Why were animals and insects so much bigger during the time before humans?
In: Biology
There are a lot of separate things going on here:
First, not everything was bigger in the past. Blue whales are probably the biggest animals to have ever lived.
Second, the above fact is kind of remarkable because you’d expect, just due to random chance, many of the biggest things to have lived in the past. Most animals that have lived are extinct, so if you just picked a random one to be the biggest, it would probably be in the past.
Third, giant insects specifically (but not other giant animals) require higher levels of oxygen in the earth’s atmosphere. This is because insects don’t have proper lungs but breathe by diffusion of oxygen through tubes. This is inefficient, and limits the size of insects. But it’s not the only limiting factor. The level of oxygen has gone up and down through history, and while insects were huge the first time levels got high, the second time it got high they didn’t get huge. By that point there were other flying animals around that could outcompete and probably eat them.
Dinosaurs got huge for a variety of reasons related to their biology that let them get particularly large. High oxygen levels had nothing to do with it, in fact oxygen levels were actually lower at some points. But dinosaurs had efficient birdlike lungs which is one reason they could get huge at all. Many dinosaurs also had air sacs in their bodies and bones which probably helped them stay lighter and get larger. There’s a lot of factors involved in their huge size, probably.
Ice age mammals were larger than modern fauna for yet another reason….basically, the modern world has very few species of large mammals (and other land animals) compared to basically any point for millions and millions of years (since mammals first got large after they took over from the dinosaurs). Why? That’s a matter of debate….some will tell you climate, some hunting by humans. I personally think the latter answer is the better one, for various reasons, but the main point is that all the mammoths and giant sloths and the rest mysteriously went extinct recently. But they weren’t really that much bigger than our surviving large land animals like elephants and rhinos…the ones that managed not to go extinct.
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