The History of Earth's Gravity, Evolutionary Arms Race

Hi Rachel,

    Thank you for presenting such an interesting problem in paleontology. A theory like yours, if validated, would totally upend the scientific community. Unfortunately, there is no evidence that gravity has ever significantly deviated from is present quantity. A significant impact would have had to occur for gravity to restrict evolutionary growth the way you describe- far greater than Chixculub, which supposedly ended dinosaur evolution (with the exception of birds).
    Applying Occam's Razor to the issue, a more likely explanation is that because CO2 levels were much higher during the Triassic, there was an overabundance of plant life that allowed the herbivorous dinosaurs to consume the astounding quantities of energy needed to support the metabolism of large bodies. Larger bodies are advantageous because it is harder to kill them. Likewise, carnivorous dinosaurs would have needed to evolve larger bodies along with them, to help bring them down for consumption. As one columnist put it, it was an evolutionary arms race. 

     But the biggest inconsistency in your theory is that while there were many gargantuan dinosaurs, there also some very small dinosaurs- far smaller than humans. There were also small mammals, small insects and amphibians- all on the same level as today's sizes. In fact, many of today's species have survived since the Mesozoic: crocodiles, nautilus, jellyfish, bees, and the platypus, to name a few. If gravity really has been changing, you'd think there'd be a shift in the fossil record during this period.

    I think we are underestimating the ability of life to reach the greatest of extremes in any given environment. Life evolves because doing so is advantageous, otherwise it dies out. Long necks, long tails, huge horns, spines, and jaws took millions of years to co-evolve because there was an overabundance of competition in the Mesozoic. Reptiles were especially capable of doing this because their body plan allowed air sacs and hollow bones. There have been giant snakes and turtles at other periods in history. Even insects and mammals like the wooly mammoth grew to pretty large sizes before we ran them off the planet. We are their replacement. What we lack in size we make up for in intelligence. Without us, some of the largest species would still be around. After we're gone, if there aren't any intelligent species to take our place, large animals will again take up the vacant niches we possessed.

    A greater mystery to me is the Faint Young Sun paradox, which is the contradiction between a weak sun in Earth's early history and the relatively warm global conditions at the time. In early Earth history, solar output was about 25-30% less than it is now, but there is no evidence that the Earth was frozen during this time. Earth has managed to maintain its "Goldilocks" status ever since it was born, which to me suggests it might be regulating its own atmosphere. And that's as close to a crackpot theory as I will venture.