others werepreparing to migrate to the land; they were already present by accident
and becameselected traits only when they imparted an advantage to the fish on
land.
The early land-dwelling amphibians were slim-bodied with fishlike
tails, butthey had limbs capable of locomotion on land. These limbs probably
developedfrom the lateral fins, which contained fleshy lobes that in turn
contained bonyelements.
The ancient amphibians never became completely adapted for
existence onland, however. They spent much of their lives in the water, and their
moderndescendants, the salamanders, newts, frogs, and toads–still must return
to water todeposit their eggs. The elimination of a water-dwelling stage, which was
achievedby the reptiles, represented a major evolutionary advance.
The Reptilian Age Perhaps the most important factor contributing to the becoming of
reptilesfrom the amphibians was the development of a shell- covered egg that
could be laidon land. This development enabled the reptiles to spread throughout the
Earth’slandmasses in one of the most spectacular adaptive radiations in
biological history.
Like the eggs of birds, which developed later, reptile eggs
contain acomplex series of membranes that protect and nourish the embryo and help
itbreathe. The space between the embryo and the amnion is filled with an
amnioticfluid that resembles seawater; a similar fluid is found in the fetuses
of mammals,including humans. This fact has been interpreted as an indication that
life originatedin the sea and that the balance of salts in various body fluids did not
change verymuch in evolution. The membranes found in the human embryo are
essentiallysimilar to those in reptile and bird eggs. The human yolk sac remains
small andfunctionless, and the exhibits have no development in the human embryo.Nevertheless, the presence of a yolk sac and allantois in the human
embryo is oneof the strongest pieces of evidence documenting the evolutionary
relationshipsamong the widely differing kinds of vertebrates. This suggests that
mammals,including humans, are descended from animals that reproduced by means ofexternally laid eggs that were rich in yolk.
The reptiles, and in particular the dinosaurs, were the dominant
landanimals of the Earth for well over 100 million years. The Mesozoic Era,
duringwhich the reptiles thrived, is often referred to as the Age of Reptiles.
In terms of evolutionary success, the larger the animal, the
greater thelikelihood that the animal will maintain a constant Body Temperature
independentof the environmental temperature. Birds and mammals, for example,
produce andcontrol their own body heat through internal metabolic activities (a
state known asendothermy, or warm-bloodedness), whereas today’s reptiles are thermally
unstable(cold-blooded), regulating their body temperatures by behavioral
activities (thephenomenon of ectothermy). Most scientists regard dinosaurs as
lumbering,oversized, cold-blooded lizards, rather than large, lively, animals with
fast metabolicrates; some biologists, however–notably Robert T. Bakker of The Johns
HopkinsUniversity–assert that a huge dinosaur could not possibly have warmed
up everymorning on a sunny rock and must have relied on internal heat
production.
The reptilian dynasty collapsed before the close of the Mesozoic
Era.Relatively few of the Mesozoic reptiles have survived to modern times;
thoseremaining include the Crocodile,Lizard,snake, and turtle. The cause of
the declineand death of the large array of reptiles is unknown, but their
disappearance isusually attributed to some radical change in environmental conditions.
Like the giant reptiles, most lineages of organisms have
eventually becomeextinct, although some have not changed appreciably in millions of
years. Theopossum, for example, has survived almost unchanged since the late
CretaceousPeriod (more than 65 million years ago), and the Horseshoe Crab,
Limulus, is notvery different from fossils 500 million years old. We have no
explanation for theunexpected stability of such organisms; perhaps they have achieved an
almostperfect adjustment to a unchanging environment. Such stable forms,
however, arenot at all dominant in the world today. The human species, one of the
dominantmodern life forms, has evolved rapidly in a very short time.
The Rise of Mammals
The decline of the reptiles provided evolutionary opportunities
for birds andmammals. Small and inconspicuous during the Mesozoic Era, mammals rose
tounquestionable dominance during the Cenozoic Era (beginning 65 million
yearsago).
The mammals diversified into marine forms, such as the whale,
dolphin,seal, and walrus; fossorial (adapted to digging) forms living
underground, such asthe mole; flying and gliding animals, such as the bat and flying
squirrel; andcursorial animals (adapted for running), such as the horse. These
variousmammalian groups are well adapted to their different modes of life,
especially bytheir appendages, which developed from common ancestors to become
specializedfor swimming, flight, and movement on land.
Although there is little superficial resemblance among the arm of
a person,the flipper of a whale, and the wing of a bat, a closer comparison of
their skeletalelements shows that, bone for bone, they are structurally similar.
Biologists regardsuch structural similarities, or homologies, as evidence of evolutionary
relationships.The homologous limb bones of all four-legged vertebrates, for example,
areassumed to be derived from the limb bones of a common ancestor.
Biologists arecareful to distinguish such homologous features from what they call
analogousfeatures, which perform similar functions but are structurally
different. Forexample, the wing of a bird and the wing of a butterfly are analogous;
both areused for flight, but they are entirely different structurally. Analogous
structures donot indicate evolutionary relationships.
Closely related fossils preserved in continuous successions of
rock stratahave allowed evolutionists to trace in detail the evolution of many
species as it hasoccurred over several million years. The ancestry of the horse can be
tracedthrough thousands of fossil remains to a small terrier-sized animal with
four toes onthe front feet and three toes on the hind feet. This ancestor lived in
the EoceneEpoch, about 54 million years ago. From fossils in the higher layers of
stratifiedrock, the horse is found to have gradually acquired its modern form by
eventuallyevolving to a one-toed horse almost like modern horses and finally to
the modernhorse, which dates back about 1 million years.
CONCLUSION TO EVOLUTION
Although we are not totally certain that evolution is how we got
the way weare now, it is a strong belief among many people today, and scientist
are findingmore and more evidence to back up the evolutionary theory.