Last week, I got to catch a talk and spend some time with Nicholas Holland, a biologist from the Scripps Oceanographic Institute at UC San Diego. Dr. Holland discussed the history of the “homology” concept in science. Today, I want to start with a primer on the current debates regarding homology, and a little bit about the history of the concept.
A Homology Primer
The textbook definition of homology is simple: homologies are characters that are shared by two organisms because they both inherited that characteristic from a common ancestor. For example, the reason that humans, cats, and hedgehogs all have fur is because they share a furry common ancestor. Fur is a homologous trait among mammals.
However, it is not always so easy to determine what characteristics are really homologous. Humans have complicated eyes, and so do octopi. Are these structures homologous?:
Some aspects of these eyes are similar, for example they both have a lens and a retina. But some other characteristics are quite different. Human eyes develop as outgrowths of the brain, while octopus eyes develop from invaginations of the animal’s surface. In humans, nerve fibers pass in front of the retina, creating a blind spot, while in octopi, the nerve fibers are behind the retina, so they don’t have that same limit to their vision. These many differences, combined with the fact that many relatives of humans and octopi lack complex eyes, has led most scientists to conclude that human and octopus eyes evolved independently, or that they are evolutionarily convergent.
And the concept gets further complicated by the fact homology can vary depending on what level of organization you are talking about. For example, the wings of bats and birds are convergent; each has independently evolved wings from an ancestor that had forelegs. However, at the skeletal level the wings of birds and bats are homologous; they share the same bones in their wings (such the humerus) because the last ancestor of these animals had the same bones in its forelimb:
Homologies can exist at the genetic level as well. In the 1960’s, before genetics was well understood, many prominent scientists thought that animals gain and lose genes too quickly during evolution for homologous genes to exist (Dr. Holland had a great quote from the prominent, and in this case very wrong, biologist Ernst Mayr, “the search for homologous genes is quite futile except in very close relatives”). We now know that most genes important to development are shared in animals as diverse as humans, fruit flys, and jellyfish. This discovery has been critical to biology, earning the 1995 Nobel Prize in Physiology or Medicine, and leading to the birth of evo-devo, but it has really complicated the idea of homology. The genes Pax6, Eyes absent, sine oculis, and dachsund form a network that is critical not only to the eyes of humans and cephalopods like octopi, but also the compound eyes of insects, which look nothing like our “camera-type” eyes. In fact, these genes are so conserved, that if a mouse has a faulty version of any of these genes, scientists can “rescue” eye development by injecting the mouse with the fruit fly version of the gene! Some evo-devo scientists have called this “deep homology”; that at the level of genetics these structures are derived from the same ancestral gene networks (e.g. Shubin, Tabin, and Carroll 1997).
But what does deep homology really mean? In the 90’s many scientists took a very literal view. If two animals shared an “eye” gene like Pax6, then their last common ancestor must have had Pax6 as well, and therefore an eye. Many papers were using shared genes to construct complex animal ancestors (e.g. Arendt and Nubler-Jung 1996, De Robertis and Sasai 1996). Dr. Holland included a great image of this idea in his talk, which was taken from Veraska et al (2000). This shows the complex ancestor of humans, octopi, and frut flies, and the genes used to justify the complexity:
In the last decade, this idea has become hotly contested. Some animals that do not share these complicated structures do share these genes, and some animals with these complex structures do not use the same genes to build them. Currently, there is no consensus about what homologous genes tell us about the physical nature of the common ancestor. I think this is the biggest question in evo-devo today. In my next post, I’ll go into detail on how Dr. Holland tackles this problem.
Papers Cited
Arendt, D. and Nubler-Jung, K. (1996). Common ground plans in early brain development in mice and flies. BioEssays 18, 255-259
De Robertis, E.M. and Sasai, Y. (1996). A common plan for dorso-ventral patterning in Bilateria. Nature 380, 37-40.
Mayr, E. (1963) Animal Species and Evolution. Harvard Univ. Press (p. 609)
Shubin, N., Tabin, C. & Carroll, S. (1997). Fossils, genes, and the evolution of animal limbs. Nature 388, 639–648
Veraksa, A, Del Campo, M. and McGinnis, W. (2000). Developmental patterning genes and their conserved functions: From model organism to humans. Molec. Gen. & Metabolism 69: 85-100.
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