Chapter 6

Major Transitions

Synopsis

The tree of life represents everything that we know about the past and the present of life on Earth. The evolution of diverse life forms can be explained by mutation, drift, and natural selection. Even though such population level changes are responsible for the origin of species, the complexity of biological forms has also undergone a dramatic increase throughout the history of our planet. The increase in complexity of life is explained by a series of ‘major transition events’ – i.e. events that shifted the way in which the (biological) information transfer has shifted from a lower-level to a higher-level unit (e.g. origin of protocells, eukaryotes, multicellularity). Furthermore, during the history of life, there were “major” evolutionary innovations that also dramatically altered the way in which the biomass produced on the planet (e.g. the innovation of oxygenic photosynthesis). In this chapter, we explore such ‘major transition’ and ‘innovation’ events in evolution. Here we restrict our focus on four transitions: origin of the last universal common ancestor (LUCA), innovation of oxygenic photosynthesis, origin of eukaryotes, and transition to multicellularity. This selection is based on two observations. First, from the perspective of historical contingency and ecological constraints, the relationship between each of these subsequent event is more obvious - e.g. oxygenic photosynthesis promoted niches for aerobic eukaryotes, and complex multicellularity evolved only from within the eukaryotic lineages. Second, these events had primarily altered the planet by changing its geochemical composition and biomass distribution.

Specifically, we first look at the evolutionary transition from protocells to the last universal common ancestor (3.8-3.5 Ga) - from which all known life forms originated. Second, we explore the innovation of oxygenic photosynthesis (2.4 Ga?). Oxygenic photosynthesis not only led to the oxygenation of the oceans and the atmosphere, but it also altered the subsequent evolution of life on the planet itself. For instance, changes in the oxygen levels permitted the ecological success of eukaryotic cells, and much later, it has permitted the evolutionary radiation of complex multicellular organisms (e.g. animals and plants). Following the oxygenic photosynthesis, next, we move onto the origin of complex (eukaryotic) cells (~2.2-1.6 Ga). The origin of eukaryotic cells was the result of an inter-kingdom symbiosis event: archeal and alpha-proteobacterial cells merged and this symbiotic relationship evolved into the most recent eukaryotic common ancestor (i.e. LECA). The evolution of these complex cells led to the next transition event, i.e. the evolution of complex multicellularity. Multicellularity has led to an unprecedented increase in biomass production. Moreover, the evolution of complex multicellularity paved the way for the evolution of more complex biological organizations, such as eusociality in insects or large social organizations seen in human species, which we discuss very briefly in the end.