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nebularain
2005-Sep-30, 11:19 AM
Study: Oxygen helped mammals grow

Thursday, September 29, 2005; Posted: 2:31 p.m. EDT (18:31 GMT)

WASHINGTON (AP) -- Mammals, once tiny creatures scampering on the forest floor, grew larger as the amount of oxygen in the air increased over millions of years, a new study says.

(snip)

They found that the air contained only about 10 percent oxygen at the time of the dinosaurs.

By 50 million years ago the oxygen level had risen to 17 percent and it was 23 percent 40 million years ago, they reported. Currently the air contains about 21 percent oxygen.

The rise of oxygen "almost certainly contributed to evolution of large animals," the researchers reported. The oxygen needs of mammals and birds are three to six times as high as reptiles. . . .

For full article: Click here (http://www.cnn.com/2005/TECH/science/09/29/oxygen.mammals.ap/index.html)


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I was reading a book called Oxygen (forgot the author's name), and in it he speculated that it could have been the increase in oxygen levels that was the real death-nell for the dinosaurs.

That's what made this article stand out to me.

Halcyon Dayz
2005-Sep-30, 12:27 PM
... in it he speculated that it could have been the increase in oxygen levels that was the real death-nell for the dinosaurs.What mechanism is he proposing? I don't see how there could be to much oxygen.

Donnie B.
2005-Sep-30, 01:05 PM
Ever tried hyperventilating? *

Too much oxygen is poisonous. An early symptom is a form of intoxication.

* Don't, especially if you're standing up and/or you value your brain cells. (From one who was once too foolish to heed that advice.)

grant hutchison
2005-Sep-30, 01:53 PM
Well ....
Oxygen is indeed bad for you, but only if you breathe it at a couple of atmospheres' pressure - not something you can achieve by simple hyperventilation.
Hyperventilation is bad for you because it reduces the blood supply to your brain (a reflex driven by low carbon dioxide levels). So you actually pass out from *lack of oxygen* in your brain cells.

Grant Hutchison

Halcyon Dayz
2005-Sep-30, 02:01 PM
That's my point.
If there is more oxygen in the atmosphere than you need, just breath less.

Donnie B.
2005-Sep-30, 03:16 PM
But (IIRC) breathing is regulated by the amount of carbon dioxide in your blood, not by the amount of oxygen.

Of course, if the atmospheric changes occur slowly enough, one would expect most (if not all) air-breathers to be able to adapt.

Swift
2005-Sep-30, 03:40 PM
A guess as to how too much oxygen could be bad for something - more oxygen is great for increased metabolism. The problem is that it also generates a lot of oxidative damage to such things as DNA. Animals (just mammals?) have a variety of means to control these oxidizing species. If other animals didn't have these, maybe this damage did them in.

However, if I understand what they are saying, the higher oxygen didn't kill off the dinosaurs, the theory is that they were still killed off by the meteor strike. Their demise allowed mammals to become the dominant animal on the planet and the higher oxygen levels allowed mammals to become big.

Nowhere Man
2005-Sep-30, 04:34 PM
Study: Oxygen helped mammals grow
...
They found that the air contained only about 10 percent oxygen at the time of the dinosaurs.

By 50 million years ago the oxygen level had risen to 17 percent and it was 23 percent 40 million years ago, they reported. Currently the air contains about 21 percent oxygen.

The rise of oxygen "almost certainly contributed to evolution of large animals," the researchers reported.
I think the larger dinosaurs (like brontosaurus and t rex) might have something to say about this.

Fred

nebularain
2005-Oct-01, 02:02 PM
My memory on the book is a bit fuzzy, but this kind-of hits on it:


A guess as to how too much oxygen could be bad for something - more oxygen is great for increased metabolism. The problem is that it also generates a lot of oxidative damage to such things as DNA. Animals (just mammals?) have a variety of means to control these oxidizing species. If other animals didn't have these, maybe this damage did them in.


I know the meteor strike is the popular theory. The author disagreed with it - I think it had something to do with the time frame - that the death of the dinosaurs occured over a long time (what was it, centuries or thousands of years? sheesh, some memory I have!). This was an alternative theory.

Just interestin stuff to chew on and speculate over and discuss, you know?

Maddad
2005-Oct-02, 12:34 AM
http://www.geocities.com/davidmadison01/coll700.htm
I wrote a paper in college debunking a minor theory of dinosaur extinction tied to a decrease in oxygen.

nebularain
2005-Oct-02, 10:47 AM
That's very good!

