The Evolution Of Mammals: We’re Getting Warmer
What’s that down there, between the dinosaur’s feet?
It looks warm and fuzzy.
Mammals. We are biased, but they are pretty great. To the little shrews that ran in between the dinosaurs’ feet trying to avoid being squashed to the graceful giant whales that thrive in the freezing arctic. It’s just amazing what we’ve managed to adapt to. Don’t get us wrong, we are no bacteria, we can’t survive in space and still need water and air. But we do pretty well for ourselves. Let’s dive in, to mammal evolution.
The Earliest Mammals: Extinct giants and tiny egg-laying survivers
Mammals evolved from reptiles just after dinosaurs evolved, roughly 200 million years ago. However, because mammals were a bit late, dinosaurs had already spread out and dominated a lot of ecosystems across the Earth: there wasn’t a lot of room for mammals. This meant that the mammals that evolved today, only really started to evolve once the dinosaurs died out about 60 million years ago.
Despite mammal evolution only really taking off in a post-dinosaur world, there were some really weird mammals living alongside the dinosaurs. Some of the earliest mammal-like-animals that lived alongside dinosaurs were called ‘synapsids’. Synapsids were basically dinosaur-style animals that started growing fur and developing different hips from their lizard ancestors (see our post on dinosaurs). Some of these early synapsids were very striking, they included some massive dinosaur-tusked-elephant things (see Gorgonpsia). However, the meteor event that wiped out the dinosaurs also wiped out 70% of life on Earth at the time – including these synapsids. So a number of strange unrecognizable branches of mammals completely died off. We will never know what weird dinosaur-mammal-hybrid-creatures could have ruled the planet. Mammal evolution started again.
The mammals that survived the meteor, tended to be much smaller and very different to modern mammals. Platypuses and echidnas are probably their closest relatives alive today. Platypuses and Echidnas are unique among surviving mammals, as they still lay eggs, a trait they’ve retained from their reptile ancestors. These sorts of animals, egg-laying mammals, are called ‘monotremes’. The earliest of them probably evolved about 200 million years ago, at the very beginning of the dinosaurs (see Morganucodon).
The thing that makes monotremes mammals is that they are “warm-blooded” (endothermic). This means they generate their own body heat through burning energy obtained by eating food. Reptiles are “cold-blooded” (ectothermic) their internal temperature is set by their outside environment.
There are a lot of advantages to being a cold-blooded reptile. Most reptiles can survive on much less food than we can. For instance, an adult male crocodile can go for up to six months without eating – despite being a few hundred kilograms in weight. Reptiles can also survive much greater variations in temperature than mammals. In fact, some turtles can even be frozen solid, and then be thawed out and be fine – that wouldn’t go well for us mammals. Reptiles can survive a lot, and don’t need a lot to survive.
Mammals are not as robust as reptiles. We need to constantly keep our core temperature high, otherwise, our internal chemistry breaks down and we die. We evolved fur to try and keep the heat in and need to eat a lot more food, so that we have energy to keep us warm. However, it gives us one advantage – we can be a lot more active than reptiles. In particular, we can be more active when it is cooler, like at night and early in the morning – lizards basically can’t do anything early in the day until they warm up by lying on a rock in the sun. We can also be active for longer – lizards can sprint for a bit, but can’t run and hunt for hours like us mammals. And having a more controlled internal temperature allows us to do some more sensitive chemistry – like what’s required for a complex brain.
Marsupials: Children are the future
The next big step in mammal evolution was marsupials. Marsupials don’t lay eggs. Instead, they lay a very small, very fragile offspring, and then take care of them in some form of a pouch. For instance, a mother kangaroo is only pregnant for about a month. However, the baby kangaroo only weighs about a gram when they are born. The mother then feeds them a diet of very rich milk and the baby triples in size within a few weeks.
This way of creating young gives marsupials a bit of an advantage over reptiles. Reptiles generally don’t care for their young: they lay eggs and then leave them somewhere. This means young reptiles need to fend for themselves and many of them get eaten and don’t survive to adulthood. To make up for this, reptiles generally lay a lot of eggs, to try and make sure some survive – often dozens. Marsupials have a different strategy – they take care of their young and protect them, almost like a reptile carrying their eggs with them. This means they can’t have as many young (because actually looking after your kids takes a bit of effort) but the child is much more likely to survive to adulthood with their parent’s help and protection.
