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Topics in Depth

The Theme of Animal Evolution in Animal Evolution and Diversity

Life on Earth did not 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 hang together inside membranes. This leads to cell-like structures and then cells. Simpler cells lead to more complex cells. Eventually, and it did take awhile, we get the cells that make up animals.

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. 



What's Happening



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.


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.


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.


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.


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.


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.



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.


206-144 million years ago

Dinosaurs dominate (you've seen the movie). The first known bird appears.


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.



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.


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.

Brain Snack

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

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