But the question was whether or not an increase in oxygen toxified their bodies or not.

grant hutchison
2005-Oct-02, 11:09 AM
But the question was whether or not an increase in oxygen toxified their bodies or not.You'd have to wonder if there's the slightest evidence available to answer this question, one way or the other.
Do we have dinosaur DNA samples in good enough condition to detect and identify free-radical damage, which we can compare across time as the oxygen partial pressure increased? Is it even theoretically possible that such samples might exist?
I suspect the answer is "no" in both cases.

Grant Hutchison

hammo1j
2005-Oct-02, 11:28 AM
Where did the extra concentration of O2 come from?

It can't have been from C02 since plants had been around for a long time or was it C02 absorbed in the ocean escaped and was converted to oxygen by plants?

Donnie B.
2005-Oct-02, 01:28 PM
Free oxygen in the atmosphere is an entirely dynamic system. If no new O2 were being produced, the atmospheric concentration would quickly fall to virtually nil.

So at any time, the level of atmospheric O2 is an equilibrium between the rate of production and the rate it's taken up in various oxidation processes. Both the production rate (which is almost entirely due to the amount of photosynthesis going on) and the absorption rate can vary. If either changes to a new steady state, the O2 concentration will take a new equilibrium value.

What can affect the production rate? Lots of things: the average global temperature, the amount of ice cover, amount of desertification, and relative size of shallow seas (or shallow areas of deep seas). The insolation rate is pertinent too, as well as the planet's axial tilt in the epoch in question. Generally, more plant biomass yields more O2 production.

There are environmental effects on the takeup side of the equation too, including some feedback mechanisms -- for example, if O2 concentration gets very high, land areas are likely to suffer massive fires, which consume oxygen and destroy plant material (leading to a lower production rate).

TheBlackCat
2005-Oct-02, 07:34 PM
This ties into something I read before. Some people think that around the time of the End Permian Mass Extinction there was a significant drop in oxygen concentrations, and that dinosaurs evolved as a response to this low oxygen level. That doesn't mean dinosaurs died off from high oxygen levels, but it might mean the atmospheric chemistry could have an impact on what species take over after a mass extinction screws up Earth's ecology.

publiusr
2005-Oct-05, 05:20 PM
Now I had thought that there was more oxygen in ages past, and that explained the giant dragonflies. You had the KT impact burn off a lot of oxygen in the combustion of plants, and as the CO2 was converted back to Oxygen.

The mammals grew as the oxygen content increased over time.

Maddad
2005-Oct-05, 11:33 PM
The KT burnoff was too short an event to affect evolution from an oxygen viewpoint. It would have been replaced too quickly.

eburacum45
2005-Oct-06, 07:44 AM
Free oxygen in the atmosphere is an entirely dynamic system. If no new O2 were being produced, the atmospheric concentration would quickly fall to virtually nil.



I agree it is a dynamic system, but the atmosheric oxygen would take a little while to disappear- it would be quick in geologic terms, but not in human terms.

Oxygen is produced by photosynthesis from carbon dioxide, and volcanos steadily release CO2 from the depths of the Earth. All the oxygen that is in the air is balanced by reduced carbon, either in the atmosphere or buried in rocks.

It turns out that most of the oxygen that has been produced by photosynthesis is also bound into the crust as well; IIRC seven times as much biogenic oxygen is bound into the rocks of the upper crust as remains in our atmosphere. So the surface of the crust is pretty much oxidised already; once the carbon in the biosphere was oxidised away (which would only remove a few percent of the atmospheric oxygen), the only route for removal of the oxygen which remains would be oxidation of freshly exposed rock- a slow process which would take hundreds of thousands, or millions, of years.

eburacum45
2005-Oct-06, 08:07 AM
But because the oxygen level is the result of a dynamic process it seems certainly possible that the level of oxygen can change over time;
the trouble is it seems difficult to accurately determine that level.
One set of estimates I found showed the O2 level above 30% in the Carboniferous, down to 14% in the Permian, 26% in the Jurassic, and 21% now;
http://www.pnas.org/cgi/content/full/96/20/10955/F2
this doesn't seem to agree with the original post in this thread, so now I am confused.

The high O2 levels in the Carboniferous are associated with the giant arthropods like Arthropleura, Meganeura and Megarachne; these creatures could grow larger despite inefficient breathing apparatus, so so it is said.
Recently the cat-sized giant 'spider' Megarachne has been reconstructed as an eurypterid, which is slightly disappointing
http://homepage.mac.com/paulselden/PhotoAlbum44.html