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Basically, in geology, what we have are a handful of principals and practices that allow us to date the ages of rocks. Once we have the ages of rocks, we are able to determine the ages of bones within them.
What we find throughout time are a succession of fossils. Now, this doesnt mean that in the early earth animals were less "complex" or anything like that. It just means, for the most part, they were suitable for their particular environment at a particular time in history.
So, if you go back in time, life starts out, of course as small microbes and things like that, then progresses its way up to very odd organisms. Once the major morphological features of the cambrian explosion are in existence, what we find are fossils that transition over time.
The order they transition is...well it is coherent and makes sense. Essentially we start out with fish (Ordovician), then we get amphibians (Late Devonian), then reptiles (carboniferous), then you get mammals and birds springing out from reptiles in the early to mid Mesozoic (Jurassic and Triassic).
If you think about what an amphibian is, when i hear amphibian i think of frogs. Animals that breathe air but live in water, or certain types of fish that breathe both air and water.
Which, if you think about earth history, fish came first (water) (450 million years ago) , then amphibians (water+land) (400 million years ago), then reptiles (land) (300 million years ago). It intuitively makes sense. And there are a plethora of amphibian fossils that hold the traits of fish and amphibians.
Everyone take notice of the timescale here too, it took a good 150 million years before we get land animals from non land animals. Its a significantly long time.
http://www.talkorigi...rt1a.html#amph1
What we find are these, animals that will have fins and scales like fish, but they will have neck bones and wrist bones like reptiles, enabling them to turn their necks to look around or twist their bodies to turn as if they were on land. What fish has neck bones which allow it to turn to look around? What fish has wrist bones and muscles that allow it to lift its body with its fins? They're transitionals. Adapted for their environments.
Ok, moving on.
So now we have these land critters who like the water.
Then you get these amphibian to reptile transitions. Animals that have traits of reptiles and amphibians. Which isnt unreasonable. Think about the last time you have went to a zoo or aquarium. Whats the difference between a salamander and a lizard? A snake and an eel?
These guys arent too much different, morphologically.
The way, taxonomy in paleontology works is, essentially what you will have is an animal like a fish. And someone will say, ok a fish will be anything with fins, without certain neckbones that has X type of jaw and Y types of organs. And a reptile will be any animal that has Z type of Jaw, W types of neck bones, V types of backbones and skull bones etc.
so, what we use are these "cladistics" to determine what we call an animal. Its like when you program your tv. If X then proceed to step 2, if Y then proceed to step 5. Does it have fins? yes, ok proceed to step two. step two, ok does it have these types of wrist bones? No, ok therefore it is a fish. or therefore it is an amphibian etc. I hope this makes sense, im not the best at explaining.
http://tolweb.org/Te...rtebrates/14952
So, as we go throughout time, we then find these part "mammal like - reptiles" (more reptile than mammal), then later on these reptile like mammals (more mammal than reptile).
http://www.talkorigi...nal/part1b.html
If you look at the link above, you just have this massive number of fossils that are somewhere in between. And this is just a small collection of information. The actual number of these fossils goes up into the hundreds. There are so many, and a lot of them have so many mammal traits and reptile traits, that people arent even sure whether we should be calling them mammal or reptile, because theyre morphologically, both mammals and reptiles. Theyre in between, theyre transitionals.
http://tolweb.org/Synapsida/14845
http://en.wikipedia.org/wiki/Synapsid
http://tolweb.org/Diapsida/14866
http://en.wikipedia.org/wiki/Diapsid
And the same goes for the transition from reptiles to birds. Remember earlier, i mentioned how reptiles split into mammals and birds.
And we start finding fossils like these.
Is it a bird? Well it has sharp teeth and huge claws like a reptile. It has the skull shape of a reptiles, thats strange. And yet it has massive feathery wings. It has hollowed out bones just like modern day birds. So what is it? Again. Reptiles (carboniferous, 300 million years ago), Birds (Late Jurassic, 150 million years ago), ok and whats in between? We get fossils like this guy.
http://tolweb.org/Ar...uromorpha/14916
http://www.talkorigi...nal/part1b.html
http://en.wikipedia....osaurs_to_birds
And, we go on and on.
In todays time we have modern forms of birds and mammals (us).
And you can see the same thing with human evolution as well. A few million years ago, you have these small hominids with small brain capacities and, their neck bones dont allow them to look forward when standing upright, and their hipbones are made as if they should be walking on 4 legs, and they have these stumpy tails and massive canine teeth and...
It really becomes very detailed cladistics. Thats what it all comes down to.
http://tolweb.org/Mammalia/15040
And if anyone is actually playing around with the taxonomy website above, take notice of how many organisms have gone extinct as well.
Anyway, this has been a rough overview of some transitional fossil info. Someone made a comment about something in another thread, so i felt i may as well make a post.
If anyone has any questions, feel free to ask.
, but even humans mutate. Our DNA, sequences proteins and proteins build our morphology. So if DNA mutates, then the proteins it sequences, change, then we subsequently, physically change.
