This chapter is about the evolution of birds from a possible candidate reptile and
then suggests an evolutionary pathway from a bird to a monotreme (platypus).
The requirement for sustained flight has meant that birds have evolved many derived
characteristics essential for mastering this process. The most important legacy of
this adaptation was the emergence of endothermy (bodily heat generation from within).
This feature alone allowed survivability in changing climates and also a wider distribution
of species across the planet. Perhaps without this spin-off benefit for the requirement
for flight, land animals would still be at the reptile stage - and no mammals (including)
ourselves would have appeared on this Earth. This was, perhaps, another one of those
tenuous, fluke-like happenings which went along to determine our very existence today.
Fossils of the proto-bird, Archaeopteryx, discovered in Germany, have been dated
to around 160 million years old. The fossils clearly show an array of feathers, many
of which resemble those of modern birds. Archaeopteryx, however, does not appear
to have a developed keel bone, which in modern birds, is used to attach the powerful
muscles needed for sustained flight. So we can assume that this bird could only glide
or soar. The fossil remains also show a significant bony tail and what appears to
be a heavy skull and mandible with teeth - not the lightweight features associated
with true birds.
In evolutionary form, Archaeopteryx appears to have been just one stage on from another
proto-bird known as Microraptor, fossils of which have been discovered in China.
Their fossils show flight like feathers as in Archaeopteryx but in addition they
also have long feathers on their legs, giving a ‘double wing’ configuration. Microraptor
had an even longer bony tail and it seems apparent these animals only used their
feathers to enhance their gliding capabilities.
If we extrapolate further, we may want to consider what type of reptile could have
been the forerunner of Microraptor? Heterodontosaurus is a possible example of an
ancestral type of reptile. It was a small bipedal ‘dinosaur’ and fossils discovered
in South Africa show it had powerful legs. These could have enabled it to jump up
into trees in search of food resources or escape from predators.
South Africa, in early Jurassic times, was on an equatorial line which cut northwards
through the African continent towards western Europe. The day time temperatures would
have been quite warm but the night time temperatures would drop significantly. It
appears that some dinosaurs may have evolved into large animals in order to withstand
these lower temperatures as there is greater heat conservation within a larger body
mass. However, some dinosaurs like Heterodontosaurus may have evolved a form of insulation
- such as downy scales to retain heat in their bodies. Heterodontosaurus would need
to remain a small animal if it exploited food resources within trees and there would
be advantages in being able to hop up into a tree and from there move up onto successively
higher branches. Being able to leap considerable distances back down to the ground,
without injury, would be also have been a big advantage and these downy feathers
may have provided incremental improvements in this ability.
We have to bear in mind the type of trees which were common in Jurassic times. These
were tree ferns, conifers, gingko and palms. The branching systems were different
to modern species but these trees will have been accessible for Heterodontosaurus.
If longer and longer downy scales which enhanced gliding and reduced fall rates were
selected for over many thousands of generations we can come to imagine how flight
assisting feathers could have been derived. The feathers on the bony tails of these
early creatures would initially serve as a sort of rudder, so that dangerous obstacles
such as branches or rocks outcrops could be avoided during descent. The glide path
to another tree could be also be navigated.
There would naturally come a stage where gliding or soaring on thermals could be
assisted by the occasional arm movements. This in turn would lead, by natural selection,
to longer and longer flight time. This would be a reflection of stronger arm muscles
and an increased metabolic rate to cope with energy requirements.
So, perhaps by about 120 million years ago, true birds existed which could achieve
an effortless sustained flight.
Some modern looking birds, therefore will have co-existed with dinosaurs in warmer
climes but their warm-bloodedness, an adaptation for the power of flight, held them
in good stead for the colder regions too. Flight has the enormous benefit of allowing
the bird to cover considerable distances, often over great expanses of water or difficult
terrain, during migration. Migration allows access to seasonal food resources in
remote areas and the escape from changing weather patterns.