Wonderful essay in Nature
by Olivia Judson, laying out the broad outlines of Earth's history:
The history of the life–Earth system can be divided into five ‘energetic’ epochs, each featuring the evolution of life forms that can exploit a new source of energy. These sources are: geochemical energy, sunlight, oxygen, flesh and fire. The first two were present at the start, but oxygen, flesh and fire are all consequences of evolutionary events. Since no category of energy source has disappeared, this has, over time, resulted in an expanding realm of the sources of energy available to living organisms and a concomitant increase in the diversity and complexity of ecosystems. These energy expansions have also mediated the transformation of key aspects of the planetary environment, which have in turn mediated the future course of evolutionary change. Using energy as a lens thus illuminates patterns in the entwined histories of life and Earth, and may also provide a framework for considering the potential trajectories of life–planet systems elsewhere.
It's a very clear and readable essay and I love the model it develops. Plus it has lots of fascinating details about each of the five epochs:
On the geological side, the flourishing of animals had at least four major impacts. First, the evolution of predation rapidly led to the evolution of armour—shells, scales, spikes and carapaces built from materials such as calcite and silica. Although, as noted above, the first protective coverings (on algae) date back to around 770 Ma, it's not until the evolution of flesh-eating animals that shells and other forms of protection became widespread. This development would eventually result in vast deposits of materials such as radiolarite, limestone, coquina and chalk and would also produce changes in ocean chemistry, as organisms removed dissolved materials such as silica and calcium and used it for themselves.
Second, animals produce faeces, which have important effects on the way that nutrients are distributed around the globe. For example, in the ocean, zooplankton faecal pellets sink more rapidly than individual algal or bacterial cells, and thus transport organic matter from the surface to the seabed. Today, the faeces of sperm whales bring iron from the deep sea to the ocean surface; the faeces of birds like cormorants transport nutrients from the ocean onto land, sometimes in fantastic quantities.
Highly recommended for anyone interested in science.
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