how did colonial organisms form after the eucaryotic cells?
Colonial organisms are formed with organisms of same species in order to form mutual groups. After the evolution of the eukaryotes on earth (~3 billion years ago), they need to have sources of energy in the form of chemicals present on earth in those times. The current condition of the earth is very different form the past, which means, life is not easy to thrive in those particular environments.
Scientists believed that colonial organisms happened under cooler environments such us under the rocks and soil. They try forming colonies in order to reduce death. It is because the outer environment might be harmful or toxic to their metabolism.