why ancient animals were bigger but humans weren’t
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You kind of have it the other way around. The reason why we no longer have so many of those ancient large animals is because they were big walking piles of meat, and humans were very good at hunting them.
The timeline of extinction for most megafauna tends to correlate pretty strongly with the spread of homo sapiens. The survival strategy of “become too big for predators to bother with” worked fairly well until humans came around with spears and coordinated teamwork which made it much harder to survive with just sheer size alone.
There are other reasons too, like the changing climate, but my understanding is that human hunting is thought to be the primary cause.
The more human culture evolved, the less we had to physically evolve. As mega-fauna evolved to exploit the changing environment, our ancestors didn’t have to get bigger to match them, we just had to invent better hunting techniques.
Other commenters have made very good points but I also want to point out that in addition to these, we experience a lot of selection bias through the fossil record. There WERE plenty of smaller animals throughout history: birds, squirrels, bats, etc. but their bones are small and so more likely to be obliterated before fossilization can take place. The resultant sample size we draw from is thus skewed toward megafauna.
I should also point out that we do have megafauna today: the blue whale is far larger than any dinosaur ever was. It’s just that it exists only in the ocean, where people tend not to go/
If you are referring to dinosaurs, humans and dinosaurs didn’t exist in the same time period. We can only speculate about the condition of the world during the age of dinosaurs. If the giant dinosaurs were brought back today, Jurrasic Park style, they would be crushed under their own weight. Whatever was different with the Earth back then, if humans existed, there is a good chance we would have been bigger too because of it.
Well, humans are technically classified as megafauna. But, being bipedal puts a pretty hard limit on how large we could be before our legs or spine give out.
In the grand scheme of things, humans haven’t been around long enough. We’ve been around only a million years, maybe three to four million if you count all the hominids and australopiths. The giant insects were buzzing around 300 million years ago; the giant dinosaurs were around 65 million years ago, etc. If you compared a horse to a horse a million years ago, it probably hasn’t changed much, either.
However, you’re looking at the wrong thing — in the last few million years, we HAVE grown an awful lot. Not our bodies, but our brains. The chimpanzee has the biggest cortex of any non-human brain, and their brains are 300cc; as our evolution diverged from theirs 6 to 7 million years ago, our brains have grown in size to 1400/1500cc, around 5x as big.
Brainwise, humans are enormous.
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