Many changes occurred to the mammal-like reptiles before they eventually evolved into true mammals. But what made the first true mammals different? Mammals have larger brains than snakes and lizards, and give birth to live young (except for echidnas and platypuses, which lay eggs). What really makes cats, rats, you and me different from therapsids, lizards and snakes (apart from not usually being especially slithery and scaly) is that we are far more active and create our own high body temperature. We don't need to wear a solar panel on our back. We just need to eat a lot more food, in fact about ten times as much as a reptile of the same size.
By having legs slung under their bodies, later therapsids could travel further and faster. The trouble with being on the go all the time was that the animals needed to eat a lot more to keep the energy levels up. So they had to spend a lot more time looking for food. To be so active they also needed to breathe in more oxygen. Therapsids evolved the knack of being able to
eat and breathe at the same time, an ability they passed on to mammals. The evolution of the different types of chewing, gnawing and biting teeth meant that food could be broken up and chewed more effectively before being swallowed. This reduced the chances of getting indigestion. So when your parents tell you not to bolt your food, but to chew it properly before you swallow, they're following some good advice first developed more than 200 million years ago. (No, this does not mean that you can accuse your parents of being that old.)
Changes in the way that later therapsids and mammals fed meant changes to the shape of the jaw. D I
As well as chomping up-and-down, you and I can also move our jaws sideways while chewing. Reptiles can't do this. The jaw of a reptile is made up of lots of different bones. Mammals evolved single, large upper and lower jaw bones, strongly hinged together.
Some of the smaller bones didn't disappear completely, though. They ended up in your ear, of all places, where they got a new job, forming the three bones of your inner ear. This gave mammals much better hearing, a useful thing to have if you are a meat-eater listening for its prey, or its prey listening out for a pair of flying fangs trying to bite you in the bum. *
Therapsids eventually lost out to dinosaurs in the battle for survival. But they had the last laugh.
They passed on all these useful features to some tiny, shrew-like mammals that were forever getting under the dinosaurs' feet. And when the dinosaurs died out, 65 million years ago, mammals took over. So therapsids set the stage for the stunning explosion of mammals that took place during the last 60 million years.
In this new world dominated by mammals, including humans, the flying insects were joined by birds, and flowering plants grew everywhere - grasses, gum trees, carnations and, of course, cauliflowers.
So there we are. We began with bugs.
Bugs evolved into seaweeds and slime, which evolved into animals, which evolved - into you!
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