Normally I write about ancient history and I guess this could be classed as ancient history, just a bit more ancient than normal.
When the BBC's Walking With Dinosaurs series was first shown I looked at the way the large dinosaurs moved and felt intuitively that it was not right, they just seemed far too big. The Victorians had noticed this and thought that the large ones would have moved partly submerged in water, the water making them buoyant, although this turns out not to be true. So I thought what if the effects of gravity was much lower then? Obviously gravity is a constant so that could not have changed in itself, but what if the Earth suddenly became more massive? This I realised was possible, when people imagine an asteroid or comet or other object hitting the Earth they tend to think of them smashing directly into the Earth, but what if a large object didn't directly hit the Earth but instead was captured by the Earth's gravitational field and gone into orbit, or the Earth had had a second moon, which of course, is much the same thing.
If you spin a coin on a hard smooth surface as the spin decays the coin flips onto its side and the process of it coming to a stop is surprisingly gradual. As the orbit of a second moon or captured object decayed it too might be quite gradual, relatively speaking, and when it finally hit the Earth, although catastrophic, it would not in itself be enough to wipe out all life but at a stroke the effects of gravity would be much larger especially if the Earth was much smaller then, although not as small as the one in the children's story, The Little Prince.
As the two bodies fused together the tectonic plates would adjust and buckle causing a great deal of volcanic activity, something that is known to have happened. At a stroke creatures that once flew would become flightless and the large dinosaurs would simply be too big to function properly and would die off, the smaller creatures being lighter would have been less affected and more of them would have survived and this is what we see. Creatures living it water would be barely affected by the sudden change in gravity but many would have perished though the general calamity of the combining of the two bodies, acid rain from the volcanic activity, a general 'nuclear' winter, and possibly whole oceans being drained because of the massive change in the Earth's crust.
Life on the lower gravity Earth would have been very different to how we imagine it today for these giant dinosaurs would not have thought of themselves a giants at all (this is a figure of speech, it is of course doubtful if they were self-aware, especially of the possibility of alternative gravity effects) and would have been far more agile and move around far quicker than the lumbering giants we imagine. Dinosaurs such as Tyrannosaurus Rex with their absurdly small fore-limbs would have hopped like kangaroos which also have small arms.
Scientists have recently made a robot kangaroo that moves in the same way as a real kangaroo.. It has small appendages in the place where the fore-arms would be which are simply used to stabilise the hop, pushing on the ground if the hopping becomes unstable (notice how fast the real kangaroo can move, so if Tyrannosaurus Rex moved like this it would have been a ferocious fast moving predator, it is also a reason for the large tail. If the low gravity theory is correct we need to re-evaluate the dinosaurs and other animal's and plant's evolutionary niches and imagine them as normal sized organisms on Earth today) . Many birds, that are now known to have evolved from dinosaurs, continue to favour hopping while on the ground, like the humble sparrow.
These faster moving dinosaurs may have had less gravity working on them but they did face another problem, they would have been very massive so they would have had problems with momentum as momentum is independent of gravity and based entirely on mass.They might have been able to move quickly in a straight line but would have faced challenges turning because they were so massive and it is perhaps for this reason that they often had huge tails which by swinging them would help them turn in the opposite direction. This problem probably led to the evolution of flight for by growing feathers on the forelimbs these 'wings' could act as air brakes and rudders giving the creatures greater manoeuvrability and a competitive advantage and of course the lower gravity environment would make flight a much easier proposition.
I am not a scientist so I have not been able to calculate the size of the object that would be needed to achieve the effects I have described but the Earth could have been considerably smaller than supposed for scientists seem to have overlooked that the Earth is naturally growing bigger, just like any massive body in parts of the Universe where there is plenty of stray matter. The Earth is continuously being bombarded with space dust and meteorites etc. so it must be getting bigger but it happens so slowly that evolution can keep up. I have to wonder if this effect has been taken into consideration when the life span of stars like the Sun are calculated.
Why this theory might be true is that when life recovered on Earth after the mass extinction there was never the same tendency to gigantism again, it simply would not work in a higher gravity environment, the exception being whales who live in water where gravity is not an issue. There is one last piece of the jigsaw puzzle, if the effects of gravity on Earth has permanently and dramatically changed why did creatures take to the air again, most especially the birds? Partly they evolved to be smaller and their bones became lighter but I have just realised that with the increased gravity the atmosphere would have become more dense than previously and so provide more 'buoyancy' needed for flight. It would also have concentrated the oxygen much like turbo-chargers in cars so air breathing animals would have got more oxygen into their systems and therefore convert energy more efficiently.
It is a bold theory but
one that might be true so it would good to get it out there for others to
think about. This theory requires that a large external object could have
combined with the Earth to dramatically increase the mass of the Earth and
more work needs to be done to see if this is actually possible, but there
is another scenario and that is the Earth was struck by many separate objects
which increased the mass of the Earth faster than evolution could keep up
with, but I prefer the single object theory as it is more elegant.