Homology vs. Analogy

  Above we have a squid and a snail.  I'm sure when you think of a squid you think of a big, slimy, deep sea creature that used to terrorize pirates and take down large ships in fairy tales.  When you think of a snail you also probably think slimy, but small and slow, a harmless creature that likes to hide in a shell and graze on your vegetable garden. The slime factor isn't the only thing they have in common.  Snails ancestors are one of the oldest known animals in the world.  There is fossil evidence that their ancestor gastropods date back to around 500 million years ago!!  Although snails are gastropods and squids are cephalopods, they are both mollusks.  This means they both lack an inner skeleton.  Squid ancestors have also been know to date back pretty far in history.  Ancestors of the squid known as ammonites date back to about 400-65 million years ago, they swam underwater like squids and they had shells just like snails! The ammonites show evidence that they could be relatives of both the squid and snail, so we understand that perhaps these are the ancestors the two species share.  

  Now although their bone structure (or lack thereof) is similar, these two species, though once connected have drifted apart.  The squid uses their bendable agile body to swiftly move through the ocean, snatch their food, and quickly maneuver through the water away from predators.  Snails use their squishy structure to curl into their shells and hide from predators.  They don't need to move fast like the squid.  They rely on their slowness to not call attention to preying birds above in the sky.  The squid and the snail both live in such different environments, you wouldn't think they were so alike.  Both use their mollusk bodies to survive but both are very different, together they are a homologous structure.  

  Now look who we have here.  On the left we have a painted turtle and on the right we have an adorable armadillo.  And guess what, both have shells! I never would have thought about the fact that these two are similar in that regard.  The turtle is an aquatic reptile and the armadillo is a land loving mammal, the only mammal to have a shell actually.  But the turtle develops his shell through the fusing of rib and back bone.  Turtles with shells are believed to have some of the oldest ancestors, dating back to possibly 260 million years ago.  They have ancestors that possibly date back further than that, but they were still genetically developing the shell.  Unlike the turtle, the armadillo shell is made up of bone fibers and skin fibers with a scale like texture.  Ancient armadillos used to be the size of a Volkswagon Beetle, whereas now they are normally the size of a cat or rabbit.  Through generations they have downsized, but scientists who have studied their ancestors fossils believe that they have always had their shell.  When the Aztecs discovered the armadillos they named them Azotochtli, which means "turtle-rabbit."  The armadillo's shell is more flexible than the turtles.  The turtle pulls itself into its shell for protection, whereas the armadillo pulls its shell around itself for protection. But both clearly use their hard exteriors to protect themselves from predators. 

  

  Even though they live in different environments, they still use their shells for the same function.  Both of their hard exteriors make it more challenging for predators to snag them.  At some point, very far back in history, both of their ancestors may not have had shells.  And then due to convergent convergent evolution they both genetically developed shells, generation after the next.  Perhaps their ancestors both lived around the same time period, where environment caused them to develop hard protective shells that allowed their species to continue to develop millions of year later and still surviving today.  Because both of their ancestors date back to around the same time, the turtle we know today and the armadillo we know today don't seem to be genetically connected at all.  Turtles have been believed to always be aquatic or at least partially aquatic, whereas the armadillos ancestors do not have much evidence proving them to ever being aquatic.  So, although these two species seem very different, somewhere down the line their ancestors picked up a great "suit of armor" to keep them around long enough for us to see them both today.