Asked by: Ronan Conway, Belfast
For many animals, there was an intrinsic selective pressure in favour of increasing body size. Since you must share your evolutionary niche with the other members of your species, being slightly larger than your peers helps you eat the higher leaves, catch bigger prey and defend a larger territory. Even more importantly, the larger you are, the safer you are from predators.
Over time, this selective pressure has caused many animal lineages to gradually increase in size. In 2009, scientists in South Africa reported evidence that dinosaurs became larger as they switched from walking on two legs to four.
But being large also means you need more food, can’t escape from natural disasters so easily and reproduce more slowly. As such the fossil record is littered with examples of animals that slowly increased in size before going abruptly extinct.