Eli5. – How were prehistoric species able to be so much larger than current species?

Question

I feel that there must be some physiological stressors or environmental factors that inhibit modern organisms of all domains from reaching the size, of say, a megaladon or brontosaurus.

In: 18

Answer 1

One theory is that as humans colonized the globe, we hunted every large animal we came across to extinction. Africa is the one big exception because the large animals there coevolved with humans. Everywhere else, we wiped out the largest species, leaving only species that could reproduce fast enough to keep up with human predation.