How Old are Polar Bears?

You’ve probably heard the news: Polar bears have been around a lot longer than we thought. “About 600 (338 to 934) thousand years” in fact. The research used to determine this involved molecular clocks, however, which has prompted an article by Brian Thomas: Circular Reasoning in Polar Bear Origins Date. Brian has incorrectly accused scientists of this fallacy before – what is it this time?

Biologists agree that polar bears, brown bears, and black bears all descended from a prototype of the bear kind. Some researchers infer from the biblical record that bears rapidly and recently diversified from a representative bear pair on board Noah’s Ark, while those who reject the biblical record derive age information from other sources.

Yes, that’s creationist science for you. And the first sentence is wrong – biologists don’t “agree” that a ‘kind’ is even a thing, or a prototype.

One group of researchers pinpointed polar bear divergence at around 150,000 years ago, beginning with mitochondrial DNA. They converted DNA base pair differences into years by assuming evolutionary time in their estimated DNA change rate. The similar date assignment paleontologists gave to the oldest polar bear fossils bolstered their confidence.

However, using nuclear DNA instead of mitochondrial DNA, another group reported in Science that polar bear’s “divergence time was estimated at 603ka [a thousand years] (median estimate), with 95% credibility intervals”—four times the earlier estimate. Both ages cannot be correct, and perhaps neither one is true. One or both of the research groups must have made a mistake. A closer look at the procedures that the evolutionists used to convert DNA base sequences into time sheds light on the age discrepancy.

The abstract of this paper tells us that:

[Our result] provides more time for polar bear evolution and confirms previous suggestions that polar bears carry introgressed brown bear mitochondrial DNA due to past hybridization.

In other words the mitochondrial genome previously used has been ‘polluted’ by interbreeding. It provides a “last common ancestor” date, but not the split that has been determined here. Oddly, Brian provides no reference for the group that found “polar bear divergence at around 150,000 years ago.”

The supplemental material attached to the Science report provided the first clue about how the researchers estimated the polar bear divergence time: “Protein-coding sequences from polar bear and giant panda were added to the alignment of 15 other Laurasiatherian mammals from Hallström et al.” Essentially, they hung their new DNA data into a historical scaffold from a previously published evolutionary timeframe that included many mammals.

They based their research on others – they checked their data to see if it lined up with what was already known. What’s so bad about that? The Hallström paper is here – unfortunately I can access nothing else referenced in this article.

Hallström and his colleagues derived their time scale by building a tree diagram showing evolutionary lines of descent between different kinds of creatures, forcing amino acid sequences into it, and adding dates to the diagram. “Divergence times were estimated from overall best amino acid (AA) ML [Maximum Likelihood] tree using 6 calibration points and the nonparametric rate smoothing method on a logarithmic scale (NPRS-LOG) as implemented in TF [Treefinder software].”

Yes, we’re digging here. Still nothing amiss.

What was the source of those dates? The Hallström-led study referenced an authoritative technical book titled The Timetree of Life, which postulated “Current [evolutionary] tree reconstruction and dating techniques are ‘relaxed’ in that they allow analysts to assume any number of local molecular clocks within a phylogeny.” Since some “local” molecular clocks tick faster and others slower, each researcher chose which clock rate they deemed best.

That quote is out of its context and makes little sense, at least to me. From what I can tell, Brian’s last sentence is like saying “Since some rulers are longer than others, each researcher used the one that was of the correct length for the task.” I think Brian is trying to make his reader pick up on the ‘assumed’ in there, but I really don’t know.

Instead of referring to reliable, unbiased, data-driven information to anchor their particular clock and evolutionary timescale to reality, The Timetree of Life researchers attached their dates to fossil ages: “Flexible calibration data, expressed in line with paleontological reality, can interact to cancel out a great deal of the uncertainty.”

They are “reliable, unbiased, data-driven information.” Anchoring molecular clock dates to reality is precisely what they do well. Brian may not agree with them, but that’s his problem.

The Timetree of Life referenced the standard geologic column, with its rock layers correlated to millions of years. And as has been well-documented elsewhere, researchers routinely hand-pick those dates to conform to long ages.

His reference for this is Ten Misconceptions about the Geologic Column, an Acts & Facts article from 1984. Amusingly, that article actually talks about the opposite. You don’t really need to do anything to make a million year old rock date to more than 6000 years.

In so doing, they also ignore scientific evidence that points to very young ages.

He gives three references here:

1. RATE
2. “See related articles in Fresh Tissues Show That Fossils Are Recent” – in other words, the whole soft tissue stuff that crops up from time to time.
3. 101 evidences for a young age of the earth and the universe, which you can see utterly refuted here.

Hardly convincing.

The polar bear geneticists made assumptions about evolutionary time, chose dates and published reports that fit these assumptions, and assumed that the millions-of-years age assignments in their “paleontological reality” correspond to historical reality. But they don’t. Each step in the process of dating the divergence of polar bears was a point somewhere inside a giant and twisted circular argument.

It’s not in any way circular, not so far as Brian has shown anyway. Their conclusion is not the same as their premise – did they begin by assuming that polar bears evolved 600 thousand years ago? I don’t think so.

The researchers would achieve more historically responsible results if they, instead, chose dates that conform to the most reliable written history of the world: Scripture.

That, on the other hand, is an exercise in circular reasoning. “The bible is the word of God, therefore the bible is inerrant, therefore the bible is the word of God” is really the textbook example of the fallacy – and Brian wants to base science on those sandy foundations. Good luck with that…