And why are animals a lot smaller now?
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Evidence suggests that in the early days of life on this planet there was a lower concentration of oxygen and a higher concentration of carbon dioxide. This lead to an abundance of plant life. This plant life evolved to grow large because the carbon dioxide would allow it to make huge broad leaves to take in as much of the CO2 as possible. Massive broad leaves need massive trunks and branches.
At the same time animals were continually evolving. As the trees grew bigger and taller so did the herbivores that lived off them. Even those that didn’t grow tall grew bigger bodies to support larger stomachs that could hold and digest more plant matter. Digesting plant material isn’t easy. Modern herbivores often have a slow digestive process which is needed to break down the plant fibers into useable calories. Which suggest prehistoric animals must have had to deal with a similarly slow digestion. So the abundance of plant matter gave the enviromental “allowance” so to speak for the animals to grow bigger.
At the same time carnivores had to evolve as well. Larger prey means a bigger predator is needed if they want to take it down alone. So the carnivores had evolutionary pressure to grow large, and the massive size of their prey would support their evolution. Unfortunately this all lead into a massive evolutionary trap.
As the environment changed, from natural disaster ow what have you, it could no longer support megaflora or megafauna. Large plants began to die off or evolved into smaller versions. Smaller plants couldn’t support larger animals so they died off as well. Without enough prey the carnivores also died off.
After that period there were a few animals that found environments which they could evolve into large sizes like whales, but the earth as a whole would no longer support anything so massive on such a scale.
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