Animals are made up of eukaryotic cells, which have a nucleus and other organs enclosed by a membrane. Each organ handles a specific function for the whole cell.
Eukaryotes evolved from prokaryotic ancestors. The evolution of eukaryotes from prokaryotes meant a tremendous increase in complexity. A eukaryotic cell can do a lot more at the same time with all its separate parts. It's a big multitasker.
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 has not 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 era (the era of "well-defined" or apparent "life"). Below are some highlights.
|Paleozoic||Cambrian 543 -490 million years ago||It's warm and wet most places and an event called the Cambrian Explosion happens where a huge increase in the number and kinds of animals occurs. No one quite knows why, 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. Lots of 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 get 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 famous end of the dinosaurs.|
|Cenozoic||Tertiary 65-1.8 million years ago||Once dinosaurs disappear, mammals take over their spots 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 also the emergence of new species.
- When one group of animals goes extinct, someone else will generally come and take its place. Resources do not go unused for long.