Earlier in the chapter, we discussed the key biological requirements for complex multicellularity. There is a critical environmental requirement as well: the presence of oxygen. There are other potential electron acceptors for respiration, like sulfate and ferric iron. However, they not only occur in much lower abundances than oxygen, but they are not gases and so do not accumulate in air. Only the oxidation of organic molecules by O₂ provides sufficient energy to support biological communities that include large and active predators like wolves and lions. And only oxygen in concentrations approaching those of the present day can diffuse into the interior cells of large, active organisms. On our planet—and, probably, on all planets—no other molecule is both sufficiently abundant and has the oxidizing power needed to support the biology of large complex organisms.

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On the present-day Earth, lake or ocean bottoms with oxygen levels below about 8% of surface levels support only a limited diversity of animals, and those that they do support tend to be tiny. Large animals with energetic modes of life—carnivores, for example—live only in oxygen-rich environments. The clear restrictions on animal form and diversity observed at lower oxygen levels today have important implications for the distribution of large, active, and diverse animals through time. We noted in Chapter 26 that oxygen first began to accumulate in the atmosphere and surface oceans about 2400 million years ago, but the chemistry of sedimentary rocks deposited after that time tells us that oxygen levels remained low for a very long time. Our modern world of abundant oxygen came to exist only 580–560 million years ago, about the time when fossils first record large multicellular organisms (Fig. 28.14).

Scientists continue to debate the causes of this environmental transformation, but the correspondence in time between oxygen enrichment and the first appearance of large, complex animals (and algae) suggests that more oxygen permitted greater size and more energetic modes of life. Greater size in turn created opportunities for tissue differentiation, leading to transport of nutrients and signaling molecules by bulk flow. The evolution of bulk flow in turn permitted still larger size, setting up a positive feedback that eventually resulted in the complex marine organisms we see today.

With morphological complexity came new functions, including predation on other animals. Protozoan predators capture other microorganisms, but do so one cell at a time. In contrast, animals that obtain food by filtering seawater gather cells by the thousands, and larger animals can eat smaller ones, opening up endless possibilities for evolutionary specialization and, therefore, diversification. Among photosynthetic organisms, multicellular red and green algae were able to establish populations in wave-swept coastlines and other environments where simpler organisms lacked the mechanical strength to hold their position along the shoreline.

The key point is that complex multicellular organisms didn’t succeed by doing the same things as simpler eukaryotes. Complex multicellularity spread through the oceans because it opened up new and unprecedented evolutionary possibilities.