Why were prehistoric era animals so gigantic in comparison to animals from the “modern era”
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Didn’t it have something to do with O2 levels being much higher than in modern times? I hope someone else can fill in my gap in knowledge.
High temperatures, and 02 rich atmosphere and subsequent abundant food made it an evolutionary arms race between predators and prey to become as large as possible since simply being big was a great defense. Eventually certain physical and practical limitations came in to play to pretty much limit further growth beyond a certain size.
The question to ask is why did megafauna species suddenly start going extinct when humans appeared on the scene. The answer is pretty obvious – we killed or domesticated as many of them as we could. The largest and juiciest hunting targets have generally suffered the worst when it comes to cohabitation with humans.
The climate during the age of dinosaurs was very different than now.
The Earth back then had higher average temperatures and the Oxygen content of the atmosphere was much higher.
Plant life was also more abundant in that era so there was much more food.
These factors supported much larger animals and insects, and an evolutionary arms race between predators and prey likely drove dinosaurs to reach immense size.
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.
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Beginning a few million years ago, these were drastic drying and cooling trends across the globe, causing the decline and extinction of many species of megafauna. On the tail of this, a species of funny hairless ape learnt how to make pointy sticks and throw rocks, and then spread across the globe, causing mass extinctions among the already fragile ecosystems.
What we see today are the animals still surviving from those ongoing extinction events, which are generally far smaller and more generalist that the fauna which went extinct.
Also, people keep claiming it was the high oxygen which allowed large dinosaurs to survive, but this is incorrect. Much of the mesozoic had lower or comparable oxygen levels to today, yet these massive dinosaurs still existed. Sauropods were the largest group of land animals to ever exist, but the groups in 2nd and 3rd place are both mammals. Ancient rhinos and elephants got to larger sizes than any theropods or hadrosaurs.
Survival of the fittest.
Basically, the large animals evolved in the first place because being large, stronger, sturdier, and more robust gave environmental advantages to creatures that were smaller, weaker, and more fragile.
However, after a certain point, changes in the environment made it less helpful to be so big and strong, in favor of other genetic selections, such as better births, more dexterous limbs, or better senses.
Fittest means whatever works best for the current environment.
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The higher 02 atmosphere was only important for the massive invertebrates during the carboniferous.
The reason that dinosaurs were so big was mainly because they had hollow bones and better breathing systems, they had many airsacs and could get O2 both when breathing in and out.
For things like mammoths its mainly because we killed all of the big animals.
Humans seem to have killed off the biggest animals available first almost everywhere they went. Gigantic animals everywhere was more or less typical for most of the fossil record. Then some bastard invented the spear. It’s a miracle that elephants and lions are still with us.
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