The Origin and Evolution of Mammals - Moodle

collection, ingestion, and assimilation must rise, with
implications for the design of the sense organs, the
locomotor system, and the central nervous control.
Increase in gas exchange requires more effective ventilation
such as is provided by a diaphragm and freeing
of the ribcage from a simultaneous locomotor
function. These add to the requirements of the actual
regulatory systems, such as elaborate internal nervous
and endocrinal monitoring systems and high
blood pressure to increase the kidney filtration rate.
For thermoregulation, variable insulation, cutaneous
blood flow rates, and evaporation mechanisms are
just some of the necessary components of the system.
Organisms maintaining high chemical and temperature
gradients with the environment cannot be
very small because of the surface area to volume
consideration. Therefore, a juvenile of an already
small mammal cannot exist independently, relying
on its own regulatory mechanisms, which in
any case take a significant time to develop fully.
Therefore, parental maintenance of what amounts to
a regulated external environment become necessary.
In the first mammals this was presumably in the
form of a nest, or conceivably a maternal pouch in
which the egg and neonate existed in a controlled
temperature and humidity, with the molecular
requirements provided by lactation.
Seen in this light, there is no identifiable, single
key adaptation or innovation of mammals because
each and every one of the processes and structures
is an essential part of the whole organism’s organisation.
To regard for example endothermy, or a large
brain, or juvenile care as somehow more fundamental
is arbitrarily to focus on one point in an interdependent
network of causes and effects. Endothermy
is necessary for maintained elevated levels of aerobic
activity, but the activity itself is simultaneously
essential for collecting enough food to sustain the
high metabolic rate. The large brain causes high levels
of learning and social behaviour, but the latter are
necessary for the parental care that allows the offspring
time to develop the large brain in the first
place. Lactation is on the one hand necessary for
mammalian development, yet on the other can only
exist by virtue of the high metabolic rates and efficient
food collection. Which has ontological priority?
To return to the question posed at the start of this
section, what was the habitat, or better perhaps the
EVOLUTION OF MAMMALIAN BIOLOGY 133
niche to which the ancestral mammal was adapted?
It must have involved a significantly fluctuating
temperature range over which activity was maintained.
It would have been dry at least at times.
There would have to be abundant highly nutritious
food but which required a particularly agile locomotory
ability to acquire, and which suited a small
animal weighing only 5–10 g. The physical habitat
would have been very heterogeneous and complex
to negotiate. A small, nocturnal insectivore living
on the forest floor and capable of tree climbing
sounds about right! Certainly there seems to have
been little competition for this habitat at the end of
the Triassic. Small lizards and their lepidosaurian
relatives were presumably diurnal insectivores as
now, and the archosaurs were starting to flourish as
the large tetrapods of the day. Birds had barely even
started their evolutionary journey down what was
to prove, 60 million years later, to be a remarkably
convergent route.
How was organism-level integration maintained
during the transition?
There is a paradox when matching an evolutionary
mechanism based on single, small changes in discrete
characters to a long term, large evolutionary
change in very many, fully integrated characters.
The fossil record of the mammal-like reptiles and
the transition to mammals supports the resolution
to the paradox termed ‘correlated progression’. In
this model, all the major processes and structures of
an organism are integrated such that each is both
necessary for, and permitted by the rest. No one
attribute can evolve and yet remain functionally
useful, without being accompanied by appropriate
changes in the others. However, the functional linkage
is presumed not to be completely tight. A small
amount of change in one attribute is possible and
can be adaptive while still remaining adequately
integrated with the rest. For example, the plausibility
of a small rise in metabolic rate caused by a few
per cent increase in mitochondrial numbers without
needing any immediate increase in the ventilation,
feeding, or vascular systems has already been
proposed. The sensitivity of the middle ear might
well increase a little, even within the constraints of
the existing level of central nervous organisation.
However, only small degrees of change in single