Although it makes sense to stick with a good idea when you’ve found it, you wouldn’t stubbornly stick to the same design even when it wasn’t quite working properly, would you? And yet this is just the pattern that appears in nature. Life on Earth shares a remarkable list of features, from protein-handedness and membrane structure to the DNA code. While some of these features, such as protein-handedness (which way around proteins are formed), are inconsequential, others are not. The genetic code is almost completely universal across all life. This is the reason GM can work, because a gene coded for in the genome of one species can be read by the translation machinery in another species’ cells. This does not necessarily have to be true, however, as the code is arbitrary and there are many possible configurations which would work equally well. Furthermore, the code is actually detrimental for some species living in extreme environments, since certain codes are more volatile than others. Despite this, the code is shared by all.

Homologies (shared traits) also appear at a smaller scale, between closely related species. For example, all vertebrates have the same design for an eye, even though there are many other possible designs that are functional. Within groups of related species, traits shared amongst the group are sometimes carried even by species that do not need them. For example, the southern cave fish (Typhlichthys subterraneus) is a cave-dwelling fish which carries vestigial eyes. These eyes are non-functional (and they don’t need to be when you live in pitch-black!) but they are still there – every generation eye structures are being grown despite the fact that they serve no purpose. What designer would incorporate that into their plans?

More stark similarities exist between certain groups of species, such as vertebrates, who all share a remarkably similar skeletal structure and basic body plan, as well as internal similarities right down to the cellular level. All vertebrates have a five-fingered (pentadactyl) hand (or foot!), and although this basic design has been modified by some species (such as the fused digits of the panda), common features are shared by all. Even whales have modified the pentadactyl limb from a hand into a flipper, and bats into a wing. And yet regardless of its use, it has five fingers. Snakes, although lacking limbs, still grow vestigial structures of the pelvis, indicating their close relatedness to quadrapedal creatures such as lizards.

The key point is that these similarities occur despite hugely disparate ways of life. When we look at the natural world, the structural similaries are not grouped according to the uses they are put to – whether you walk, swim, fly or slither, whether you eat meat, or fruit, or plants, or sunlight – the similarities reveal relatedness rather than common purpose.

These grouping of similarities, even when the shared trait is not the most beneficial for all group members, strongly suggest the branching pattern of relatedness between species that evolution predicts. Some traits are shared by all life, and these traits were probably also present in the last common ancestor of all life. These traits are at the cellular level; the DNA code, the machinery for translating genes into proteins, the design of the phospholipid cell wall. Other traits are shared by smaller groupings of species, and these traits appeared sometime before their last common ancestor. All eukaryotes (fungi, plants and animals) share internal cellular features such as mitochondria. All vertebrates share a remarkably similar skeletal structure, and all warm-blooded animals use a substance known as keratin to produce hair and fur. And so on. There is no logical reason to expect that intelligent design would create a pattern like this. But it is exactly what we would predict from the theory of natural selection. Decent by modification should produce a tree of life in which more closely related branches are more similar. Intelligent design should produce structures informed by use, but this is not the pattern we observe in nature.

Articles in this Series:

1. Intro: Reasons Why Evolution is True
2. Part One: The Panda’s Thumb
3. Part Two: Parasitoid Wasps
4. Part Three: Ring Species
5. Part Four: Galapagos Finches
6. Part Five: The Quirky Human Eye
7. Part Six: Homology
8. Part Seven: Coevolution
9. Part Eight: PreCambrian Rabbits
10. Part Nine: DIY Evolution
11. Part Ten: Convergent Evolution

Want to Know More?

• The Natural History Museum: The Pentadactyl Limb
• Striedter and Northcutt (1991) Biological Hierarchies and the Concept of Homology
• The Top 10 Useless Limbs
• Diogo et al(2008) From fish to modern humans – comparative anatomy, homologies and evolution of the head and neck musculature
• Diogo et al(2009) On the origin, homologies and evolution of primate facial muscles, with a particular focus on hominoids and a suggested unifying nomenclature for the facial muscles of the Mammalia
• Rational Wiki: Common Decent
• Clack (1993) Homologies in the Fossil Record: The Middle Ear as a Test Case 
• Bloch, McArthur and Mirrop (1985) tRNA-rRNA sequence homologies: Evidence for an ancient modular format shared by tRNAs and rRNAs
• McCune and Schimenti (2012) Using Genetic Networks and Homology to Understand the Evolution of Phenotypic Traits