Tales of Evolution: The rise of the mammals
In part 1 of this series we looked at how our fish ancestors colonised the land 360 million years ago (mya), and gave rise to the early amphibians. Here in part 2 we’ll be looking a bit later in the evolutionary journey, at the transition from reptiles to mammals.
The early amphibians had several key adaptations that allowed them to thrive on land – including sturdy limbs and air-breathing lungs. However, like almost all modern amphibians, they would have had to lay their eggs underwater – otherwise the eggs would dry out and the offspring would die. This limited them to areas with ponds and rivers. However, one group began to evolve adaptations which were better suited for dry land. They produced hard, waterproof eggs which could be laid on land, allowing them to live in hotter, drier areas. By 320 mya, this group had evolved from amphibians into early reptiles.
Early reptiles: sauropsids and synapsids
These early reptiles soon split into two groups: the sauropsids and the synapsids. The two groups were quite similar, but with slight differences in skull structure, which affected how the jaw muscles attached to the skull. The sauropsids would give rise to all modern reptiles and birds, and the synapsids would give rise to the mammals. From 320 to 250 mya, the synapsids were the dominant group of early reptiles, with most of the top land predators being from this group. Then came the end-Permian extinction– the worst mass extinction event the world has ever seen. It lasted 60,000 years, during which time up to 95% of all species became extinct. Most synapsid species also became extinct, and one group of sauropsid reptiles took their place as the dominant land predators – the dinosaurs. The few synapsids that survived were small insect-eaters, and much of the evidence suggests that they were almost all nocturnal – being active at night and sleeping during the day. This is likely due, in part, to the fact that those which were active during the day would have been eaten (or outcompeted) by dinosaurs. However, being nocturnal comes with a lot of problems, including sensing nearby prey when it’s too dark to see.
Reptiles and mammals are quite different in terms of the way their ears work, and this is partly due to the nocturnal ancestry of mammals. Reptiles have a single, small bone connecting the eardrum to the inner ear (where sound waves are converted to electric signals and sent to the brain). Mammals, on the other hand, have three small bones connecting the eardrum to the inner ear. These are called the malleus, incus and stapes. Having three bones rather than one allows for a more efficient transmission of sound waves to the inner ear, allowing mammals to hear higher frequencies than reptiles can. This would have been particularly useful for the nocturnal, insect-eating mammal ancestors, as it would have enabled them to hear the high frequency sounds produced by their insect prey. But how did these two extra ear bones evolve? Strangely, these ear bones used to be jaw bones. The lower jaw of reptiles connects to the rest of the skull via two large bones called the angular and the quadrate. Over the course of synapsid evolution, these large bones became much smaller, and the lower jaw became much larger – enough that it could directly connect to the rest of the skull. The angular and the quadrate were then free to take on a new role in helping the existing ear bone connect the eardrum to the inner ear. Morganucodon, a small mouse-like synapsid that lived 200 mya, shows us how this transition happened. The relative sizes of the lower jaw, angular and quadrate bones were halfway between those of reptiles and of mammals, and so the lower jaw connected to the rest of the skull in two places: via the angular and the quadrate (as in reptiles), but also directly (as in mammals). Whilst this transition from jaw bones to ear bones may seem odd, the jaw bones and ear bones develop from the same cluster of cartilage in the embryo. And so, the evolution of one to another would only have required minor changes in development to resize and reposition the bones. Interestingly, while mammals have improved hearing compared to reptiles, we have poorer colour vision. Most reptiles have four different colour receptors, whilst most mammals only have two. It is likely that the two other receptors were lost in evolution during this period of nocturnal living, as the ability to distinguish a wide range of colours was no longer useful.
Evolution of the mammal ear bones: Dimetrodon (300 mya), an early synapsid, has the lower jaw connected to the upper skull via the angular and quadrate bones. The stapes is the only ear bone. Morganucodon (200 mya), a later synapsid, has the lower jaw connected directly to the upper skull, as well as via the angular and quadrate bones – there are two jaw joints. Hadrocodium (195mya), an early mammal, has the lower jaw connected directly to the upper skull. The angular and quadrate bones have become much smaller and are now part of the ear rather than the jaw. Image redrawn and adapted from reference.
Being active only at night also raises another problem – that of staying warm. Most reptiles are heavily dependent on sunlight to keep them warm, and will spend long periods of the day basking in the sun to keep their bodies at the right temperature. So how did our synapsid ancestors keep warm when they were only active at night? The answer is that they began to produce their own heat. Mammals, when compared with reptiles, are very inefficient when it comes to using oxygen and food for energy. In fact, mammals have to eat much more food than reptiles do in order to survive. This is because mammal cells “lose” a lot of the energy they make as heat. But this, combined with another unique trait, means we can keep ourselves warm without needing to be in the sun. This unique trait is called hair. Our hair traps a layer of air around our skin, which helps to keep in the heat our bodies produce. It is likely that hair first evolved as whiskers, used for sensing prey rather than for keeping warm. Only later would hair evolve to cover the rest of the body and provide insulation. Although soft tissues like hair don’t fossilise as easily as hard tissues like bone, there are some fossils which give us an indication of when body hair evolved. Castorocauda and Megaconus, which both lived about 164mya, appear to have had dense coverings of hair.
By around 160 mya, we begin to see true mammals emerging, with the appearance of the three mammal groups: monotremes, marsupials, and placentals. The monotremes, like the platypus, are the most reptile-like mammals, as they lay eggs rather than give birth to live young. The marsupials, like kangaroos, give birth to very underdeveloped young, which then live in the mother’s pouch. The placentals, which include ourselves, give birth much later as our young remain inside the mother’s womb and are nourished via the placenta. However, despite the diversification of these different groups, mammals were still predominantly small, nocturnal creatures. But then, around 65 mya, the age of the dinosaurs came to an end. With dinosaurs extinct, the mammals were able to increase in size and diversify further, and have continued to do so over the last 65 million years. Although highly diverse, we still share many traits which we can trace back to our small, nocturnal ancestors. Our advanced hearing, reduced colour vision and warm, hair-covered bodies are all features we have inherited because of their long evolutionary fight to survive.