But the electron microscope showed that plant and animal cells are virtually identical, except for the presence or absence of chloroplasts. Neither one could be the forebearer of the other.
Bacteria contain no organelles.
Endlessly shifting evolutionary pressures reshuffled collections of cells into organisms of every conceivable size and shape. Every fundamental trend eventually runs up against an equally potent counterforce. In the case of the trend toward greater complexity, the constraint was the cost of coordination. Within the limits imposed by a fixed quantity of resources, 3.5 billion years of evolution came up with organic designs that struck workable compromises between the benefits and costs of size and complexity. In business, the drive for greater efficiency and the attempt to avoid the intense competition in crowded niches also propels a spontaneous evolution of larger and more complex organizations.