In the era of dinosaurs, why was everything big?

Georgia Mills gets to grips with Alpesh's question.

Georgia - To start with, the dinosaurs were around for hundreds of millions of years and I know we think of the really exciting ones like Diplodocus and T-rex. But actually, a lot of animals around then weren't so big. So insects, their biggest time was around the carboniferous period which was actually a long time before the dinosaurs were around. And I think the largest animal that was ever recorded is of course, the blue whale and that's still around today. Saying that though, a lot of dinosaurs did get very big and there are several theories about why this might be. The main reason is that the ecosystem could just support larger animals and being big is often a really good idea if you're competing for mates. It can help you to fight off the smaller males and if you're competing for food as well. One idea is that the plants that were around during the dinosaur times were quite tough and were quite hard to eat. So, for herbivores, it became quite a good idea to have longer guts so that they could process this food more easily. And so, they could get bigger and then of course, the carnivores could get bigger as well. Another thing to consider is that dinosaurs could've had less physical constraints on them from becoming bigger. So, a problem with warm blooded animals becoming really big is that they could overheat quite easily and dinosaurs, it's not quite known for sure if they were warm blooded or cold blooded, but it's likely they were somewhere in between. So, they wouldn't have had this constraint and they also had quite efficient bird-like lungs which meant that they could take in oxygen and distribute it around the body much more easily.

Max - There's actually a lot of evidence in the fossil record for really large mammals as well after the so-called era of the dinosaurs, after the dinosaurs went extinct and mammals began to become much more prevalent. There's also loads of what we call mega fauna - really large animals. There are some terrifying things - if you look it up - that existed in north and south America, and now, in Australia and Africa as well. The reason we think these are no longer around, is a combination of being over hunted by humans and also, a degree of climate change.

Chris - Equally, did not humans go through a massive phase as well? I remember seeing when I was in Johannesburg, I went to Wits University there. This is the department that Raymond Dart worked in and he was one of the people who was one of the big forefathers, the godfather if you like of sort of palaeoanthropology. There are skeletons there of humans from maybe 300,000 years ago or so and they would have made a pro basketballer look like a dwarf. There are people there who were absolutely huge and I asked Professor Lee Berger who was showing me around, why did people evolve to get so big because the cost of growing to such a big size is extremely costly in the sense that you've got to have enormous amounts of energy to go into growth to get that big. You've got a huge body to maintain. It makes you easier as a target to hit doesn't it? He said, "They got big because everyone got big. If everyone's big, you've got to get big to defend yourself."

Georgia - Yeah. It's interesting that these giant people aren't really around so much anymore. Chris - Not in my case.

Max - There's a few people, like it's generally regarded as a disease now I think, that you do get people topping 7 feet. Huge people do still exist. They're just very rare.

Georgia - Small people still exist though. I'm an example of that, but being big like you said, it's a cost. And also, this is really obvious when you look at the fossil record, when mass extinction events happen, big animals are always the first to go because they're slower breeders. It's easier for them to run out of food. If you look at the animals that are endangered today as well, it's some of the most big ones.