhabitants of the Artic sea ice and had adapted very rapidly to their current and unique ecology at the top of the Arctic marine food chain. As such, polar bears provide an excellent example of evolutionary opportunism within a widespread mammalian lineage.

I've attached a copy of their phylogeny to this post, over to the right there. You can see the clearly 'nodal' (Basal) position of the polar bear they found. My first instinct was that this blew my pet hypothesis out of the water - that polar bears first underwent cultural evolution to adapt to ice based lifestyles, and then subsequently underwent phenotypic evolution. This would explain the the fact that ABC brown bears are paraphyletic to the rest of the brown bears (but only with mtDNA markers!). However, there's something that's... mmm... niggling? I think that's a word.

See, there are two issues. First is that identification is based on morphology. It looks like a polar bear skull, therefore progenitor polar bears looked like modern polar bears. On the other hand, you would see nothing if progenitor polar bears looked like modern brown bears, because we'd mis-classify them as arctos instead of maritimus. Therefore, the study could only find that polar bears were more like modern polar bears in morphology. Connected to this issue is that given the only the left mandible (jawbone) could be recovered. Granted, that's more than what we'd have otherwise, as polar bear remains are rare (As things that hangs out around water tend to be!). Still, it may be premature to state that this is substantial morphological evolution.

My second issue is that bone are far less static than we commonly think of them. Bone is an incredibly dynamic tissue, like muscle, that grows and whithers based on whether it's used, and how it's loaded with stress from the muscles. Look at the bones of people using percussive machines such as jackhammers, for example! In biologist talk, this is what we call "Environmental induction", where your environment causes you to have some feature, not genetics or heredity. To begin with, the differences between maritimus and arctos is subtle, as I've illustrated to the right with skulls from our teaching collecting here at UAF.  Compare the right skull to the picture of the skull I have below. Ignoring the intra-individual differences (and the differences from differential weathering), the skeletal differences between the two species is incredibly subtle. There's few characters you can use to differentiate them, but it takes a trained eye. No one, to my knowledge, has checked to see if varying brown bear diet varies cranial morphology. Ditto for polar bears.

There are messages I'm taking home, though. First, the date of the split of the species. I'll buy it based on the stratigraphy, the tree they generated, and how well it jives with the rest of the evidence. This makes polar bears a really recent species (not that we had any suspicions to the contrary!), which is interesting. Second, whole mtDNA genome sequencing for ancient remains: even cooler than I thought it was. Third, polar bears had a polar bear diet when they split. Pretty dang cool right there. As for the course of evolution of the polar bear, I remain somewhat sceptical and hesitant to accept their conclusions. 

Edit to add: Oh, wow, am I embarrassed! I posed the wrong skull comparison! The one I have above is arctos and americanus! That's entirely wrong! Here's a proper polar bear skull!

Lindqvist, C., Schuster, S., Sun, Y., Talbot, S., Qi, J., Ratan, A., Tomsho, L., Kasson, L., Zeyl, E., Aars, J., Miller, W., Ingolfsson, O., Bachmann, L., & Wiig, O. (2010). Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear Proceedings of the National Academy of Sciences, 107 (11), 5053-5057 DOI: 10.1073/pnas.0914266107