By Rosalyn Brady
October 19, 2023
UPDATED 12:00 PM EST
[Image Credit: Artistic reconstruction of early mammal ancestors (species: Hydrocodium wui) shown hunting insect prey, illustrating how the adoption of an insectivorious diet and miniaturization played an important role in mammal evolution (image by Dr. Stephan Lautenschlager, University of Birmingham)]
Although mammals only truly emerged after the extinction of the dinosaurs, the beginning of mammalian evolution was already taking place during the late Carboniferous period, about 312 million years ago. It was at this point that synapsids, a classification which eventually grew to include all mammals, split from sauropsids, which now include all reptiles and birds. What distinguished synapsids was the presence of temporal fenestra, a single hole behind their eye sockets to which jaw muscles attach. The production of amniotic eggs, which contain amnions - thin fluid membranes enclosing and protecting the embryo - are common between synapsids and sauropsids, and distinguish them as a subclass of Tetrapoda (four-limbed vertebrates).
Most early synapsids looked similar to lizards, and had carnivorous or insectivorous diets and perhaps even scales, if not reptilian scutes on their skin as the synapsid Dimetrodon had. Many mammalian traits, such as bony secondary palates separating noses and mouths, complex dental patterns, lower jaw bones being incorporated into the middle ear, and upright limbs are shown to have evolved during the age of non-mammalian synapsids.
Mammalian fur and the scales synapsids may have had come from a similar source: placodes, patches of thick skin created by columnar cells. The presence of hair differs from mammal to mammal, and evolved for a variety of reasons: as tactile sensory receptors, protection against too much or too little heat, or, in the case of quills, as a defence against predators.
Milk secretion from mammary glands originated soon after the split between sauropsids and synapsids as a way for synapsids to keep their drying-intolerant eggs moist, and the mammary gland has its origins in an ancient apocrine gland, which produces sweat, is associated with the hair follicle, and opens into hair follicles in modern mammals. The existing presence of lactose, anti microbial agents, and nutrients including calcium, phosphate, and iron-binding proteins allowed this primitive milk to provide nutrients to eggs.
Eventually, milk replaced the nutrients in eggs as a primary food source for young mammals, and gradually, egg laying was abandoned in favour of live birth. Until then, the composition of the milk was primitive at first, as a way to provide calcium and protein to eggs, before gradually becoming more complex after hatching. However, the advent of live birth meant that milk composition became more complex and nutrient rich during the entirety of lactation.
Warm blood, another mammalian feature, fully evolved around 233 million years ago and evolved quickly, over less than a million years. Even some of the earliest non-mammalian synapsids were developing warm blood in order to increase aerobic capacity to reach higher speeds and higher levels of exertion, over longer times.
With the development of these traits, a group called therapsids eventually became distinguished from synapsids, with upright limbs, larger temporal fenestrae, and specialized teeth - molars, canines, and incisors. While most of these became extinct at the end of the Triassic period, a group called cynodonts, which had already begun evolving many mammalian features, survived and eventually gave rise to modern mammals, the first of which, Brasilodon quadrangularis, existed about 225 million years ago.
For most of our existence, mammals have been small creatures, living in the background of larger ones such as the dinosaurs. But after the asteroid impact that eradicated dinosaurs occurred, these mammals were able to survive due to their generalist, adaptable diets allowing them to eat anything available and their ability to burrow underground and protect themselves from things such as tsunamis and extreme temperatures. After this, an open world with more space and no dinosaurs allowed for a great deal of genetic diversity to emerge - leading the evolution of mammals to where we are today.
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