"Living Fossils": A Dead Idea
Monday, March 23, 2015
In an essay titled "The
Rise and Fall of the Living Fossil", Ferris Jabr argues that, "The
idea that some species are relics that have stopped evolving is
finally going extinct." Jabr focuses on the crocodile for most of the
piece, noting some of the errors that have resulted from the idea:
Intrigued by this puzzle, a post-doctoral research fellow at the University of Washington named Jamie Oaks began collecting DNA samples from all 23 living crocodilian species, comparing sections of the genome where mutations were most likely to have appeared. Although the fossil record had confirmed that ancient crocodilians were more diverse than previously realized, it also demonstrated that, on the whole, crocodilians were not particularly swift evolvers compared to mammals and other vertebrates. Even accounting for this slower-than-average evolution, Oaks did not find nearly as many differences between the modern crocodilian genomes as one would expect had those species diverged all the way back in the Cretaceous. He concluded that modern crocodilian species split from their last common ancestor between 8 and 13 million years ago, not long before ancient hominins split from their last common ancestor with chimpanzees. The living fossil theory of crocodiles had overestimated their evolutionary age by about a factor of 10.Jabr's piece relays fascinating details as to how this idea was shown to be false in the case of crocodiles, and it provides interesting historical background on both how the idea arose from Darwin's "fanciful" phrase and how it and other notions shape popular conceptions of evolution.
-- CAV
2 comments:
It's definitely an interesting study, one I'll have to read more on. That said, it shows more the power of words to influence our perceptions than anything profound in paleontology. It's been known for as long as I've been in the business (and longer, most likely) that terms such as "living fossil" are mere metaphores. There's no requirement for them to not speciate, nor for them to remain unchanged; it's simply a short-hand way of saying that they retain a surprising number of primative traits ("primative" being synonymous with "ancestral", not with "simple"). The use of this metaphore has, without a doubt, been a roadblock to a better understanding of organisms to which we have given the name--but at least I have never encountered any systematic definition of the term, implying that the influence is more subtle than the article suggests.
The thing I am most curious about is what the results would be had a different portion of the DNA of modern species been used. The issue is, no DNA analysis uses the whole sequence; you can't, there is too much data and too many issues with comparison. So we pick and choose which genes to look at. While I do not doubt the validity of the researchers' choice in the slightest, I do wonder if choosing other gene sequences for this analysis would have much of an impact, and if so how they would explain the discrepencies.
I am happy that crocodilians are getting increased recognition. While my first love remains predatory dinosaurs, even afte six years of studying mammals, crocodilians are definitely in the running. They are weird and wonderful monsters, everything that draws people into paleontology. I can't wait for the day when I see a small child in a museum look up at a crocodilian with the same wonder I've seen in the eyes of children looking at T. rex fossils, or mammoth fossils.
Diwar,
"That said, it shows more the power of words to influence our perceptions than anything profound in paleontology"
True, and I suspect that one could probably argue productively that, early on, it may have been better anyway for paleontologists to study more obvious cases of evolution than crocodiles. Still, it's interesting that even in the field, at least as far as I can tell from this article, there was some resistance to the idea that anything could be gained from studying them.
Gus
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