THE PALEOZOIC ERA This era happened over a 290-million-year time period and started with what's called the Cambrian Explosion. This was when the amount and diversity of life on earth really exploded. It ended with the Permian extinction, which likely destroyed most of those life-forms for all eternity. This era had six distinct periods that we know of currently. There were two types of bacteria living in the millions of years before this eon began. Some were called heterotrophs, which survived on other living things —mostly other bacteria. Others were called autotrophs —they did not need other living things but relied on the sun's energy to get their energy. Autotrophs were so important because they used photosynthesis and made oxygen as a toxic byproduct. At one point, a smaller bacteria must have been eaten by a larger one. The smaller one produced energy and became symbiotic enough to become the energy factories in eukaryotic cells we now call mitochondria. These are all part of nearly every eukaryotic life form and cell today. As cells became more specialized, they formed multicellular organisms that were probably initially tube-shaped water dwellers without any mobility. The Cambrian explosion meant that many new species were formed. Most phyla that existed during that time have since become extinct. You would have seen a lot of arthropods like trilobites, mollusks, brachiopods, and others. You have probably seen many brachiopod fossils, even if you haven't known it. Figure 12 shows a brachiopod fossil:
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