December 23, 2008 | David F. Coppedge

Bat Evolution: The Play’s the Thing

According to the Darwinian script, each animal evolved its particular adaptations from an ancestor lacking those adaptations.  Take bats.  They must have evolved their wings and sonar from mouse-like ancestors that lived on the ground.  Is it enough to imagine these things, or should we expect science to provide evidence that is what really happened?
    Scientific American published a story entitled, “Taking Wing: Uncovering the Evolutionary Origins of Bats.”  It sounds like the evidence has been uncovered, now to be revealed for the first time, and we are about to look at it.  Actually, reporter Nancy B. Simmons ended with this remarkable admission:

Despite many new discoveries about the rise of bats, mysteries remain.  Bat ancestors must have existed prior to the Eocene, but we have no fossil record of them.  Likewise, the identity of the closest relatives of bats is still unknown.  Investigators are also eager to learn when the bat lineage first became distinct from that of the other laurasiatheres and how much of early bat evolution and diversification took place in the northern continents versus the southern continents.  We therefore need fossils that lie even closer to the beginning of bats than Onychonycteris does.  With luck, paleontologists will find such specimens, and they will help solve these and other riddles about the origins of these fascinating animals.

Obviously this points back to Onychonycteris and the other “many new discoveries” that will have to support the evolutionary story across the remaining gaps.  What did the article say about these?  Simmons started off by discussing the wonder of bats as we see them today.  She admitted that “their ascension was hardly a foregone conclusion: no other mammal has conquered the air” with powered flight, though several mammals can glide on outstretched flaps of skin.  Powered flight puts severe requirements on many organs, though, and the echolocation found in 85% of these “superb fliers” puts additional anatomical constraints on the skull, mouth, ears and throat.  She spent some time describing all the factors involved after saying, “Indeed, exactly how these rulers of the night sky arose from terrestrial ancestors is a question that has captivated biologists for decades.”
    Then she went into her discovery this year of Onychonycteris finneyi in Wyoming, “the most primitive bat ever discovered” (see 02/16/2008 discovery report).  This bat, though possessing shorter forelimbs and longer hindlimbs than extant bats, was still fully capable of flight.  In fact, living mouse-tailed bats have a similar wing aspect ratio, she said.
    The main evolutionary question addressed by her find, then, was not how powered flight evolved, but whether it evolved first, or sonar first, or whether both flight and sonar evolved simultaneously.  Earlier fossils did not help in filling the gap, she argued, but Onychonycteris did not appear to have sonar.  The flight-first theoreticians win, she claimed.
    “Still, we lack fossils that establish how bats are related to other mammals,” she said in a section about the diversity of living bats.  Genetic studies do not show them related to other gliding mammals.  The nearest ancestors, “an ancient lineage known as Laurasiatheria” consists of “such diverse beasts as carnivores, hoofed mammals, whales, scaly anteaters, shrews, hedgehogs and moles,” – none of which are fliers (although there were flying whales in Disney’s Fantasia 2000).  This leaves a lot of evolutionary space unfilled:

Primitive laurasiatheres, however, were probably mouse- or squirrel-size creatures that walked on all fours and ate insects.  Laurasiatheres are thought to have evolved on the ancient supercontinent of Laurasia, which comprised what is now North America, Europe and Asia, probably in the late Cretaceous period, some 65 million to 70 million years ago.  The exact position of bats within this group is uncertain, but clearly a considerable amount of evolutionary change separates Onychonycteris and other bats from their terrestrial forebears.
    Some of this change from land dweller to flier may have occurred surprisingly quickly, if recent discoveries in the field of developmental genetics are any indication.  Though short by bat standards, the fingers of Onychonycteris are greatly elongated as compared with those of other mammals.  How could this elongation have evolved?

Good question.  Her answer?  Bone morphogenetic proteins (BMPs).  The genes for these limb-growing proteins are expressed differently in mice and bats.  If we can imagine gradual changes in gene expression of BMPs, then, we can imagine transitional forms, even if none are found in the fossil record:

It is therefore possible that a small change in the genes regulating BMPs underlies both the developmental and evolutionary elongation of bat wing digitsIf so, that might explain the absence in the fossil record of creatures intermediate between short-fingered, nonflying mammals and long-fingered bats such as Onychonycteris and Icaronycteris: the evolutionary shift may have been very rapid, and few or no transitional forms may have existed.

She apparently did not ask why differences in BMP expression didn’t lead to flying hedgehogs and cows jumping over the moon.  On the other hand, maybe they did; they just didn’t leave any fossils.  That the gap was filled in with imagination is underscored with her final paragraph, quoted above: “Despite many new discoveries about the rise of bats, mysteries remain….”

The only transitional bats in Darwin’s belfry are imaginary ones.  Darwin removed the requirement for hard evidence and replaced it with imagination.  That’s why nothing makes sense in biology except in the light of evolution; if you can simply imagine the transitional forms that should be there but aren’t, you can make your theory come true without data.  Suddenly everything makes sense.  Data are such contrary things, anyway.  It helps, too, when you also rule that alternative views cannot be heard.  This was Hamlet’s undoing; Charlie conquered Claudius, took the throne, exiled Hamlet along with his righteous anger, canceled the play, and produced one of his own, featuring all kinds of fanciful chimeras like imaginary transitional bats taking wing.  It was a hit.  It had to be.  The subjects dared not fail to applaud, cheer and beg for encores.  His own play’s the thing to assuage the conscience of the king.  Now you know the rest of the play within the play.

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Categories: Mammals

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