Carl Baugh (you can read his dissertation online) has got it all wrong. He's a creationist who makes wild claims about oxygen concentration fueling the greater strength and intelligence and size of the patriarchs of the Bible, and that dinosaurs were just lizards who grew huge in the oxygen-rich atmosphere before the biblical Flood. (He's also the crazy guy Sadly, No! is mocking).

A paper in this week's Science shows what happens when real scientists examine reality, instead of the fantasies invented by religious kooks. Falkowski et al. examined oxygen concentrations over the last few hundred million years. They did this by examining the isotopic concentrations of carbonates and organic material from marine sediments, and modeling atmospheric conditions from that.

What they determined was that oxygen concentrations were at a low of approximately 10% in the Triassic, rose gradually through the Mesozoic, and hit a peak of 23% in the Eocene, about 10 million years ago. We're now seeing a slow decline to the current value of 21% O₂ in our atmosphere.

So O₂ concentrations were actually substantially lower in the age of the dinosaurs than they are now. What about the idea that increasing O₂ availability would lead to more giants? That seems to be partially true, but in a more complicated way than Baugh thinks. They tried to correlate oxygen levels with major events in the evolution of terrestrial vertebrates (a messy and difficult thing to do, I think), and do claim that there is a pattern: more oxygen means more metabolically active forms, and some increase in size—but the increase is to the present (or near present) size of mammals.

This is their rather complicated summary diagram.

(click for larger image)
Mammal evolutionary events based on fossil morphological and molecular evidence, compared with oxygen concentrations in Earth's atmosphere modeled over the past 205 My using carbon and sulfur isotope data sets; O₂ levels approximately doubled over this time from 10% to 21%, punctuated by rapid increases in the Early Jurassic and in the Eocene. Changes in average mammalian body mass is taken from. Vertical black bars represent known fossil ranges, blue lines represent inferred phylogenetic branching. Only some of the ordinal-level placental mammals are shown.

They do describe a doubling of the average size of mammals after the Late Cretaceous, but I'd be reluctant to ascribe that to a simple consequence of increased oxygen availability—there was also that expansion into depopulated niches after the K-T extinction.

Whereas the relatively rapid decline in oxygen at the end-Permian and early Triassic is suggested to have been a major factor contributing to the extinction of terrestrial animals (mostly reptiles) at this time, the rise of oxygen over the ensuing 150 My almost certainly contributed to evolution of large animals. Animals with relatively high oxygen demands, including theropod dinosaurs (the group that includes living birds) and small mammals, evolved by the Late Triassic. Avian and mammalian metabolic demands are three to six times as high per unit biomass as those of reptiles. Although the reproductive strategies of the earliest mammals are not known with certainty, both the fossil record and molecular divergence indicate that superordinal diversification of placental mammals occurred between 65 and 100 Ma. This radiation corresponds to a period of relatively high and stable oxygen levels in the atmosphere. Although placental evolution is not unique to mammals, this reproductive strategy, which can facilitate geographic expansion of a species, requires relatively high ambient oxygen concentrations. In the placenta, maternal arterial blood, with oxygen levels near ambient alveolar pressure, mixes with placental venous blood in a sinuslike vascular structure. Fetal umbilical arterial (really venous) blood arrives in a capillary network in the maternal sinus where oxygen diffuses into the fetal blood. The nature of this exchanger requires the mammalian fetus to live at a very low arterial oxygen pressure. Although at low oxygen, placental hemoglobin binding affinity for O₂ is modified by pH (i.e., the Bohr effect), with exceptions, few extant mammals reproduce above elevations of 4500 m, corresponding to atmospheric oxygen levels in the Early Jurassic.

Eh, I hesitate to accept that comparison. There are other differences between a warm lowland swamp at a low partial pressure of oxygen, and a cold arid mountain top at the same partial pressure of oxygen. There are also relatively few extant lizards above those elevations.

Whereas a bolide impact at the Cretaceous-Tertiary (K-T) boundary and the ensuing extinction of dinosaurs provided ecological opportunity for the radiation of placental mammals, the rise of oxygen in the Eocene corresponds to a large increase in average mammalian body size. The density of capillaries per unit muscle scales to the 0.87 power of size in mammals; hence, larger animals require high ambient O₂ levels to obtain maximal metabolic rates. Comprehensive study of body mass of nearly 2000 fossil mammals in the North American record indicates a steady expansion in size range throughout the Cenozoic, tracked by mean body size due to the static lower limit of size. Data show a rapid increase from small to medium-sized mammals in the first few million years after the K-T event. This size contrast is blurred slightly with the recent discovery of larger Cretaceous mammals, but this trend does not appear to be driven by oxygen. A second upward surge in mean body mass is recorded for the early through middle Eocene (50 to 40 Ma), followed by further but less dramatic size increases through the Miocene. This trend tracks a change in oxygen. The early to middle Eocene, an interval characterized by the highest global mean annual temperatures and the broadest latitudinal span of warm subtropical to temperate faunas and floras for the Cenozoic, was also a time of high morphological disparity in North American placental mammals. One might infer that this indicates a proliferation of ecological roles in the North American mammalian fauna. Notably, many of the living placental orders appear in the early Eocene, and artiodactyls, the dominant large terrestrial herbivores today, underwent a massive radiation in the mid-Eocene. Data from other continents are more limited, but there is reason to argue that North America serves as a model for broader patterns, at least in the northern hemisphere. The substantially improving records in Europe and Asia, especially, will provide an interesting test of the pattern.

That's darned cool stuff. I think the correlations are murkier than they let on, but it's reasonable to infer that increasing oxygen would open up expanded metabolic opportunities for larger animals.

Falkowski PG, Katz ME, Milligan AJ, Fennel K, Cramer BS, Aubry MP, Berner RA, Novacek MJ, Zapol WM (2005) The Rise of Oxygen over the Past 205 Million Years and the Evolution of Large Placental Mammals. Science 309(5744):2202-2204.