The idea of evolution, together with the absence of spontaneous generation in the modern world leads to some simple conclusions.

Given the similarities between all living organisms, which will be described below, an obvious conclusion is that all life is descended from a single common ancestor.

Recognition of the structural similarity, and apparent relatedness of organisms, predates the theory of evolution.

In this scheme, each organism has a unique genus and a species name, such as Sahelanthropus tchadensi (both are italicized and the species name is not capitalized).

The organism is also placed in the classification hierarchy.

So Sahelanthropus tchadensi has been placed in the Kingdom of Animals, the Phylum of Chordates, the Class Mammalia, and the Order Primate (as are human beings!)

Homology and Analogy

A major problem in classifying organisms is deciding whether similar traits reflect common ancestry or independent solutions to a particular problem, that is whether the traits were developed independently.

For example, an anatomical analysis of the forelimb of the mammals suggests that they are homologous structures.

To say that two structures are homologous means that they are derived from a common ancestor.

Moreover, it implies that the features they share were already present in their common ancestor.

This type of reasoning leads to the prediction that fossils of the last common ancestor of the mammals will be found to have a forelimb with a similar structure to that of modern mammals.

The alternative is that the structures are analogous, rather than homologous. When two structures are analogous, it means that they evolved independently. Their final form is determined by their function.

Consider the wing of a pterodactyl, a flying reptile, a bird, and a bat, a mammal. Analysis of these structures indicates that they are distinct.

In the pterodactyl, the wing membrane is supported by the 5th finger of the forelimb, in the bird by the 2nd finger, and in the bat, by the 3rd, 4th and 5th fingers.

The last common ancestor of flying reptiles, birds and mammals did not have wings, although it certainly had a forelimb.

These structures formed independently. They are analogous structures.

The physics of flight are constant. Organisms of the same size must deal with the same aerodynamic problems and constraints. In general there are only a limited number of workable solutions.

Under these conditions different populations will, through the process of variation and selection, end up with similar solutions. This process is known as convergent evolution.

Convergent evolution occurs frequently, primarily because there are only certain solutions available to solve specific 'problems'.

For example, the use of a dagger is a particularly effective solution to the problem of killing another organism.

This solution has been "discovered" independently many times.

Marsupial and placental mammals diverged from a common ancestor ~100Myr ago. Since then, they have converged on a number of similar adaptations.

The evidence for a common ancestor:

Similar structures may arise independently. Apparently different structures, such as a human hand and a whale's flipper, can be related.

To distinguish between these two possibilities requires a carefully examination of structural details. The more details two structures share, the more likely they are homologous rather than analogous.

For examples, all organisms use ribosomes to translate the information stored in nucleic acids into proteins.

The ribosomes of bacteria and humans are quite similar in shape and molecular organization.

Detailed analyses of many different types of organisms reveals the presence of a common 'molecular signature' that speaks to the nature of the last common ancestor. If we look at living organisms we find they all ...

• use DNA as their genetic material
• use a common genetic code with only a few variations
• use the same molecular system to translate DNA-based information into polypeptides
• use the same amino acids and nucleotides to build polypeptides and nucleic acids
• use adenosine triphosphate or ATP to store energy.