The Rise and Reign of the Mammals

The hallmarks of mammalness evolved, piece-by-piece over the last 325m years:

• 303-307m ya - The Carboniferous Rainforest Collapse - The climate became drier, temperatures swung cold and hot, and the ice caps melted, eventually disappearing for good in the ensuing Permian Period.
• 252-251m ya - The Permian-Triassic transition. 90% of species were wiped out. Mega volcanoes erupted for several hundred thousand years. Animals that survived became smaller (the Lilliput Effect). By growing fast and reproducing earlier, they have a better chance of successfully making it through a mating season, ensuring that their genes are passed on in this harsh, mercurial world.
• 230m ya - First Mammals
• 201m ya - Start of the Jurassic - Dinosaurs got bigger, mammals got smaller. Mammals were better than the dinosaurs at being small.
• 145m ya - Start of the Cretaceous
• 125-80m ya - Cretaceous Terrestrial Revolution - Angiosperms, flowering plants appear and quickly take over 80% of the flora, bringing alongside them pollinator. Towards the end, grass starts to appear
• 66m ya - End of the Cretaceous, beginning of the Paleocene - an asteroid hit the Yucatan peninsula with the force of over 1bn nuclear bombs, punching a hole in the crust 40km deep and over 160km wide. Almost instantaneously, everything within about 1,000km was vaporized, and the nuclear winter lasted several years. About 75% of species, including all dinosaurs except birds, died out. Only disaster species survive in darkness and filth, small and omnivorous. Within about 375-800k years temperatures stabilized and ecosystems recovered, and mammals take over. Now birds get smaller and mammals get bigger
• 56m ya - Paleocene-Eocene Thermal Maximum (PETM)- hottest period since the end of Cretaceous. The exemplary global warming event in the geological record, instigated by volcanoes and causing 200k years of upheaval, and creating the PETM Trinity of mammals all over the northern continents:
  • Primates
  • Even-toed artiodactyls
  • Odd-toed perissodactyls
• 48m ya - Middle of the Eocene. Mammals start to modernize.
• 34m ya - Eocene/ Oligocene transition - global temperatures dropped and precipitation became scarcer. Grass, able to tolerate harsher conditions, replaced thirsty jungles and forests over 10m years
• 5-7m ya - humans split off from primates but continue to pass genes back and forth until about 4m ya.
• 3.3-2.7m ya - The Ice Age begins. We’re still in it. Even 10k ya about half of North America was a frozen wasteland, along with northern Eurasia. North and South America touch, causing the Great America Interchange, where mammals from the north rampaged over the south
• 300k ya - homo sapiens appear
• 100k-40k ya - our modern body shapes became fixed. Also at this time, human population sizes increased, and technological and cognitive innovations spread.
• 50k ya - tools and other artifacts had become more elaborate. There are burials, construction of dwellings and other weather-proof structures. We become modern in the way we think, communicate, worship and seek meaning. We pour out of Africa, arriving in Australiz, North America (30k-15k ya). This is also the moment of the Near Time Extinction, where we met the ice age megafauna and much of that megafauna died. This is the blizkrieg theory. The other obvious suspect is climate change
• 23k ya - wolves start to live near us and start evolving into dogs. Since then, we have domesticated more than 25 species, breeding billions of pigs, sheep, cows, etc
• 12k-10k - In the Neolithic, much of this domestication of animals and plants began, ushering in the Agricultural Revolution, where we went from nomadic hunting and gathering to a more sedentary existence, building town and villages, irrigation canals. With a ready source of ever-available food, our population expanded exponentially and since only some people had to spend time producing the food, we could diversify and have people become doctors, priests, architects, couriers, builders, teachers, politicians, and scientists.
• 10k years ago - mamths and saber-toothed tigers suddenly became rare

What Makes a Mammal?

