Fifth.

Apologies for the absence, a few projects cropped up that needed my attention. This blog will be looking at how fluctuations in atmospheric oxygen levels may have had an influence on evolution, which caught my attention thanks to an article written by Berner and his colleagues (2009):

 Throughout history there have been fluctuations in global oxygen levels, whose cause can be attributed to tectonic activity capturing carbon dioxide into underground carbon stores, volcanism spewing out a cocktail of gases into the atmosphere and, perhaps fundamentally, the evolution of photosynthesising organisms, who produce oxygen as a by product. Thus, organisms have to adapt accordingly, in order to maintain and regulate their environment in what is known as the 'Gaia Hypothesis'. 

 The above graph comes from Berner et al's (2009) article, showing atmospheric oxygen levels during the Phanerozoic, noting that several extinction events coincide with decreasing oxygen levels, such as at intervals 5 (Devonian), 9 (Permain-Triassic) and 11 (Triassic-Jurassic) on the graph. Berner (2009) denotes four major events relating to evolution and oxygen:

1. The origin of the first animal body plans coincides with an increase in oxygen (interval 1).
2. Two independent phases of high oxygen levels relate to the conquest of animals on to land, interval 4 showing the first of these conquests, mainly achieved by arthropods, and interval 7, which included the vertebrates.
3. The all time oxygen high during the Carboniferous and Permain (8) may have caused gigantism in several arthropod groups including the dragonflies and millipedes, whilst an overall increase in body size is seen in many vertebrate groups. 
4. Interval 12 may have caused the increase in mammalian body size during the tertiary.

However, the mechanisms of these points are unclear, however the presence of high oxygen levels is seen to influence size in trout (Dabrowski, 2004), and alligators (VandenBrooks, 2007).

The decrease in atmospheric oxygen during the late Triassic (interval 10), coincides with the rapid diversification of dinosaurs. This may have been due to a novel mechanism used by these organisms to extract oxygen in low oxygen conditions, and is retained by birds today due to its usefulness during high altitude flight. Known as the pulmonary air sac system, fossilised dinosaur bones are seen to have air filled spaces, where the bone has been invaded by the aforementioned system (O'Connor, 2005). Essentially, these air sacs means that inhaled air in effectively passed into the lungs twice via a bellow system, extracting as much oxygen as possible from one breath. Thus, dinosaurs had a respiratory advantage compared to other terrestrial organisms at the time, causing their rise and diversification as dominant land animal for 150 million years. 

Berner et al, 2009- http://webh01.ua.ac.be/funmorph/raoul/fylsyst/Berner2006.pdf

K. Dabrowski, K. J. Lee, L. Guz, V. Verlhac, J. Gabaudan, Aquaculture 233,383 (2004). 

J. M. VandenBrooks, thesis, Yale University (2007). 

Patrick M. O'Connor & Leon P. A. M. Claessens (2005) Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs Nature 436, 253-256

http://michigantoday.umich.edu/2008/10/aero1-lg.jpg