How did we get here?  Why do we look the way we do?  Why do animals look the way they do, and why are there so many different kinds?  We are always trying to solve mysteries about how and where we came from.  To learn the answers, we have to go back in time.  We have to go back to a time before humans walked on Earth and back to a time before dinosaurs walked on earth!  We have to go back to a simpler time.

Think of a slug.  Pretty simple, right?  It has a body, a mouth, and round eyes.  It slides around and eats plants.  That's about it.  Now think of a giraffe.  Not as simple.  It has a long neck, little horns, and a tongue that's as big as your arm.  It has orange spots to keep it hidden from lions.  A giraffe has many different parts that work together and make it much more complex than a slug.  Complexity is when something is made of a lot of different parts.  It is the opposite of simple.  This is a good word to remember when we look back at how simple living things used to be and how they changed into what they are today.

A computer microcircuit has high complexity.
The original uploader was Luestling at German Wikipedia., Public domain, via Wikimedia Commons


When life began on Earth, all the living things were very simple.  Most of them were only one cell and were so small you would not have been able to see them with just your eyes.  They stayed like this for a very long time, more than 3 billion years in fact.  Then, about 545 million years ago, things changed.  Complexity, that lovely word you just learned, came very quickly to the animals of the Earth.  It was almost as fast as lightning.  Okay, it didn't happen that quickly, but based on how old the Earth is, 5-10 million years is not very long.  The Cambrian Explosion was a time when many of the organisms we know today evolved from simple living things.  These living things had many cells and could do a lot more things, like run, swim, and bite.  This was a big change for the world.  Think of having only three colors of crayons and then suddenly having thousands of pens, pencils, and paints to choose from.  It's kind of like that.

This has happened many times in the Earth's history.  Something may cause many animals to die off, and millions of years later, many new living things may appear.  So what happened after all these new animals appeared?  A lot of the simple animals died.  It's sad but true.  The animals that lived were stronger and had armor.  An exoskeleton is hard bone-like stuff on the outside of the body like a shell or the outside of a bug.  Many animals have exoskeletons, such as crabs or ants.  With this "armor" the new animals would not get sick as easily, and it was harder for hungry things to eat them.

An exoskeleton can come in very handy when you look this good to seagulls.
U.S. Fish and Wildlife Service Headquarters, Public domain, via Wikimedia Commons


I do not need to tell you that you do not wear your bones on the outside like a crab or an ant.  Our skin is soft, and though it is strong, it's still not as strong a shell.  You have an endoskeleton, which means your bones are on the inside.  So why if having armor keeps you safe from sickness and animals that want to eat you, do we have our bones on the inside?  For one, bones are stronger than shells.  They will not break as easily.  Your ribs keep your heart, stomach and other parts safe.  Bones also help us run and move around in many different ways.  Animals with shells cannot move as quickly.  The best part is that our bones let our muscles grow bigger because they are not held in by a shell.
All animals with endoskeletons, please go to my left.
Sklmsta, CC0, via Wikimedia Commons


Fossils lead us to believe that the Earth had a big explosion of life that brought us the many animals we know today.  This is not certain.  It's only a theory or a way of explaining something.  Many questions are still up in the air.  We'll keep trying to figure out how life as we know it came to be and whether it was as fast as lightning.

References:

Biology Introduction to Animals and Plants. "Arthropods." FMCC, 2005. <http://faculty.fmcc.suny.edu/mcdarby/animals&plantsbook/animals/07-Arthropods.htm>

Fossil Museum. "Cambrian Explosion."  Fossil Museum, 2009. <http://www.fossilmuseum.net/Paleobiology/CambrianExplosion.htm>

WGBH Evolution. "The Cambrian Explosion."  PBS, 2001. <http://www.pbs.org/wgbh/evolution/library/03/4/l_034_02.html>