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In the beginning was the Big Bang.
Fast forward, and there's a planet Earth, oceans, and continents. The most complex life forms were the fishes.
390 million years ago (Ma), the fishes learned to live on land. Their fins evolved into legs, which led to paleontologists calling them tetrapods. But they still had to return to the sea to lay their eggs. They were thus amphibians.
Around 325 million years ago, we were smack dab in the middle of the Carboniferous period. This means there were lots of trees in the tropics. These trees repeatedly got buried in swamps due to cyclical rising and falling of sea levels. These trees were buried in such vast amounts in the latter half of the Carboniferous that they are responsible for a lot of our coal resources today, especially in western Europe and the eastern US.
There were warm, wet forests around the tropics and ice caps around the poles. It was an ice age: the ice spread remarkably far away from the poles. This means there were strong temperature gradients between the poles and the equator. One illustration of this is gradient is that trees near the poles have very distinct growth rings, because those reflect significant seasonal temperature differences. In contrast, trees near the equator don't show marked rings, because temperatures were more stable throughout the year.
Around 325 million years ago, the polar ice cap melted, causing rising sea levels.
This coincided with one amphibian lineage splitting into two lineages, one that would become reptiles and one that would become mammals.
2 large landmasses: Gondwana at the South Pole, Laurasia at the equator. Eventually they collide and form Pangaea. That collision forms the Appalachians.
The reason there's an ice cap is that the giant trees are pulling carbon dioxide out of the atmosphere and getting buried. Less carbon dioxide in the atmosphere means lower temperatures (no greenhouse effect). Lower temperatures means growing polar ice. Growing polar ice means lower sea levels. Lower sea levels mean less flooding. Less flooding means more trees. More trees mean warmer temperatures (apparently). Warmer temperatures means melting ice, flooding, and tree burial.
Moreover, when bacteria consume organic material and cause it to decompose, they consume oxygen. This means less oxygen in the atmosphere. So burying the trees and keeping them from decomposing not only meant less carbon in the atmosphere, it meant more oxygen.
More oxygen in the atmosphere means arthropods, like insects, can grow much larger. Dragonflies could achieve wingspans of up to .75m/2.5 feet. Millipedes got up to 2.4m/8 feet long.
All of these patterns get reversed during the Permian, when the ice age ends.
While this is going on (ca 325 Ma), some amphibious tetrapods learn to insulate their eggs with fluid. This means the embryos can grow in the egg on land, without having to get their water from the ocean. This means the tetrapods can live full-time on land, i.e. not be amphibious. These creatures are called amniotes, because they lay amniotic eggs (eggs with amniotic fluid).
These amniotes are the ancestors of reptiles and mammals, both of which are full-time land dwellers.
In the middle of the Carboniferous period, the weather changed. It switched from the Mississipian period to the Pennsylvanian period*.
These amniotes then split into the future reptile and mammal lines, which are distinguished by the number of holes behind the eye sockets for anchoring the jaw muscles. Reptiles have 2 holes; they are diapsids.** Mammals have 1 hole; they are synapsids.
If I understand correctly, synapsids split off first and evolved one hole, then diapsids evolved two holes.
Then around 307-310 Ma, the Carboniferous Rainforest Collapse happened, inaugurating the Permian Period. The ice caps melted, coal swamps replaced by seasonal, semi-arid drylands, and some deserts arise in Pangaea. Trees have to be better adapted to drier conditions. Some extinctions, especially in the plants (one of only 2 extinctions in the plant record).
Amniotic eggs are now a superpower that allow you to roam more and live in drier conditions. This helps explain why in the Carboniferous, amphibians were large and numerous, but now they are small. The larger animals became reptiles and mammals.
