• Evolution & Evidence • Evolution & Science • Basics • Evidence: Index

• Related FAQ Links --> • Site Resources • Main Site Index • What is Atheism? • Religion & Theism • Skepticism & Logic • Arguments for / against Gods • Evolution vs. Creationism • Religious Timelines • Hate Mail • Glossary • Book Reviews

• Chat Room Join others in the Agnosticism/Atheism chat!

The many homologies between living organisms offer powerful evidence for evolution in and of themselves. However, there is another way of looking at the data that ties the various homologies together into an even more powerful form of evidence, the hierarchy of life. If common descent actually happened, the general theory of evolution predicts that:

1. All of life should be groupable into a hierarchical, nested tree of organisms which shows the closeness of their relationships to each other; and
2. No matter what metric you use to develop the tree, you should get very similartrees.

To recognize what a powerful prediction this is, you have to consider a few key points. First, outside of evolutionary theory, there is no reason for species to fit into nice nested trees at all. In fact, you would expect that they would not form such trees, since many of the characteristics they would be based on would be independent variables. You would expect to find contradictions that would prevent you from organizing life into a hierarchical, nested tree which has, by definition, a "root" or common origin point. (See Figure 1.)

Figure 1. The vertical direction is time and the horizontal is change of characteristics of life forms. (a) Common descent; (b) Transformationism. Note that species can change significantly but that they have independent starting points. No common ancestry.; (c), (d) and (e) Creationism. (d) and (e) are creationist explantions for extinctions and/or appearance of new species over time such as are shown in the fossil record. Note that while the creationist paradigms are drawn as stright lines some minor variation might be allowed (i.e. within " kinds quot;.)

With a small number of attributes you might be able to make some subtrees just by chance even if life forms arose independently. However, it would be incredibly improbable for you to form a consistent set of trees from different, essentially independent characteristics. Given the number of species to group and the number of possible characteristics, there are an enormous number of possible trees that could be formed. Many times the number of atoms we would expect in the universe.

The improbability of forming fairly consistent hierarchies from characteristics of lifeforms which arose independently makes the creationist claims about the improbability of abiogenesis or the spontaneous origin of the universe seem incredibly likely by comparison. (Of course, creationist claims about abiogenesis and cosmic origins are largely flawed or irrelevant as well but, that's another article.) If we just consider large groupings of organisms, for example the 29 major taxa, there are over 10³⁶ possible ways to arrange the classes into a tree.

Possibly the most compelling evidence for common descent is that, not only do life forms fall into nice hierarchies based on just about any set of characteristics you would like to look at, but every set of characteristics you choose to group by yields trees that are very similar! The same basic phylogenetic tree (courtesy of Zeus Thibault) can be arrived at by independently looking at similarities of anatomy, proteins, DNA, or other biochemical characteristics. The only rational explanation for this is if all life originated from a common ancestor.

You would expect life forms to fall into nested trees of similarity if all life arose from a common ancestor. The fact that life forms do form these sorts of consistent trees is an amazing verified prediction of the general theory of evolution and a powerful piece of evidence that it is correct. As more genetic codes are decoded and more comparisons are made, they continue to fall into place. With the amount of work on decoding the genomes of various organisms that is going on today, new pieces of evidence for evolution are arriving regularly. The general theory of evolution has withstood the test of all the data.

Summary of Homology Evidence

The homologies among life forms offer incredible support for the general theory of evolution. These homologies come from several different sources including anatomy, embryology, and biochemistry, and include special homologies among junk DNA that only make sense from an evolutionary perspective.

All of science is provisional, so there is always room for doubt where science is concerned. However, given the lack of a functional explanation for these homologies, the similarities in nonfunctional and even detrimental characteristics, the large number of alternatives available, and the consistent set of hierarchies formed when organisms are grouped by these characteristics, any doubt moves from "barely reasonable doubt" to "unreasonable doubt." What's amazing is, we haven't even gotten to the fossil evidence yet! Evolution is extremely well supported without even considering the fossil evidence. In the next sections, we'll briefly look at biogeographical evidence and then move on to examine the fossil evidence.

(A bibliographic list of sources and recommendations for further reading will be coming soon...)

This FAQ information provided by Lance F..