Multicellularity
Archaeans, bacteria, and eukaryotes continued to diversify and to become more sophisticated and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archaea and bacteria. Around 1.1 billion years ago, the supercontinent Rodinia was assembling. The plant, animal, and fungi lines had all split, though they still existed as solitary cells. Some of these lived in colonies, and gradually some division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1 billion years ago, the first multicellular plants emerged, probably green algae. Possibly by around 900 million years ago, true multicellularity had also evolved in animals. At first it probably somewhat resembled that of today’s sponges, where all cells were totipotent and a disrupted organism could reassemble itself. As the division of labor became more complete in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die. Many scientists believe that a very severe ice age began around 770 million years ago, so severe that the surface of all the oceans completely froze (Snowball Earth). Eventually, after 20 million years, enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures. By around the same time, 750 million years ago, Rodinia began to break up.
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