Now we're in the Permian. We start to get more mammal-like animals. They become increasingly warm-blooded, meaning they generate more and more of their heat inside their body, instead of absorbing it from the air around them. They start to develop hair. The hair starts as only a few hairs for sensory purposes, like a cat's whiskers, but eventually starts to cover the whole body. If you're generating your heat internally (i.e. are warm-blooded), it's efficient to have an insulation layer. This is why most mammals (warm-blooded) have body hair, birds (warm-blooded) have feathers, and reptiles and amphibians (cold-blooded) don't have this insulation layer.
We're not sure *why* warm-bloodedness evolved, but being warm-blooded means you can run faster and farther without having to rest and soak up more sun.
These animals also started to change their locomotion and posture. Instead of crooked limbs that sprawl out to the side, like lizards and crocodiles, their legs become more and more straight and more and more directly underneath the body. Their limb joints can also move in more directions now, which means they can run, dig, and climb.
All these new traits combine to make them better hunters and better able to explore new terrain.
These increasingly mammal-like animals that lived during the Permian are called therapsids.
Then volcanoes known as the Siberian traps were so active that we had the famous Permian extinction event. Therapsids got a lot smaller, and that helped them survive. They could burrow into smaller spaces to find shelter, and they needed less food and could eat smaller food. Furthermore, they evolved to mature faster, producing lots of offspring and dying quickly. That numbers game helped the species survive very difficult conditions.
After the end of the Permian period, we leave the Paleozoic Era and enter the Mesozoic Era. That is the age of dinosaurs.
Because dinosaurs were becoming very big and tended to prey on small mammal-like creatures, these small mammal-like creatures started to become nocturnal. They lost color vision, which they no longer needed. Only a few of us, mostly primates, have since gotten it back.
Full on warm-bloodedness develops in the Triassic period (the first period of the Mesozoic Era). Warm-bloodedness and cold-bloodedness are two ends of a continuum, and the continuum is just: "How much of your body heat do you generate internally vs. depend on the external environment for?"
Warm-blooded animals can run faster and farther with their higher metabolism, but that higher metabolism consumes more calories at rest, and need more oxygen.
Around this time, the increasing shift of legs under the body and other skeletal changes means these mammal-like creatures stop undulating when they move. They now move straighter and faster. This change allows them to stop compressing one lung with each side-to-side undulation. Not compressing their lungs helps increase their stamina, because they get more oxygen when they move.
Then things get really interesting with the jaw. The jaws of amphibians, reptiles, and mammals superficially look similar, but if you start staring at the bones underneath the flesh, you see very important structural differences.
The ancestors and relatives of mammals have lower jaws made of many bones. We only have one: the mandible. Several of the other bones fused together in mammals. As the jaw bones were fusing together in early mammals, they did so in a way that allowed stronger jaw muscles to form at the new joint. This allowed stronger biting, which allowed mammals to expand their range of food.
The change also came in concert with new kinds of teeth, and tooth alignment between the upper and lower jaws. Instead of just some pointy teeth that could puncture an animal, jaws that could snap shut, and the ability to swallow prey whole, mammalian bone-muscle-teeth changes in the jaw led to them developing the ability to chew their food.
, or else became very small and moved back into the ear, where, as the malleus, incus and stapes, they help transmit sound. You may have learned about these ear bones in school under the names hammer, anvil and stirrup.
The interesting thing is
ontogeny recapitulates phylogeny
teeth can chew food instead of swallowing it whole
oxygen
greenhouse warming
tree rings
* N. Am. terminology; elsewhere Lower Carboniferous and Upper Carboniferous.
** They are called diapsids for cladistic reasons even if individual species later lost a hole.
***This ends up being so important that for a long time "mammal" was defined as "all animals that descend from this ancestor animal that developed this new joint in the jaw."
Not what you think of when you think of a mammal! But the one that makes the most sense in cladistic terms, apparently.
Recently, apparently paleontologists have decided to define mammal as "descendants from the common ancestor of all modern living animals." This means animals that were cousins but came from a clade that didn't leave any descendants are excluded, even if they did all the things we think of as mammals doing: looking, behaving, growing, metabolizing, feeding, nursing, and grooming their hair just like mammals.
