Animal Evolution

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Life on this earth simply did not start out with animals. All the 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 a while, we get the cells that make up animals.

All animals and plants are classified as multicellular eukaryotes: their bodies are made up of large numbers of cells, and microscopic inspection of these cells reveals that they contain a nucleus and a number of other organelles. Compared to prokaryotic organisms such as bacteria, plants and animals have a relatively recent evolutionary origin. DNA evidence suggests that the first eukaryotes evolved from prokaryotes, between 2500 and 1000 million years ago. That is, eukaryotes as a taxon date from the Proterozoic Era, the final Era of the Precambrian. Fossils of both simple unicellular and more complex multicellular organisms are found in abundance in rocks from this period of time. In fact, the name "Proterozoic" means "early life".