Large Animals and Evolution
Why does it seem that in the past the creatures of the
earth grew to much larger sizes than in the present (with few exceptions)?
Is the Earth's size a factor?
Evolution has historically suggested that the larger the animal, the more likely it will
survive. The dinosaurs may have been a test of this
hypothesis. Competition and predators development probably had a role in
this large phenomenon.
Update: July 2008
There are several factors at play, but none have anything to do with the Earth’s size,
which is basically constant.
First, some insects were larger, it is suspected, because there was considerably more
oxygen in the atmosphere at that early time (late Paleozoic). A paper in 2006 by Alexander
Kaiser of Midwestern University suggests that the subsequent decrease in oxygen (from 35
to 21%) puts a strict limit on an insect’s size today.
Second, the larger dinosaurs were probably cold-blooded. A warm-blooded animal of that
size would have a difficult time not overheating. This is discussed in dinosaur books.
Note that the most enormous mammal (whale) today lives in the cold ocean, and the largest
land mammal (African elephant) has enormous ears for cooling and rarely moves quickly. So
this is why mammals, at least, never get very large. And large dinosaurs are gone because
of an extinction event.
Third, you might wonder why the remaining reptiles at the time didn't re-evolve back into
large ones again after the extinction. Well, during the Mesozoic (dinosaur) era, CO2 and
temperature were higher, and vegetation flourished, so there was plenty of food. Furthermore,
there was no competition from mammals and birds as we know them today. But today, mammals and
birds have gotten such a foothold in their habitats, that something like the Komodo dragon
today would have a hard time feeding itself if it was dramatically larger.
These issues are discussed in any high-quality dinosaur book.
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Update: June 2012