Evolution

Why Birds Can Fly Over Mount Everest

uses a captivating narrative style to elaborate on a complex question. The exposition makes the scientific parts memorable.

features a good anecdote of insight favoring the prepared. On a jog in London in January, probably primed by reading Nick Lane’s Oxygen, he noticed that there were some bar-headed geese. These geese migrate between India and Kazakhstan/Mongolia, necessitating a flight over the Himalayas, sometimes at 28,000 feet.

Bar-headed geese, like all birds, have super-efficient lungs with two pathways. They also have airsacs and hollow spaces in their bones. When they breathe in, half of the oxygen-rich air goes into these spaces, and the other half goes into the lungs through the rear entrance. When they breathe out, the oxygen-rich air in the hollow spaces now also goes into the lungs via the rear entrance, while the oxygen-deprived (carbon-dioxide and water vapor laden) air is pushed out the front exit.

Time to update my Biology knowledge from high school days. notes that Aves have been folded into the Dinosauria category.

To deal with gravity in the atmosphere, plants evolved to produce lignin, a rigid molecule of carbon and hydrogen, which enabled them to grow vertically.

However, in the decomposition phase, bacteria and fungi were unable to digest the lignin, leading to 100m years of undigested lignin deposits (Carboniferous Period).

For every hydrocarbon trapped in lignin, an oxygen atom is not used in the decay process. 300m years ago, oxygen was \(\ge 30\%\) of the atmosphere. Even insects, who have inefficient respiratory systems, were able to get huge. Too much oxygen is bad because the atmosphere would be very flammable.

In the Permian Period, some micro-organisms evolved to excrete ligase, an enzyme that used oxygen to break down lignin into \(CO_2\) and \(H_{2}O\). Oxygen levels fell to \(\le 12\%\). \(95\%\) of life on Earth died (largest extinction event).

Oxidation of hydrocarbons into \(CO_2\) and \(H_{2}O\) is quite common, e.g. burning wood (very rapid oxidation), digesting food (moderate oxidation), decaying of organic matter (slow oxidation).

5m years later, dinosaurs evolved a lung system with both an entry point and an exit point. As oxygen levels crept back up, dinosaurs were able to get very big.

ties in to us burning the fossilized lignin (which has turned into fossil fuels) at a rate that releases \(CO_2\) \(500k\) times faster than it was deposited in the first place.

As much as dinosaurs were massive, the blue whale is considered the largest animal ever to have lived (33.6m long, +190 tonnes). Best estimates of the largest dinosaurs are 40m by 20m, and 77 tonnes. .

So while dinosaurs could have been larger in end-to-end dimensions, blue whales are more massive.

References

Walter Murch's Letter to His Granddaughter About Earth’s History: Why Birds Can Fly Over Mount Everest. Walter Murch. nautil.us . news.ycombinator.com . Jun 24, 2020. Accessed Oct 12, 2021.
Largest Organisms. en.wikipedia.org . en.wikipedia.org . Accessed Oct 12, 2021.