Most marsupials live in Australia. They include a lot of the typically famous Australian animals – kangaroos, wallabies, koalas, and wombats. Marsupials have died out in most other regions and been replaced with other more modern mammals. However, there are still some outside of Australia, in the Americas and surrounding islands. There were also some very large and extraordinary extinct marsupials. Many lived along humans in Australia as recently as 40,000 years ago, including marsupial versions of lions and hippos.
Placental mammals: Bearing young
The final step in mammal evolution is the evolution of the placenta. Instead of just caring for their young in a pouch, like marsupials, placental mammals raise their young inside them, providing nutrients directly through their blood. Almost all the mammals we think of today are placental mammals: rodents, carnivores, primates etc. These modern mammals all evolved really quickly filling the ecological spaces left by the dinosaurs dying out 60 million years ago.
Creating young out of your own body, makes the whole process a lot more intense. Reptiles just go and lay some eggs on a beach somewhere, and let the young develop, grow and hatch on their own after gestating for a few weeks. Mammals take months to develop and grow their young inside them: elephants are pregnant for 22 months. Then, placental mammals need to work out a way to get the young out of their body – childbirth. This is an incredibly dangerous process for a lot of mammals – even in humans. Without modern medical care, roughly one out of eight childbirths will result in a death of the mother or child.
This greater investment of the parent’s time and resources means that mammals need to be a lot more committed to their young. Most fish will eat their offspring without a second thought: they lay millions of eggs, so they are not particularly invested, it doesn’t matter if some, or even most die. Reptiles generally don’t do this, they at least lay their eggs somewhere somewhat protected. But they don’t recognize their own offspring if they run into them later in life and certainly don’t care for them in any particular way. However, almost all mammals will care for their offspring for a significant period of time after their birth. Raising them until they are able to fend for themselves. It’s a clear difference in the animal kingdom.
This new relationship of mammals with their young is the origins of something very special – familial love. Now, we don’t want to get all sappy, plenty of other animals care for their young – octopus and birds certainly do. But mammals were the first large group of animals for which it was really essential. The only way for a mammal species to succeed is if mammalian parents care for, and care about their young. And for a lot of mammals to survive, they need to care for the mothers – mothers tend to be a bit slower and more vulnerable when pregnant or caring for young children.
This new relationship between mammals and their young also changes their approach to surviving. Instead of a completely brutal survival of the fittest competition, mammals start co-ordinating and moving together. Again, it’s easy to exaggerate this – plenty of mammals are solitary hunters and plenty of other animals move together in flocks and shoals. However, a lot of mammals have developed much more complex ways of relating to each other than the rest of the animal kingdom, like social structures with designated roles in order to protect their vulnerable. Examples of these structures include the prides of lions, herds of buffalo, pods of whales – often, but not always, lead by a dominant male with a harem of females and some succesor males. Some animals even have rituals where new members are granted membership of these groups or mourning those who exit. For example, elephants shake trunks to meet up with old friends and hold funerals for their dead. Mammals are the origin of something very special in the animal kingdom.
Placental mammals: Bearing young
The evolution of mammals is a story of love. Mammals are the point when animals evolve from solo organisms that come together occasionally to reproduce, into animals that live together in groups to take care of each other. From the early monotremes that lay eggs in burrows, but take exceptionally good care of them compared to their reptile ancestors. To the marsupials, animals who evolved to carry their young with them in pouches to nurture them and keep them safe. Then finally to placental mammals, the herds of mammals cross the Earth today, that care for their young from their very own flesh.
Mammal evolution is the story of a group of animals discovering how to raise and nurture their offspring in more connected and intimate ways. And as part of that, mammals worked out that it’s good to form some social bonds and work together. In some ways, it’s the originals of civilization itself – coordinating to take care of one and other. It’s a truly beautiful thing and the start of many of the best aspects of humanity. We should cherish our mammalian companions. They have some of the best parts of us.
