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1/ The peak of Everest is infamously termed 'Death Zone'. Each breath contains ~ 30% of the oxygen found at the sea level.
Birds, however, can easily fly over the Everest. What gives them such great lungs? They got them from the dinos. But why did the dinosaurs have them?

2/ All mammals, including us humans, breathe in and out through the same opening (unlike our digestive tracts). Birds however, have a separate in and out point.
They also have air sacs and hollow spaces in their bones that helps them reduce their overall weight.
3/ When they breathe in, half of the oxygen goes into these hollow spaces, and the other half goes into the lungs. When they breathe out, the good air that has been stored in the hollow places now also goes into their lungs, and the bad air (CO2 and water vapor) is pushed out.
4/ So it doesn’t matter whether birds are breathing in or out: Good air is always going in one direction through their lungs, pushing all the bad air out ahead of it.
To understand the reason behind this great respiratory system, we need to understand their evolution.
5/ And since birds evolved from those giant dinosaurs, we need to understand why the dinosaurs had this super lung system in the first place?
And thus begins a great tale of evolution.
6/ Until about 450 mil yrs ago, plants only existed in the oceans and the Earth's surface was life less. Under water, plants didn't have to worry about gravity, they could just float. But on land, gravity pulled them down to the surface - reducing the area to absorb sunlight.
7/ So with (lots of) time, evolution gifted these plants with lignin. The stuff that lends rigidity to the plant cell walls, and helps them stack one cell upon another.
This allowed the plants to develop the trunks and branches to defy gravity and stand upright.
8/ But as we know, it is very difficult to predict the second-order effects of anything. As these plants died, the bacteria and fungi started decomposing/eating them - but they couldn't digest the hard part – lignin.
So they just ate the meat and left the bones (lignin).
9/ Eventually, another tree would die and fall on top of the first, and this continued for millions of years. All this lead to a huge heap of undigested lignin deposited under the surface of the Earth. The stuff that humans would later refer to as "coal".
10/ Now we remember from our chemistry lessons that oxygen helps in the combustion of anything. But here we had this huge heap of lignin that was not getting decomposed. So for every hydrocarbon atom that went in, an oxygen atom remained unused.
11/ All this unused massive amounts of O2 then started piling up in the atmosphere. Today, oxygen is 21% of the atmosphere. But around 300 million years ago, oxygen was well over 30%. There was so much oxygen that many creatures like dragonflies and frogs were able to get huge.
12/ But evolution is not biased to the survival of any one specie. So while all this oxygen and lignin was building up, the bacteria and fungi finally evolved a chemical that could dissolve lignin and break it into smaller parts. Chemical X.
13/ The bacteria could now eat up all the wood that was not fossilized already, using up the atmospheric oxygen in the process. The atmospheric O2 levels began to decline rapidly. It went from a high of above 30% to around 12% at the end of the Permian Period (250 mil years ago).
14/ Meanwhile the life on Earth had accustomed itself to the abnormally high oxygen levels. Oxygen depletion was bad news for them. ~95% of all life on Earth died gasping for air. However the remaining 5% got a little help from our old friend, evolution.
15/ To cope up with reduced O2 levels, these creatures came up with complicated lung systems with both an entry and exit points, hollow bones and air sacs to temporarily store oxygenated air. One of the animals who got benefited from this new design were – dinosaurs.
16/ Their new lungs were so efficient that when O2 levels crept slowly back up to 20% over the next many millions of years, dinosaurs were able to get massive in size. Their super efficient lungs could deliver oxygen to every part of their huge bodies.
17/ The birds later borrowed these respiratory systems from their ancestors, and that's why bar-headed geese can fly over the Himalayas!

I learned all this from this amazingly well written article below. Please read if you enjoyed the thread!
Just like with everything Science, there's nuance and a lot to be discovered with this as well. Some theories do suggest that higher O2 levels might not be the cause of gigantic animals after all.

h/t @Elsie_youlater

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