Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Monday, June 6, 2022

A fish out of water

A lot of us probably have moments in our education where the teacher showed us something that overturned some piece of how we saw the universe.  I can remember several:

  • Finding out in physics class that horizontal and vertical motion are entirely independent of each other -- so if you shoot a bullet from a gun horizontally, and at the same moment drop a bullet from the height of the gun barrel, both will hit the ground at the same time.
  • Learning that chemical reactions completely change substances' properties; for example, if you take sodium (a soft, silvery metal) and react it with chlorine (a yellow-green, highly toxic gas) you get table salt.
  • Seeing in math class that there are different kinds of infinity -- countable and uncountable -- and the uncountable kinds are way bigger.  Infinitely bigger, in fact.  (If you want to find out more about how this was figured out, and also learn about some other mind-blowing stuff about math, check out my post on bizarre mathematics from a couple of years ago.)
  • Learning in astronomy class that because light travels at a finite speed, any time you are looking at something, you're seeing what it looked like when light left its surface.  So you never see anything in the present -- you see everything as it was in the past.  And the farther it is away from you, the further into the past you're looking.

Another one came up in my high school biology class, during the unit that was to launch me on a life-long fascination: evolution.  Ms. Miller, our bio teacher, told us that all tetrapods (four-limbed vertebrates) came from a common ancestor, and the way we know this is that regardless of the limb function, they all have the same number and arrangement of bones.  Your arm has one upper arm bone (humerus), two lower arm bones (radius and ulna), seven wrist bones (carpals), five hand bones (metacarpals), and fourteen finger bones (phalanges).  But... so does a whale's flipper, a dog's front leg, a bat's wing, and a lizard's leg.  The sizes differ, but the bones are the same.

[Image is in the Public Domain]

I've never found a single creationist or intelligent design advocate who could give me a cogent explanation of why an omnipotent creator would give a whale articulated finger bones and then encase them in a solid flipper.

In any case, I found the idea boggling; that we have the arm and hand bones we do because we inherited the basic structure from an ancestor hundreds of millions of years ago -- and share that same structure, and that same ancestor, with all other four-limbed vertebrates.  And just a couple of weeks ago, a team led by Shigeru Kuratani of the Evolutionary Morphology Laboratory at the RIKEN Cluster for Pioneering Research in Japan, have found a good candidate for what exactly that common ancestor might be.

Called Palaeospondylus, the great-great (etc.) grandparents of all tetrapods was a fish-like animal, only about five centimeters long.  It swam around in the warm seas of the Devonian Period on the order of four hundred million years ago, so preceded the previous candidate (Tiktaalik) by a good twenty-five million years.

A reconstruction of Palaeospondylus [Image licensed under the Creative Commons Smokeybjb, Palaeospondylus, CC BY-SA 3.0]

The tipoff that this was the common ancestor of tetrapods had nothing to do with limbs, but with the ear morphology -- they had three semicircular canals, organs critical in balance and equilibrium, just like we do -- and some details of cranial anatomy.  "The strange morphology of Palaeospondylus, which is comparable to that of tetrapod larvae, is very interesting from a developmental genetic point of view," said Tatsuya Hirasawa, who co-authored the study.  "Taking this into consideration, we will continue to study the developmental genetics that brought about this and other morphological changes that occurred at the water-to-land transition in vertebrate history."

I don't know how anyone could think about this -- that a little fish found fossilized in a limestone bed in Scotland might be the last common ancestor between humans, bats, lizards, and salamanders -- and not be at least a little bit amazed.  It brings home what to me is one of the most wonderful things about the evolutionary model; that all life on Earth is connected, not only ecologically, but genetically.  You're a distant cousin to all other terrestrial species, however different they may look from you.

And if learning that doesn't alter your view of the universe, I don't know what would.


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