I'm bothered by the consistent gigantism of ancient times, or rather, the consistent dwarfism of OUR era. I used to think of dinosaurs as spectacular exceptions that caught the popular imagination, but they're not. Fish, bugs, land animals, marine reptiles, pteranodons--now the record wingspan is up to what? 55, 60 feet? No living bird has even a quarter that span. And I don't believe ancient dragonflies could fly today--hell, they'd barely be able to breathe. If they were just out-competed by mammals and birds, we'd see pteranodon sized condors, seismosaur-sized giraffes and elephants. Surely life is as ingenious now as it was a hundred million years ago.

Answer: the environment has changed, biomass has decreased and impoverished. Most species stranded on small islands shrink over time compared to their mainland cousins. Isn't the lack of land creatures rivaling the dinosaurs evidence that something has changed, that Earth Island's gotten ecologically smaller, less friendly?

A few scientists have proposed that the oxygen content of ancient air could have been higher. I guess it's just possible oxygenated air could have "supercharged" a pteranodon's metabolism, allowing flight where today it couldn't. Though they're touted as gliders not fliers--another evolutionarily unlikely position. Why bloat beyond practical flight-size, to the point where you could only glide? Because they COULD fly, full size!

How could that be possible? Simple, if the atmosphere was thicker then. The combination of bugs and dactyls convinces me that either 1) planets, or at least Earth, lose more atmosphere over time than current theory says, perhaps through some interaction with solar wind, OR 2) catastrophes like comet strikes or massive lava flows can somehow strip or fix substantial parts of the atmosphere--the flip side of the theories about meteoric impacts into limestone like Yucatan (vaporizing all that carbon leads to a long-term greenhouse period once the dust settles out). But what could cause the opposite, a change so drastic that a significant percentage of the atmosphere gets fixed in the soil or water? I don't have any answers; I don't know enough geochemistry. But I do know something about life's plasticity, and the absence of big flying creatures now, when plane-sized pteros were managing just fine... that's a glaring change in quite a short geological time. It's a niche that their close relatives the birds would quickly grow into if it were possible. So it isn't. So things have changed. The only question is how.

I know it sounds unlikely, but remember we're still Terracentric thinkers. We think of our current air pressure as normal for a world this size. Venus, slightly smaller, with lower gravity, has an atmosphere nearly 100 times ours. Look at the spread of existing pressures on rocky worlds with atmospheres: Mars with a hundredth our pressure, Titan, smaller still yet with denser air than Earth, to Mercury, with a near-Martian gravity yet almost no air, to Venus with ninety times our air pressure. Compare this to the monotony of surface gravity (you'd weigh essentially the same on five local planets--five out of nine! Five out of eight, if you're a Plutoclast). The wild variation in atmospheres is suggestive. To what extent is our present atmospheric pressure a historical accident? And what accidents could change it? Okay, sure, let's say Earth had Venusian levels of CO2, but it got locked up in shells and coalbeds... N2 was just an odd little leftover trace gas and O2 is plant waste. Voila, Earth's atmosphere! Fine. But are there other stories beyond this easy, obvious one? If you did precipitate out most of Venus's CO2 blanket, would the remaining traces really be an Earth-density atmosphere? The latest I've seen says it'd be several times denser! So why's our air so thin?

The cliche is that our present atmosphere protects us, it's near-optimal for life, and other than modest swings in CO2 it hasn't changed much in a billion years or more. But what if it's NOT optimal?

Essentially, I'm trying to point out that the only other truly Earthlike body we know has an atmosphere so thick ours is Martian by comparison--mere wisps and traces. Barely enough for life? It happened to Mars, after all. Why can't it happen here? More slowly, of course, or we wouldn't be here, but... are we sure it hasn't happened? Earth now has vast barren Martian zones--deserts and ice caps, where savanna and boreal forests once grew. In the context of a TEN THOUSAND-fold difference between Venus and Mars' air pressure, the idea our atmosphere could once have been a few times the current pressure is no more outrageous than drifting continents or comet strikes or survival of the lucky instead of the fittest--all of which were heresy in the orderly, static, linear view of evolution I was taught as a kid.

I'm not insisting this is true. I am insisting it explains a lot of biological oddities, like the inefficiency of spider and insect breathing and the size of ancient insects, as well as the glaring case of the pteros, and life's earlier exuberance generally. All those sails on dinos have been assumed to be solar heaters and/or radiative cooling fins, but if winds were once thicker, thermal transfer via the air, not via radiation, would have been much faster than today. Does that explain the relative paucity of air-fins and frills among modern animals? If air was once more like a sea, so many fins make a certain intuitive sense.

If ancient air was thicker, the very oldest trees, would, I expect, show on average smaller, narrower, more finely divided leaves, since winds would pack more punch. The oldest flying insects wouldn't just run larger than modern ones; even the small ones would have a heavier wing-load and proportionally smaller wings than modern fliers.

But we won't test the hypothesis, or examine scenarios by which atmospheric pressure could decay, if we don't even consider it.

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