Why were prehistoric era animals so gigantic in comparison to animals from the “modern era”

Question

Why were prehistoric era animals so gigantic in comparison to animals from the “modern era”

In: 1281

Answer 1

Edit: it’s been brought to my attention that I am delusional and completely misread the title question. This comment answers the question why did the huge animals go extinct, not why they got so big to begin with sorry

Assuming you’re talking about the huge horses and koalas and stuff, and not dinosaurs:

A curator at the American Museum of natural history wrote a book called End of the Megafauna if you want a deep dive. But basically, there are three theories about why megafauna disappeared so completely: overkill, overchill, and overgrill (massive kudos to the naming committee)

Overkill refers to the theory that humans hunted megafauna into extinction before they had a chance to adapt to humans. The best argument for this theory is probably Australia, where humans first appeared 50k years ago and all the megafauna went extinct around 46k years ago.

Hunter humans obviously targeted the biggest prey possible. They’d band together and take down a huge meat source that couldn’t move very fast because of its size. The megafauna had no natural defences again humans, and because they breeding cycles were so long (the bigger the animal the longer gestation tends to be), they didn’t have enough generations to evolve adaptations before they were wiped out.

The overchill theory argues that megafauna in the main continents of Eurasia and the Americas were wiped out by an ice age. During an ice age, temperatures drop and a lot of fresh water becomes locked up in ice, creating changes to geography and geology, therefore affecting the plant ecosystem makeup. Animals that depended on a certain biome of plants (most megafauna were herbivores) were therefore wiped out.

The overgrill theory argues that an asteroid impact set off wildfires that killed of most megafauna, especially in the Americas.

There are arguments agains all three theories that I won’t get into. There is basically no consensus and all three theories have merit.