Let's try to think of these numbers in a way that is easier to understand. One day is 86,400 seconds. How long does it take for one million seconds to tick by? --Eleven and a half days. One billion seconds? --Almost thirty-two years.
Geologic time is divided broadly into eras. Eras are further divided into periods and periods into epochs. The units most commonly used are periods, at least for the Paleozoic and Mesozoic eras. Epochs are more commonly used when referring to the Cenozoic era. Epochs for the Paleozoic and Mesozoic are seldom used by the layman. Modern time back to one million years ago is known as the Holocene, but many texts also refer to it as Recent.
For the first three billion years, the highest form of life on earth was single-celled blue-green algae. Then, about 700 million years ago, many multicellular species began to appear and by 500 million years ago, the ancestors of virtually all the major modern groups of invertebrates had appeared.
