Topic 3 - Cambrian Explosion

Major Evolutionary Transitions

Evolution of cells (specifically eukaryotic cells): precursors to life would’ve already been present in the Precambrian, meaning fundamental processes of DNA replication, protein synthesis, respiration, cellular division, etc, all would have emerged prior to the first evidence of cells.

Evolution of multicellularity (cells working together): no single emergence of multicellularity, we actually have a lot of convergent evolution in a lot of different lines of our evolutionary tree. This means this multicellularity evolved multiple times in different lineages. There are 2 hypothesis for this:

• Coming together hypothesis (free living unicellular cells that were not necessarily genetically related that were moving bumping into each other which led to a joining)

• Staying together hypothesis (cells in an ancestral lineage would have split but remained together after cellular replication, as we assume they have some sort of biological fitness advantage)

Evolution of individuality: when all these cells come together and become an integrated whole (fundamental life). To become an individual organisms, we have to see specialization of different cell parts (therefore losing their totipotency). We think that these cells came together and formed multicellularity aggregations (change of mutation) which allowed them to become specialized.

Evolution of group - living: This requires some degree of sociality (basic communication and coordination between individuals). This created a lot of complex behaviors in these groups, which formed benefits such as foraging and avoiding predators. However, there was a problem, the evolution of cheating was formed, which lead to the evolution of consequences. There was also a competition for resources, increased risk of pathogens and parasites.

Major Innovations/Precursors in the Precambrian

Increase in disparity (range of morphological differences observed between different species): the radiation of bilateral symmetry was driven by a lot of innovations in foraging behavior which allowed many niches to be occupied.

Increase in diversity (number of different species that we see): is a result of the interplay of the combinatorial libertarian development system (start out simple and go to complex) and an increase in the number of needs that the first bilaterians had to meet as complex ecological interactions developed.

Time onset: the onset would’ve occurred before the explosion, meaning it would’ve occurred with the ediacaran fauna. We don’t see this explosion earlier because of something going on in the environment.

Duration: considering that massive diversity and disparity were occurring, this was not actually a long period of time. The duration was primarily driven by the developmental radiation/innovation (how quickly new things could develop).

Uniqueness: we have neve seen another event like this. The uniqueness is a consequence of developmental entrenchment (crucial role of the environment for some developmental systems and development itself) and ecological niche saturation (ability for organisms to expand out and exploit niches).

Theories for the Cambrian Explosion

Changes in the abiotic environment: 3 separate ideas:

• Complex life needed an environment that allowed it to form (ozone layer, which was already there, it just needed time to reach sufficient oxygen levels).

• Snowball Earth explains that there was a massive environmental shift that prompted some evolutionary innovation for the ediacaran fauna and then to the Cambrian

• Negative carbon isotopic anomaly suggests that there was some sort of environmental disturbances going on at the time which killed the ediacaran fauna and opening up niches to be filled.

Changes in the biotic environment: There was an evolution arm’s race. When there’s lots of niches to fill, you can have a lot of evolution, but once these niches are filled, it becomes more difficult to find a structure/body that is more successful (competition).

Changes in the genetic or developmental capacity: Organisms cannot evolve if the genes responsible for specific traits/components don’t exist. It is suggested that the origins of bilateral symmetry along with the evolution of the HOX genes are primarily responsible for the explosion. Changes in the way that genes are themselves are wired could have been a major source of morphological modification, which can lead to significant ranges of gene expression.

The Light Switch Theory

A simple explanation for the explosion of life. It is the evolution of visions (ability to form an image on a retina or in some way). Being able to detect movement allows organisms to hunt and to be hunted, allowing more niches to be open to organisms. This may have triggered an evolutionary arms race between predator and prey, which would’ve caused a need for evolutionary innovation in terms of body forms and morphological adaptations.