Yes, Nick.
My research, based on the work of many people, past and present, indicates that Earth's electromagnetic fields could have been orders of magnitude greater or less than they are today, thus influencing its gravity field. The electromagnetic nature of gravity is highly controversial. However, I see a lot of evidence for a lower gravity field on Earth at some time in the past.
I wrote a Picture of the Day called,
Tyrannosaurus Rex: Prima Ballerina several years ago that discusses the idea. However, I think there are even better examples.
Consider the pterosaur called
quetzalcoatlus, as big as a modern giraffe with a wing span of at least 12 meters, and weighing about 500 kilograms. Its skull was so large that it could swallow a full-grown man. By no stretch of the imagination can one think that a 500 kilogram pterosaur could ever fly in today's gravity field.
I know that there are glosses from the scientific community that say they soared, lived in high aeries, and caught fish on the wing. However, the delicacy of the wing structure belies that idea. They would have folded up like a broken kite from their own weight. This ties in with Ted Holden's analysis of brontosaurus: in todays gravity field, it would have been unable to even lift itself off the ground.
That tells me that gravity can fluctuate in strength. And what about the megafauna in North America's past? Bison with horn spans of 4 meters; dire wolves 8 feet high at the shoulder; armadillos as big as a Volkswagon Beetle -- it could be that Earth's force fields have changed many times in the past, perhaps into the time of human beings. "There were giants in the Earth in those days..." might not be such a fanciful idea. There are a lot of reports from various sources that talk about giant human skeletons, although that's anecdotal.