Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Thursday, November 17, 2005

The Mosasaur and the missing link

A new fossil mosasaur, one of a group of non-dinosaurian reptiles that return to marine existence, has been found by an amateur fossil hunter (see? Science can be done by non-professionals) near Dallas Texas, appropriately called Dallasaurus turneri after the location and discoverer Van Turner who found it 16 years ago.

This fossil is interesting because it is one of your classical "missing links". Mosasaurs, which ended up 40 feet long (12m) at the end of the Cretaceous when they and dinosaurs and a whole lot of other life went extinct from a bolide impact, evolved fins from their limbs, and many of the primitive mosasaurs had partial limbs/fins.

D turneri, however, has the complete set of limbs it shared with its reptilian ancestors and cousins. This is interesting for mosasaur specialists of course, but it also allows me an opportunity to talk about two often-misunderstood terms in evolution - "missing link" and "primitive".

Our mosasaur is "primitive" because he (she?) shares the ancestral state rather than the later derived state. "Highly evolved" mosasaurs like the Mosasaurus monster that lived at the Cretaceous end, were not "more complex" or "more advanced" except in the sense that they had changed from the ancestral condition. Had the later mosasaurs re-evolved feet from their fins (and we can be pretty sure they would not be a simple reversion, since so many developmental paths and genes would have been deactivated) then that state would have been "more evolved" or "advanced", and the fin condition "intermediate" and the original feet "primitive". The very word "primitive" means "came first". That's all it means. Only because of a progressionist view of history due to the pre-Darwinians does it mean "less complex" or "less developed" in our ordinary use, and Darwinian evolution does not imply that it had to be simpler.

That brings us nicely to "missing link". In the view of history and being known as the scala naturae, or "ladder of nature", things grade slowly from simple to complex, in a worldview dubbed The Great Chain of Being, after an Alexander Pope poem, Essay on Man (Epistle 1, 1732):
See thro' this air, this ocean, and this earth
All matter quick, and bursting into birth:
Above, how high progressive life may go!
Around, how wide! how deep extend below!
Vast chain of being! which from God began;
Natures ethereal, human, angel, man,
Beast, bird, fish, insect, who no eye can see,
No glass can reach; from infinite to thee;
From thee to nothing.--On superior powers
Were we to press, inferior might on ours;
Or in the full creation leave a void,
Where, one step broken, the great scale's destroyed:
From Nature's chain whatever link you like,
Tenth, or ten thousandth, breaks the chain alike.
If the chain is a continuous gradation as Pope and those who influenced him, all the way back to Aristotle but particularly in the renaissance and after, believed, then a gap means a "missing link". And clearly, if the Great Chain is a temporal sequence, then the earlier is simpler, and the later more complex. So it followed, and this was the way it was understood by Lamarck, the first widely-read evolutionist, that if humans evolved from earlier forms, they were simpler, and there should be no gap in the sequence. Hence, to show that we did evolve over time, there needed to be evidence of our immediately prior, simpler ancestors. If they were missing, they were a missing link.

But that wasn't Darwin's version of evolution. No simple ladder of nature, no chain. Evolution was, for him, a tree, and it didn't follow that any later branch was smarter, stronger, bigger, or more complex. In fact, he documented cases of vestigialism and regression himself, particularly in his barnacles book.

Darwin's tree diagram from The Origin of Species.

So a missing link might in fact be very similar to the earlier and the later forms, or it might be radically different from either one. Moreover, an intermediate form might be a mosaic of two groups rather than a smooth blend, as Archeopteryx and the recent Chinese feathered dinosaurs are between birds and (other) theropod dinosaurs. A smooth blend form is only to be expected on the Great Chain version of evolution. Moreover, if a period of geological history (shown in his diagram as "layers" of strata I...XIV) is not recorded for a branch or lineage of the tree, then there most certainly will be missing links. We expect them.

So I hope this helps explain some of the unintuitive aspects of evolution. Common sense is informed here by centuries of doctrines that we no longer think are true.