In the case of our own emergence
there are more than two dozen transitional fossils
intermediate between modern humans
and our own ape-like ancestors.
But it is not enough to simply find the transitional fossils.
They also must satisfy the sequence test.
That is, the ages of the fossils must match the progression
from primitive form to a more modern form.
Dating fossils is a relatively straightforward process
using several methods to cross-check the ages determined.
Some care must be taken to ensure that dates considered
are the earliest appearance of each fossil
since there's no reason a transitional form must go extinct
before a modern form appears.
And there is no reason a successful form cannot endure for long periods of time
with very little change.
The modern crocodile can boast of one hundred million years
and the cockroach
which has been around for two hundred and eighty million years.
But back to sequencing
If we consider our reptile-to-bird example
and date the intermediate forms
we find that they are in precisely the right order.
A wider view also passes the sequence test.
Prokaryotes appear first,
followed in order by sponges
and starfish, fish,
amphibians,
reptiles,
and mammals.
We will return to this point later.
In the examination of structured organisms
we frequently find organs that either have no function or have a modified function
completely out of character with the structure.
For example:
wings are very complex structures specifically adapted for flying.
So why do ostriches have wings?
Ostrich wings are certainly not useless to the ostrich,
but they're certainly useless as wings.
The vestigial blind eyes of the mole and cave-dwelling fish and salamanders
are easily explained only by reference to previous ancestors
who had a need of eyes.
Snakes are clearly the descendants of four-legged reptiles.
Most pythons still carry vestigial pelvises
hidden beneath their skin.
These useless pelvises are not even attached to the vertebrae,
but simply float in the abdominal cavity.
Many legless lizards carry rudimentary vestigial legs underneath their skin
undetectable from the outside.
Dandelions reproduce without fertilization.
Yet they retain flowers and produce pollen
useless to the dandelion,
but not to its ancestors.
Many flightless beetles still carry the fully formed wings
of their ancestors, but carry them underneath fused wing covers.
Human herbivore ancestors are responsible for our useless wisdom teeth
and our vermiform appendix.
And our tailbone
is the developmental remnant tying us to our ancestors
who had external tails.
There are no vestigial structures that were not previously functional
in an ancestor.
All vestigial organs make sense only in the framework of evolution.
And of course we do not find vestigial organs that argue against evolution.
No nipples in amphibians,
or vestigial feathers on mammals,
no primates carried vestigial horns or degenerate wings.
We do not find arthropods with leftover backbones,
no snakes have wing parts and no humans have gizzards.
If creatures evolved from one another
there should be a geographical trail.
Marsupials are a good example.
With few exceptions marsupials inhabit Australia.
The possum and some South American species are explained by continental drift.
South American and Australia used to be connected
and split apart only after marsupials evolved.
Conversely placental mammals were virtually absent in Australia
until we introduced them there.
South America, Africa and Australia share lungfishes, ratite birds like ostriches and emus and toothed frogs,
all of which occur nowhere else.
Human's closest living relatives are the great apes indigenous to Africa.
So it is no real surprise to find that our own origins are there as well.
A test closely related to vestigial sections
addresses similarities in structure in spite of differences in the functions
of that structure.
If species evolve from another species,
the new species must often adapt and modify existing body parts
to do different tasks.
If we examine the bone structure in primate hands
and bat wings
and bird wings and pterosaur wings
and whale flippers
and penguin flippers
and horse legs and mole forelimbs and frog legs,
we will find they all have the same bones in the same relative positions.
The standard tree diagram easily demonstrates why these different species
have the same structure.
That is they have common ancestors who possess these structures.
The fossil record again confirms this conclusion
as it readily provides a chronological progression of intermediate forms.
We can occasionally find explanations for what appears to be bad design
by tracing the source of body structure to our ancestors.
The mammalian respiratory and gastro-intestinal tracts
cross one another,
so we cannot swallow and breathe simultaneously.
Why does this less than optimal anatomy exists?
An ancient ancestor from the Devonian,
a lungfish, swallowed air to breathe
and the bladder that held the air was the precursor to our lungs.
Humans have inherited this original design with the mouth connected
to both the digestive and respiratory system,
even though it now causes problems.