Evolution: The Super-Duper Big Stuff

There is nothing inherent in the process of evolution by natural selection that necessarily leads up to a buildup of complexity over time. Indeed, along some branches of the tree of life, there’s likely been very little increase in complexity for billions of years. But along other branches of this grand tree of life, we see trends like an increase in body size, an increase in number of different cell types, an increase in number of protein-coding genes and in increase in total genome size.

Over and over, during what biologists John Maynard Smith and Eors Szathmary have dubbed the major transition in evolution, we see such trends.

My colleague Carl Bergstrom and I discuss these major transitions in detail in our textbook Evolution (WW Norton, 2012), but, in a nutshell, they include:

1. The origin of self-replicating molecules.
2. The origin of the first cells.
3. The emergence of more complex (eukaryotic) cells that include a nucleus and a suite of organelles housed inside a membrane.
4. The evolution of sexual reproduction (from asexual reproduction).
5. The evolution of multicellular organisms from single-celled ancestors.
6. The evolution of germ cells, a specialized line of cells that became gametes.
7. The evolution of groups, including the evolution of extreme sociality, like that seen in some species of bees, ants, and wasps, with their division of labor and sterile castes.

It might seem that each of these major transitions is unique. And indeed some, such as the origin of the genetic code and the evolution of eukaryotic cells, were one-of-a-kind events. Other transitions, such as the evolution of multicellularity and the evolution of group living, have evolved independently numerous times. Regardless of whether a major transition has occurred just once or many times, evolutionary biologists hypothesize that many of the major transitions in evolution involve cooperation between what were formerly competitive entities. There are two ways this tends to happen during major transitions:

Individuals give up the ability to reproduce independently, and they join together to form a larger grouping that shares reproduction. For example, early in the history of life, independently replicating molecules joined together within a membrane to form proto-cells. Later, along numerous branches on the tree of life, unicellular organisms joined together to form multicellular creatures. Solitary individuals started living together in colonial groups, sometimes even giving up the possibility of independent replication, as we see in many species of social insects. In each of these cases, formerly autonomous, competitive units, merged and shared their reproductive fate. Star Trek fans--think Borg.

Once individuals aggregate into higher-level groupings, they can take advantage of the cooperative benefits associated with economies of scale and efficiencies of specialization. Economies of scale come about when groups can perform a task more efficiently than single individuals. Efficiencies of specialization come about because when groups are collectively engaged in a task, they can benefit from a division of labor, allowing different individuals to specialize in different tasks. For example, we see the benefits of both economy of scale and efficiency of specialization when we compare multicellular organisms to their single-celled counterparts.

The big changes--the major transitions in the history of life on our planet--are, in one way or another, about the evolution of cooperation.

Additional reading: Maynard Smith, J., and E. Szathmary. 1997. The Major Transitions in Evolution. Oxford University Press, New York.