I’ve remarked in these pages before that there appears to be a bottleneck in biological evolution, that is, true multi-cellular tissue architecture organisms did not evolve until the Precambrian Explosion about a 500 Myr ago. (You know, the Burgess Shales and all that). Prior to that, the only known semi-specialized colonial organisms were the stromatolites, and they are more akin to sponges (a transitional form?), not really what you would call a precursor to a true metazoan, like say a Cnidarian.
I’m particular interested in this result because it has severe implications for the distribution of intelligent life in the cosmos. However, just today I stumbled on this abstract that suggests that perhaps I was overly pessimistic.
Sometimes its good to realize that maybe you’re wrong about something. I feel better already.
Kent C. Condie, in Earth as an Evolving Planetary System (Third Edition), 2016
Origin of Metazoans
Metazoans (multicellular animals) appear to have evolved from single-celled ancestors that developed a colonial habit. The adaptive value of a multicellular way of life relates chiefly to increases in size and the specialization of cells for different functions. For instance, more suspended food settles on a large organism than on a smaller one. Since all cells do not receive the same input of food, food must be shared among cells and a “division of labor” develops among cells. Some concentrate on food gathering, others on reproduction, while still others specialize in protection. At some point in time when intercellular communication was well developed, cells no longer functioned as a colony of individuals but as an integrated organism.
The trace fossil record suggests that metazoans were well established by 1000 Ma (Figure 9.10), and the great diversity of metazoans of this age suggests that more than one evolutionary line led to multicellular development. Leaf-shaped fossils in North China suggest that some form of multicellular life had evolved by 1.7 Ga (Shixing & Huineng, 1995). Based on their size (5-30 mm long), probable development of organs, and possible multicellular structures, these forms are likely benthic multicellular algae (Figure 9.11). Recently discovered centimeter-sized structures from 2.1-Ga black shales from West Africa are interpreted as colonial organisms, also probably metazoans (El Albani et al., 2010). Although metazoans appeared by about 2 Ga, because of an inadequate fossil record we cannot yet trace these organisms back to their unicellular ancestors.