Life on Earth didn't start out with animals. All life on earth started from the primordial ooze, where inorganic molecules produced organic molecules, organic molecules formed bigger molecules, and, eventually organic molecules hung together inside membranes. That led to cell-like structures and then cells. Simpler cells led to cells that are more complex. Eventually, and it did take a while, we would up with cells that make up animals.

Animals are made up of eukaryotic cells, which have a nucleus and other membrane-bound organelles. Each organelle handles a specific function for the cell.

Eukaryotes evolved from prokaryotic ancestors. The evolution of eukaryotes from prokaryotes meant a tremendous increase in complexity. A eukaryotic cell can multitask much better than a prokaryote because of all its separate parts.

Evolution is the basis for the diversity of life. Evolution is the process of change that has occurred to life over time. Life on Earth hasn't always been the same. Organisms change and organisms come and go. It keeps things interesting.

The oldest fossils of multi-cellular animals we've found are over 600 million years old, so it took a little while to get where we are now. The eon covering the time from when animals really get going until now is called the Phanerozoic eon (the era of "well-defined" or apparent "life"). Below are some highlights.

Era	Period	What's Happening
Paleozoic	Cambrian 543–490 million years ago	It's warm and wet most places, and something called the Cambrian Explosion happens. This is a couple-million-year period where there was a huge increase in the number and kinds of animals. No one quite knows why it happened, but of course there are a few ideas. All animals live in the water at this point. All of the major types of marine invertebrates that now exist appeared at this time, as did the forebears of some current groups of terrestrial invertebrates and marine vertebrates. Many other groups appear as well, but not everyone gets to stick around.
	Ordovician 490–443 million years ago	It's still warm and wet and a few organisms take to land, including some plants and the first animals on land, the arthropods. Near the end of the period, it gets very cold and many marine invertebrates go extinct.
	Silurian 443–417 million years ago	Glaciers that formed at the end of the last period do some melting and the oceans rise. Some new vertebrates show up and animal diversity on land increases.
	Devonian 417–354 million years ago	The first forests are forming and land animals continue to diversify. Tetrapods appear (animals with four limbs). Fish really diversify. Near the end of the period, we get another major extinction event.
	Carboniferous 354–290 million years ago	This is a cooler period, and animals and plants continue to diversify. Some insects grow wings and fly. Reptiles show up.
	Permian 290–248 million years ago	Most of the land on Earth is one big mass called Pangaea. (Yep, even the land changes over time.) Reptiles become the dominant vertebrates and mammal-like reptiles appear.
Mesozoic	Triassic 248–206 million years ago	We get the first dinosaurs and the first mammals. Global warming at the end of the period leads to yet another round of extinctions.
	Jurassic 206–144 million years ago	Dinosaurs dominate (you've seen the movie). The first known bird appears.
	Cretaceous 144–65 million years ago	Flowering plants appear, which is a boon for insects and other animals that adapt to eat them. Another mass extinction occurs before the current era. This is the infamous end of the dinosaurs.
Cenozoic	Tertiary 65–1.8 million years ago	Once dinosaurs disappear, mammals take over the valuable resources and quickly, in geologic time, diversify. Towards the end of this period, hominids, including our ancestors, appear.
	Quaternary 1.8 million years ago until now	Humans dominate. There are extinctions of large mammals. A few ice ages occur. We'll have to wait and see what's next.

There are some general trends here.

• Life started in water and spread to land later.
• When the climate changes drastically, we see big extinction events and the emergence of new species.
• When one group of animals goes extinct, another will come and take its place. Resources don't go unused for long.

Brain Snack

Why did the Cambrian Explosion occur? If you find out, let us know. In the meantime, here are some theories.