Sunday, January 10, 2010

Tetrapods Evolved Earlier Than Previously Thought...Cue the Creationists!

Paleontologists recently described a fossilized trackway discovered in Poland that dates to 395 million years ago.

Illustration and photograph by Grzegorz Niedzwiedzki, courtesy of National Geographic

What's cool about it is that paleontologists previously believed tetrapods evolved around 370 million years ago based on finds such as the Tiktaalik rosa which seemed to be a transitional form between fish and amphibians.  However, this new find reveals that tiktaalik was in fact an evolutionary holdover from a transition that occurred at least 20 million years earlier.  Unfortunately, we need more fossil evidence to properly pin down when the transition took place, but at least this find gives us a better idea of where to look in the rocks. Clearly, 395 million years old is too young!

Of course, the usual suspects are tripping over themselves to show that this invalidates tiktaalik's place as a transitional fossil and disproves evolution.  PZ Myers does a nice job explaining why that's simply not the case.  In short, evolution isn't a ladder, but a branching tree.  Creatures that were once transitional forms can continue to survive for millions of years as long as they can continue to reproduce (look at the coelacanth).  In this instance, tiktaalik is the descendant from a population of creatures that were the transition between fish and tetrapods.  Some of that ancestor population continued to survive in their transitional form, eventually becoming the tiktaalik, while others continued changing into tetrapods, including the ones that left their footprints behind in prehistoric Poland.  None of the scientists involved ever claimed tiktaalik was a definite ancestor of all tetrapods.  It was just a prime example of what the direct ancestor might have looked like.

It might be easier to explain using human evolution.  Imagine alien paleontologists are studying the Earth 350 million years in the future, and they have only a rough understanding of human evolution and few fossils to go on.  Then they find a fossilized chimpanzee skeleton followed a couple of years later by a more ancient set of Homo erectus footprints.  Would the alien paleontologists have to rule out evolution entirely because the more human-like footprints come before the apparent transitional form in the geologic record?  Of course not.  As we know, the chimpanzee is little changed from the transitional form between humans and all other primates.  However, we also know it is not our direct ancestor, just a good example of what the common ancestor between apes and us might have looked like.  On the other hand, Homo erectus obviously lived before modern chimpanzees, but was well on its way to becoming human while still living at the same time as the chimp's ancestors.

Applied to the recent discoveries of tetrapod evolution, the tiktaalik is like the chimpanzee.  It's an approximate representation of a common ancestor, which continued to coexist among the ever-changing tetrapods.  When those early amphibians walked across an ancient beach in Poland, somewhere in the shallow waters of the world were the ancestors of the tiktaalik.  They had changed little from the common ancestor to both and would remain virtually unchanged for several million more years.

Anyway, this showcases part of what I love about science.  It's always changing in response to new evidence, bringing us closer to an accurate understanding of the world around us.  Isn't reality awesome?

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