Teaching in Cambridge: the origin of mammals





The mammals of today – rats and horses, elephants and monkeys, whales and bats and many more - are very distinct from their closest living relatives, the reptiles. They have an extraordinarily high-energy lifestyle manifested by a metabolic rate and sustainable activity level around ten times higher; far more adaptable behaviour driven by a brain 5-10 times larger; and a unique means of nourishing their developing young by milk. Taken together these biological attributes endowed mammals with the ecological potential to occupy almost the entire range of terrestrial, and several freshwater and marine habitats. In these four lectures we will build up a comprehensive picture of how, why and when this radically new kind of organism first evolved. We will draw on evidence from (i) the excellent fossil record of the lineage that culminated in mammals, (ii) the geological and palaeoecological circumstances of the times, and (iii) our broad understanding of the biological nature of mammals and of the evolutionary process.



Lecture 1. The fossil history of the synapsids (stem-mammals)


A series of unique characters shows us that modern mammals are a monophyletic group, distinguished from their living sister group Sauropsida (reptiles+birds). The stem mammals, or synapsids, diverged from close to the very base of the amniote phylogenetic tree in the Carboniferous. The initial phase of synapsid evolution consisted of the the Upper Carboniferous-Lower Permian pelycosaurs, which established the first terrestrially-based tetrapod fauna, but remained restricted to tropical, humid conditions. The second phase of synapsid evolution commenced in the Middle Permian with the sudden appearance of therapsids. These had higher metabolic rates and activity levels, which adapted them for cooler, seasonal climates of higher latitudes. They rapidly diverged into several taxa of herbivores and carnivores, and achieved world-wide dominance on land for the rest of the Permian Period. However, the great end-Permian mass extinction wiped out almost all the therapsids. The third phase of synapsid evolution resulted from one of the few surviving groups, the cynodonts. These evolved increasingly mammal-like forms through the Triassic, and culminated in the emergence of the ancestral mammals by the end of that Period. The first mammals were small bodied, nocturnal insectivores living in a world increasingly dominated by dinosaurs.



Lecture 2. The evolution of mammalian biology: jaws, teeth, and sense organs.


The sequence of synapsid fossils shows how the jaw-closing musculature gradually reorganised to create a combination of a very large yet at the same time very precisely applied bite force that is unique amongst vertebrates. Correlated with this, occluding teeth evolved that worked together, uppers against lowers, which increased the rate of food assimilation and extended the range of potential foodstuffs. The tooth-bearing dentary bone of the lower jaw increased in size relative to the postdentary bones. This had the surprising effect of increasing the sensitivity to air borne sound picked up by the back of the jaw, and transmitted to the inner ear via the jaw hinge bones, the precursors of the mammalian middle ear ossicles. The fossils also provide indirect evidence for an increasing sense of smell, and of eyes able to see at night.



Lecture 3. The evolution of mammalian biology: locomotion, ventilation and brain.


Another radical change between mammals and basal amniotes was the loss of lateral undulation, and evolution of a new gait with the feet underneath the body and the belly off the ground. The vertebral column evolved the characteristically mammalian differentiation into a highly mobile neck, a ventilatory thoracic region, a force transmitting lumbar region, an elongated sacral region for attachment of the pelvis, and a reduced tail. The functional significance of these changes is a combination of increased ventilation and increased agility of movement. The therapsid skeleton indicates an intermediate stage in which the basal pelycosaur sprawling mode and the mammalian parasagittal gait were both possible, as seen by analogy in modern crocodiles. The secondary palate in the skull, and the form of the ribcage indicate an enhanced ventilation rate. The brain enlarged, initially the cerebellum in the cynodonts correlated with fine neuromuscular control. Only later, in the ancestral mammals did the forebrain expand significantly, which suggests they had evolved more complex behaviour.



Lecture 4. The origin of mammals: endothermy and the whole picture.


Endothermy, and the high energy turnover associated with it, plays a pivotal role in understanding mammals. There are several conflicting theories for why endothermy first evolved, attributing it to one or another single selective force. In contrast, the correlated theory proposes that endothermy evolved as an integrated system, in which the several individual functions associated with the constant elevated body temperature and the increased aerobic activity gradually evolved together in a co-ordinated fashion. The essence of mammalian biology centres on the higher metabolic rate necessary for accurate physiological regulation of the animal’s internal environment – thermoregulation and chemo-regulation.  This regulation is both necessary for, and at the same time facilitated by the functioning of a complex brain and the associated behaviour. The correlated progression model accounts for the origin of the mammal’s high level of independence from external environmental fluctuations in temperature, humidity, and physical terrain. The potential to increase the range of geographic regions, habitats, niches, and lifestyles that occurred during the subsequent radiation of mammals was a consequence.

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