Fast forward, and there's a planet Earth, oceans, and continents. The most complex life forms were the fishes.
390 million years ago (Ma), the fishes learned to live on land. Their fins evolved into legs, which led to paleontologists calling them tetrapods. But they still had to return to the sea to lay their eggs. They were thus amphibians.
Around 325 million years ago, we were smack dab in the middle of the Carboniferous period. This means there were lots of trees in the tropics. These trees repeatedly got buried in swamps due to cyclical rising and falling of sea levels. These trees were buried in such vast amounts in the latter half of the Carboniferous that they are responsible for a lot of our coal resources today, especially in western Europe and the eastern US.
There were warm, wet forests around the tropics and ice caps around the poles. It was an ice age: the ice spread remarkably far away from the poles. This means there were strong temperature gradients between the poles and the equator. One illustration of this is gradient is that trees near the poles have very distinct growth rings, because those reflect significant seasonal temperature differences. In contrast, trees near the equator don't show marked rings, because temperatures were more stable throughout the year.
Around 325 million years ago, the polar ice cap melted, causing rising sea levels.
This coincided with one amphibian lineage splitting into two lineages, one that would become reptiles and one that would become mammals.
2 large landmasses: Gondwana at the South Pole, Laurasia at the equator. Eventually they collide and form Pangaea. That collision forms the Appalachians.
The reason there's an ice cap is that the giant trees are pulling carbon dioxide out of the atmosphere and getting buried. Less carbon dioxide in the atmosphere means lower temperatures (no greenhouse effect). Lower temperatures means growing polar ice. Growing polar ice means lower sea levels. Lower sea levels mean less flooding. Less flooding means more trees. More trees mean warmer temperatures (apparently). Warmer temperatures means melting ice, flooding, and tree burial.
Moreover, when bacteria consume organic material and cause it to decompose, they consume oxygen. This means less oxygen in the atmosphere. So burying the trees and keeping them from decomposing not only meant less carbon in the atmosphere, it meant more oxygen.
More oxygen in the atmosphere means arthropods, like insects, can grow much larger. Dragonflies could achieve wingspans of up to .75m/2.5 feet. Millipedes got up to 2.4m/8 feet long.
All of these patterns get reversed during the Permian, when the ice age ends.
While this is going on (ca 325 Ma), some amphibious tetrapods learn to insulate their eggs with fluid. This means the embryos can grow in the egg on land, without having to get their water from the ocean. This means the tetrapods can live full-time on land, i.e. not be amphibious. These creatures are called amniotes, because they lay amniotic eggs (eggs with amniotic fluid).
These amniotes are the ancestors of reptiles and mammals, both of which are full-time land dwellers.
In the middle of the Carboniferous period, the weather changed. It switched from the Mississipian period to the Pennsylvanian period*.
These amniotes then split into the future reptile and mammal lines, which are distinguished by the number of holes behind the eye sockets for anchoring the jaw muscles. Reptiles have 2 holes; they are diapsids.** Mammals have 1 hole; they are synapsids.
If I understand correctly, synapsids split off first and evolved one hole, then diapsids evolved two holes.
Then around 307-310 Ma, the Carboniferous Rainforest Collapse happened, inaugurating the Permian Period. The ice caps melted, coal swamps replaced by seasonal, semi-arid drylands, and some deserts arise in Pangaea. Trees have to be better adapted to drier conditions. Some extinctions, especially in the plants (one of only 2 extinctions in the plant record).
Amniotic eggs are now a superpower that allow you to roam more and live in drier conditions. This helps explain why in the Carboniferous, amphibians were large and numerous, but now they are small. The larger animals became reptiles and mammals.
