For about three billion years, unicellular organisms dominated Earth until roughly one billion years ago when persistent cell-grouping enabled evolution of complex animals, plants and fungi. Multicellularity has arisen at least forty times across life, while animals appear to have become multicellular only once. Key multicellular functions include cell adhesion, molecular signaling and coordinated gene regulation that drives cellular specialization and spatial organization. Many unicellular relatives of animals express proteins involved in these functions, indicating that the molecular toolkit for multicellularity predated the first animals. Multiple unicellular eukaryotic lineages, including choanoflagellates, filastereans, ichthyosporeans and corallochytreans, can serve as models.
The prevailing view held that a flood of genes had to evolve to enable the key properties of multicellularity: the ability of cells to stick together, communication using molecular signals and the coordinated regulation of gene expression that causes each cell to specialize and take its position in the organism. But studies found that some unicellular organisms express a slew of proteins that control key properties of multicellularity in animals,.
For some three billion years, unicellular organisms ruled Earth. Then, around one billion years ago, a new chapter of life began. Early attempts at team living began to stick, paving the way for the evolution of complex organisms, including animals, plants and fungi. Across all known life, the move to multicellularity happened at least 40 times, suggests one study. But, in animals, it seems to have occurred only once.
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