Eli5 How did prehistoric animals get so big?

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And why are animals a lot smaller now?

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Well, first of all, the biggest animal ever is the blue whale, which is still around. There are still plenty of large animals.

More generally, though, it’s true that there seem to be more big animals in the past than there are now. Unfortunately, the evidence is a bit incriminating for us. Large animals (termed “megafauna”) tend to disappear from an environment shortly after humans arrive. We see this in the fossil record in the Americas, Europe, and Australia. Within a few thousand years of humans arriving, many megafauna species go extinct.

There is wide agreement that human hunting and human-made alterations to the environment are responsible.

I read a theory once that it had to do with oxygen content in the atmosphere?

Evidence suggests that in the early days of life on this planet there was a lower concentration of oxygen and a higher concentration of carbon dioxide. This lead to an abundance of plant life. This plant life evolved to grow large because the carbon dioxide would allow it to make huge broad leaves to take in as much of the CO2 as possible. Massive broad leaves need massive trunks and branches.

At the same time animals were continually evolving. As the trees grew bigger and taller so did the herbivores that lived off them. Even those that didn’t grow tall grew bigger bodies to support larger stomachs that could hold and digest more plant matter. Digesting plant material isn’t easy. Modern herbivores often have a slow digestive process which is needed to break down the plant fibers into useable calories. Which suggest prehistoric animals must have had to deal with a similarly slow digestion. So the abundance of plant matter gave the enviromental “allowance” so to speak for the animals to grow bigger.

At the same time carnivores had to evolve as well. Larger prey means a bigger predator is needed if they want to take it down alone. So the carnivores had evolutionary pressure to grow large, and the massive size of their prey would support their evolution. Unfortunately this all lead into a massive evolutionary trap.

As the environment changed, from natural disaster ow what have you, it could no longer support megaflora or megafauna. Large plants began to die off or evolved into smaller versions. Smaller plants couldn’t support larger animals so they died off as well. Without enough prey the carnivores also died off.

After that period there were a few animals that found environments which they could evolve into large sizes like whales, but the earth as a whole would no longer support anything so massive on such a scale.

Sorry for format; on mobile.

IIRC, it’s something to do with greater carbon dioxide levels, therefore making a much warmer climate. As a result of this, plant life has pretty much no growth limit. It only makes sense, then, that you’re gonna have herbivores evolve to huge sizes to eat these massive plants… and as the carnivores’ prey got bigger, so did they (but slightly smaller than herbivores).

As for why are animals smaller today, they evolved to be more energy efficient. It required a lot of energy to be a 1135kg, 12.8m long Titanoboa, for instance. Nowadays, the record for the heaviest snake belongs to the green anaconda, averaging at 70kg, and 4.5m long. The largest specimen ever found was a 5.21m long female weighing 97.5kg. The title for longest snake goes to the reticulated python. They can grow up to 6.5m (although specimens over 6m are rare), and weigh up to 75kg.

A very interesting question!

If you were a caveman and it’s winter – you might have 3 options:

1. Stow away some seeds and berries in the summer and hope they don’t rot.
2. Eat a bunch of tiny ants, crickets, and other tiny insects.
3. Kill one big animal, light a fire, and have a barbecue with all your friends and family.

Humanity chose option 3 – a 300+ lb giant animal killed by one person with a bow and arrow can provide your friends and family with enough food for weeks potentially.

They found that more convenient than trying to find hundreds of pounds of insects or hundreds of pounds of fruit/veggies on the off season.

Some of the smaller animals could reproduce fast enough. The bigger animals often don’t reproduce as quickly so many died out.