Asked by: Matt Evans, Gloucester
There is a tendency in evolution for animal lineages to get larger over geological time scales. This is known as Cope's Rule, after 19th-century palaeontologist Edward Cope. Horses, elephants, bears and even humans all evolved from smaller versions of essentially the same body shape. The reason for this is that nearly all animals are competing with other members of their own species for resources. Being slightly larger than your peers allows you to fight them off to defend a kill, territory or mate, and also makes you less vulnerable to predation.
Over evolutionary time scales, this selective pressure tends to drive animals to become ever larger - but, of course, larger animals also need more food so the trend can't continue indefinitely. They also have longer reproductive cycles and this makes the species as a whole more vulnerable to environmental change because it can't adapt so quickly. This often leads to animal lineages growing steadily larger before abruptly becoming extinct - whereupon a smaller species takes over the niche, and begins growing larger. And so on.
However, Cope's Rule is not inviolable and plenty of lineages have evolved in the opposite direction. Birds, for example, are descended from dinosaurs. What's more, the largest animal ever - the blue whale - is still alive today.
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