• Three are three modern groups of mammals:
  • Placentals - The placenta acts as the baby's pantry, lungs, and excretory system at the same time. Four main groups:
    • Afrotheria - Most of whom live in Africa today
    • Xenartha - Most South American species like anteaters, sloths and armadillos
    • Laurasiatheria - Dogs, cats, pangolins, odd and even-toed hoofed mammals, whales, and bats
    • Euarchontoglires - Us, other primates, rabbits, and rodents
  • Marsupials
  • Monotremes
• Warm-bloodedness
• Hair likely began as a sensory aid (like whiskers, a display structure, or as part of a gland-based waterproofing system, and was later repurposed as a body coating to retain heat. Once an animal has a lot of hair on its body, it's a tell-tale sign that it is producing at least some body heat internally, and doing its best to keep it from escaping. Making heat is expensive. Iv you're going to run your furnace at full blast, you'll want to close the windows, too, lest your gas bill will become unsustainable.
• Small size, warm-blooded metabolism, and stronger and more efficient bites developed together as part of a package deal.
• Mammals can sustain running speeds 8x faster than lizards and can forage for food across larger areas.
• They have to eat a lot more calories than cold-blooded animals of similar body size, and they have to breathe a lot more oxygen.
• Mammals developed a secondary palate - a hard roof of the mouth - which divides the mouth from the nasal passage. Lungs could take in more oxygen.
• The dentary-squasmosal joint allowed a completely new, more efficient way of eating. By chewing their food to mush, early mammals could do most of the processing in the mouth, beginning digestion before the food hit the stomach.
• The first Triassic mammals developed the neocortex - becoming brainier as they became better chewers.
• Mammary glands - Feeding babies with milk gives a steady source of food and fosters a bond between mother and child, which is important for cognitive and social development. For lactation to work , the babies must have diphyodonty (two sets of teeth, milk and adult), a hard palate and a muscular throat for sucking.
• Milk and larger brains evolved around the same time.
• Ear bones are jaw bones that evolution repurposed for hearing.
• New types of molars grind and shear food - come from eating insects
• Carnivorans, with razor teeth move to the top of the food chain
• Waif dispersals - small mammals on foliage rafts cross water and colonize new lands.
• The blue whale is the heaviest animal of any kind ever, probably 30-40 tons more than the biggest dinosaur
• Elephants are the largest land mammals today
• From 100m-20m ya, Africa was almost entirely isolated
• There may be some limit to overall land mammal body size, dictated by diet (normally herbivores), temperature (the risk of overheating gets more acute the bigger you get).
• Why didn’t mammals get as big as the dinosaurs? Probably because of lungs.
• The big brain club (where brain in comparison to body size is about primate level) include whales, elephants
• The only mammals that has ever flown are bats. There are 1,400 bat species and 20% of mammals alive today are bats. Echolocation likely evolved after flight
• Tall teeth allows mammals to graze and wear down their teeth for longer. Rabbits and rodents make the most of this, 10m years before horses, though by 5m ya horses are the experts. The growth of grasslands led to an enormous diversification of mammals.
• Wooly mammoths are the best understood extinct species, with DNA preserved in frozen mummies. They seem to have had a social life, recorded in cave paintings.
• LA Brea Tar Pits have amazing fossils of sabre-toothed tigers
• Now there are 6,000 species of mammals.
• Fruit-eating may have been pre-requisite for bigger brains. Grasping and fruit-eating seem to be linked. long-fingered hands, flexible wrists and ankles, and gently cusped molars are vestiges of lunging for fruit in trees. The primate ancestors became better at climbing trees and became smarter and sharper eyed. Brains got bigger and reorganized, with a larger neocortex for sensory integration and larger visual regions that developed as the olfactory regions truncated, reflecting a trade of smell for eyesight. Their bulbous forward-facing eyes could see in 3D and some in technicolor.
• Plato called. humans « featherless-bipeds ». Apart from birds and some dinosaur antecedents, we are some of the only animals that habitually move on two feet, freeing our hands to do other things. Other defining components
• Walking on two legs came before big brains and learning to shape tools from stone, at least 3.6m ya.
• Around 3.5m ya there were many hominin species living together, but now we are the only one left.
• Around 3.4m ya we find butchered animal bones with cut marks from stone tools, followed by the tools themselves. From this point, the archaeolical record begins.
• Eating meat meant more calories than leaves and bugs. Teeth and chewing muscles got smaller and more idle time meant more opportunities for socializing, communicating, teaching, learning.
• Making tools meant that we didn’t need to wait for natural selection to produce new teeth and claws for obtaining new foods. We could do it ourselves.