Now we're in the Permian. We start to get more mammal-like animals. They become increasingly warm-blooded, meaning they generate more and more of their heat inside their body, instead of absorbing it from the air around them. They start to develop hair. The hair starts as only a few hairs for sensory purposes, like a cat's whiskers, but eventually starts to cover the whole body. If you're generating your heat internally (i.e. are warm-blooded), it's efficient to have an insulation layer. This is why most mammals (warm-blooded) have body hair, birds (warm-blooded) have feathers, and reptiles and amphibians (cold-blooded) don't have this insulation layer.
We're not sure *why* warm-bloodedness evolved, but being warm-blooded means you can run faster and farther without having to rest and soak up more sun.
These animals also started to change their locomotion and posture. Instead of crooked limbs that sprawl out to the side, like lizards and crocodiles, their legs become more and more straight and more and more directly underneath the body. Their limb joints can also move in more directions now, which means they can run, dig, and climb.
All these new traits combine to make them better hunters and better able to explore new terrain.
These increasingly mammal-like animals that lived during the Permian are called therapsids.
Then volcanoes known as the Siberian traps were so active that we had the famous Permian extinction event. Therapsids got a lot smaller, and that helped them survive. They could burrow into smaller spaces to find shelter, and they needed less food and could eat smaller food. Furthermore, they evolved to mature faster, producing lots of offspring and dying quickly. That numbers game helped the species survive very difficult conditions.
After the end of the Permian period, we leave the Paleozoic Era and enter the Mesozoic Era. That is the age of dinosaurs.
Because dinosaurs were becoming very big and tended to prey on small mammal-like creatures, these small mammal-like creatures started to become nocturnal. They lost color vision, which they no longer needed. Only a few of us, mostly primates, have since gotten it back.
Full on warm-bloodedness develops in the Triassic period (the first period of the Mesozoic Era). Warm-bloodedness and cold-bloodedness are two ends of a continuum, and the continuum is just: "How much of your body heat do you generate internally vs. depend on the external environment for?"
Warm-blooded animals can run faster and farther with their higher metabolism, but that higher metabolism consumes more calories at rest, and need more oxygen.
Around this time, the increasing shift of legs under the body and other skeletal changes means these mammal-like creatures stop undulating when they move. They now move straighter and faster. This change allows them to stop compressing one lung with each side-to-side undulation. Not compressing their lungs helps increase their stamina, because they get more oxygen when they move.
Then things get really interesting with the jaw. The jaws of amphibians, reptiles, and mammals superficially look similar, but if you start staring at the bones underneath the flesh, you see very important structural differences.
The ancestors and relatives of mammals have lower jaws made of many bones. We only have one: the mandible. Several of the other bones fused together in mammals. As the jaw bones were fusing together in early mammals, they did so in a way that allowed stronger jaw muscles to form at the new joint. This allowed stronger biting, which allowed mammals to expand their range of food.
The change also came in concert with new kinds of teeth, and tooth alignment between the upper and lower jaws. Instead of just some pointy teeth that could puncture an animal, jaws that could snap shut, and the ability to swallow prey whole, mammalian bone-muscle-teeth changes in the jaw led to them developing the ability to chew their food.
, or else became very small and moved back into the ear, where, as the malleus, incus and stapes, they help transmit sound. You may have learned about these ear bones in school under the names hammer, anvil and stirrup.
The interesting thing is
ontogeny recapitulates phylogeny
teeth can chew food instead of swallowing it whole
oxygen
greenhouse warming
tree rings
* N. Am. terminology; elsewhere Lower Carboniferous and Upper Carboniferous.
** They are called diapsids for cladistic reasons even if individual species later lost a hole.
***This ends up being so important that for a long time "mammal" was defined as "all animals that descend from this ancestor animal that developed this new joint in the jaw."
Not what you think of when you think of a mammal! But the one that makes the most sense in cladistic terms, apparently.
Recently, apparently paleontologists have decided to define mammal as "descendants from the common ancestor of all modern living animals." This means animals that were cousins but came from a clade that didn't leave any descendants are excluded, even if they did all the things we think of as mammals doing: looking, behaving, growing, metabolizing, feeding, nursing, and grooming their hair just like mammals.
