The Origin and Evolution of Mammals - Moodle

elevation of maximum aerobic activity. Virtually all
authors have made the assumption that such a
complex arrangement could not have evolved in its
entirety in one step, and therefore must have arisen
by a simpler route consisting of two successive
steps. The argument then shifts to which function
was initially selected for, and which only subsequently.
The underlying assumption is arguably
false, however, and therefore the ensuing question
is the wrong question. The correct question is not a
matter of the order in which the parts of the complex
whole changed, but rather of how the various
evolving characteristics are interrelated such that
every stage in the transition from fully ectothermic
organism to fully endothermic organism remained
a viable, integrated entity. This approach to
understanding the origin of mammals is taken
up a fortiori later, but for the moment the particular
case of the origin of endothermy can be illustrated
by way of a plausible scenario (Fig. 4.13). Suppose
a mutation occurred that caused a small increase in
number of mitochondria in all the cells. This would
increase the maximum level of sustainable aerobic
activity a trifle, permitting a few extra minutes in
the chase. Simultaneously and unavoidably, the
greater heat produced by the extra mitochondria
would incrementally increase the animal’s average
body temperature by a few degrees, perhaps enough
to permit an extra half an hour’s activity before torpor
set in at nightfall. Not much else can happen
now until maybe a small incremental increase in
oxygen uptake, sufficient to support another increment
in BMR, or perhaps a modification to the skin
vascularisation reducing its conductivity slightly.
A hypothetical model like this indicates that both
the main functions of endothermy can in principle
evolve simultaneously and incrementally, with neither
having primacy over the other. Furthermore,
within the constraints imposed by the functional
integration of organisms, it is actually a much simpler
explanation than that based on the serial accumulation
of separate functions.
Meanwhile, in this light the question of when
endothermy evolved becomes transmuted into the
question of what level of endothermy had been
achieved by this or that particular stage. There is little
doubt that the pelycosaur-grade synapsids were
ectothermic, as witness particularly the dorsal sail
EVOLUTION OF MAMMALIAN BIOLOGY 129
of Dimetrodon and Edaphosaurus. At the other
extreme, a case has already been made that cynodonts
had achieved a relatively high level of
endothermy. The largest problem is assessing the
status of pre-cynodont therapsids such as gorgonopsians,
therocephalians, and dicynodonts. The
impressive modification of the locomotory mechanics
at the basal therapsids level supports the view
that there were enhanced aerobic activity levels.
Their widespread abundance in temperate regions
of the world suggests a reasonable degree of
thermoregulation.
As to the temperature physiology of the earliest
mammals themselves, the story is complicated by the
process of miniaturisation that occurred in the lineage
which led to them. Very small organisms face an
array of biological constraints and potentials differing
in several respects from those of otherwise comparable
larger ones, as will be discussed presently
(page 135). Nevertheless, it is hard to doubt that animals
with the complete anatomy of modern mammals
did not share their complete general physiology
as well.
An integrated view of the origin of
mammalian biology
Having reviewed what can be inferred from the
fossil record about the evolution of the mammalian
condition of various separate functional systems,
it is now appropriate to take an integrated view
of the evolution of mammalian biology as a whole.
Comparison of a typical modern reptile such as a
lizard or crocodile with a primitive living mammal
reveals immediately just how great was the transition
from their last common ancestor to the first
mammal, and how it affected virtually every physiological
and anatomical feature. The evolutionary
paradox implied by this observation has been long
and widely appreciated; indeed, ever since evolution
by natural selection became accepted as the
overarching explanation for the diversity of life. On
the one hand, evolutionary change is caused by
mutations in genes that affect discrete features, yet
on the other hand organisms must remain complex,
tightly integrated individuals in which all the structures
and processes are designed to interact with
one another to generate the overall biological nature